Merge branch 'main' into hanif/preview-drag-detail

A profile with no top or bottom shell layers, or no walls, leaves the infill on the surface,
so hiding it changed the picture. The viewer now reads the print's default region settings
when it loads a print and leaves the infill in place for such a print; per-object overrides
are not consulted.

docs/HLSD/gcode-preview-dragging.md

@@ -0,0 +1,80 @@
+# G-code preview while dragging
+
+The sliced preview draws every toolpath segment of the plate as an instanced box. On a large
+plate that is tens of millions of segments, and the frame is GPU-bound: the cost is the number of
+instances drawn, not anything the CPU does per frame. Dragging the camera over such a plate cannot
+keep up. The `preview_reduced_detail_mode` preference (*Graphics > G-code Preview*, off by
+default) lets the preview draw less while the user drags and put the full toolpaths back when they
+let go.
+
+| Preference | Values | Effect |
+|---|---|---|
+| `preview_reduced_detail_mode` | `off`, `solid`, `shell` | what is drawn while dragging |
+
+libvgcode (`src/libvgcode`) builds and binds the reduced toolpath set, `GCodeViewer` maps the
+preferences onto it and draws the solid model, and `GLCanvas3D` decides when the user is dragging.
+The OpenGL ES path keeps a single set and ignores the preference.
+
+## Two sets, one walk
+
+`ViewerImpl::update_enabled_entities()` walks the visible vertex range once and fills two segment
+index buffers side by side: the **full** set and the **reduced** set (segments and options).
+Building them together is what makes switching free: starting or ending a drag is a buffer
+binding, never a rebuild. A change of mode does rebuild. Nothing is built while the mode
+is off, and the reduced buffers are then uploaded empty so that the last set does not stay
+allocated.
+
+Whatever the mode leaves out, the bottom and top layers of the visible range are kept whole: they
+are the faces the range cuts open, and the top is what the user is looking at.
+
+### Modes
+
+- `EndLayersOnly` (`solid` in the preference) keeps only the two end layers. `GCodeViewer` then
+  draws the sliced objects and the prime tower as opaque solids, see below.
+- `ShellOnly` (`shell`) keeps every role but the sparse infill, internal solid infill, internal
+  bridge infill and gap fill, which lie under the skins or between the walls. Walls, top and
+  bottom surfaces, bridges, supports and the prime tower are drawn whole, so from outside the
+  print looks as it does at rest. The roles come from
+  the slicer, so the classification is exact wherever the slicer's is; what it cannot express is
+  the inside of the prime tower and of a support, which share one role with their outside and are
+  kept whole. A profile with no top or bottom shell layers, or no walls, leaves the infill on the
+  surface, so `GCodeViewer` turns the hiding off for such a print when it loads it, from the
+  print's default region settings; per-object overrides are not consulted.
+
+## The solid model
+
+The preview already loads the sliced objects as shells for its translucent ghost.
+`GCodeViewer::render_solid_model()` draws those shells opaque, in their filament colors, with the
+`gouraud` shader, whose z range cuts them to the visible layer range. The two toolpath layers of
+the reduced set are drawn afterwards and cap the cut with what was really printed there. The
+shells hold only the objects, so while this mode is on the prime tower is added from its sliced
+mesh, positioned as the print placed it. It is added or removed on its own when the mode changes,
+without reloading the objects, keeps its opaque color so that it never appears among the
+translucent shells, and stays out of their bounding box. Supports have no mesh and are not shown,
+and `load_shells()` drops every non-model-part volume, so a negative volume is not cut out.
+
+A plate whose shells are not loaded keeps drawing toolpaths, since the solid model would leave
+only the end layers.
+
+## Deciding that the user is dragging
+
+`GLCanvas3D::_update_preview_interaction()` runs at the top of every preview frame, before the
+canvas decides whether to reuse its cached scene, so that the switch lands in that frame. Dragging
+is `GLCanvas3D::is_user_interacting()`, the same answer the scene cache reads: the camera, the
+navigator, a gizmo, the rectangle selection or either slider being held. A slider reports this from
+ImGui's active id rather than its dirty flag, which is raised and consumed inside one frame. A
+wheel step has no duration, so it holds the reduced set for a 150 ms settle time instead, and the
+frame that restores the toolpaths is scheduled for when that time runs out, since the render timer
+only wakes the idle loop. A drag cut short by focus or capture loss is ended explicitly, and a
+button release wakes the idle loop, because on some platforms nothing else would until the next
+input.
+
+## Reused scene frames
+
+`GLCanvas3D` keeps its last scene pass for frames that only rebuild the overlay (`SceneCache`). Its
+key covers the canvas size, the camera and hover state, not what the toolpath sets draw, so a frame
+that reuses the scene must never be one on which the set is switched.
+`_update_preview_interaction()` therefore reports whether the bound set changed, and a frame on
+which it did redraws the scene. The canvas neither captures nor reuses the scene while the user
+drags, so no reduced frame outlives a drag, and the frame that ends a wheel's settle time is
+requested as a full frame.

src/libslic3r/AppConfig.cpp

@@ -205,6 +205,13 @@ void AppConfig::set_defaults()
     if (get("seq_top_layer_only").empty())
         set("seq_top_layer_only", "1");
 
+    // what the preview draws while the user drags it
+    {
+        const std::string mode = get("preview_reduced_detail_mode");
+        if (mode != "off" && mode != "solid" && mode != "shell")
+            set("preview_reduced_detail_mode", "off");
+    }
+
     // ORCA: darken the layers the preview layer slider is not scrubbed to
     if (get("preview_dim_previous_layers").empty())
         set_bool("preview_dim_previous_layers", false);

src/libslic3r/ClipperUtils.cpp

@@ -1,11 +1,7 @@
-#include <algorithm>
-#include <cmath>
 #include <limits>
 #include <numeric>
 #include <unordered_map>
 
-#include <tbb/parallel_for.h>
-
 #include "ClipperUtils.hpp"
 #include "Geometry.hpp"
 #include "ShortestPath.hpp"
@@ -817,69 +813,6 @@ Slic3r::ExPolygons intersection_ex(const Slic3r::Surfaces &subject, const Slic3r
     { return _clipper_ex(ClipperLib::ctIntersection, ClipperUtils::SurfacesProvider(subject), ClipperUtils::SurfacesProvider(clip), do_safety_offset); }
 Slic3r::ExPolygons intersection_ex(const Slic3r::SurfacesPtr &subject, const Slic3r::ExPolygons &clip, ApplySafetyOffset do_safety_offset)
     { return _clipper_ex(ClipperLib::ctIntersection, ClipperUtils::SurfacesPtrProvider(subject), ClipperUtils::ExPolygonsProvider(clip), do_safety_offset); }
-
-namespace ClipperUtils {
-    std::vector<ExPolygonsTile> tile_expolygons(const ExPolygons &expolygons, size_t per_tile)
-    {
-        BoundingBox              extent;
-        std::vector<BoundingBox> bboxes;
-        bboxes.reserve(expolygons.size());
-        for (const ExPolygon &expoly : expolygons) {
-            bboxes.emplace_back(get_extents(expoly));
-            extent.merge(bboxes.back());
-        }
-        if (! extent.defined)
-            return {};
-        const int     tiles  = std::clamp(int(std::sqrt(double(expolygons.size()) / double(std::max<size_t>(per_tile, 1)))), 1, 32);
-        const Point   size   = extent.size();
-        const coord_t tile_w = std::max<coord_t>(1, size.x() / tiles + 1), tile_h = std::max<coord_t>(1, size.y() / tiles + 1);
-        std::vector<ExPolygonsTile> out(size_t(tiles * tiles));
-        for (size_t i = 0; i < expolygons.size(); ++ i) {
-            const Point c = bboxes[i].center();
-            ExPolygonsTile &tile = out[size_t(std::clamp(int((c.y() - extent.min.y()) / tile_h), 0, tiles - 1) * tiles +
-                                              std::clamp(int((c.x() - extent.min.x()) / tile_w), 0, tiles - 1))];
-            tile.members.emplace_back(i);
-            tile.bbox.merge(bboxes[i]);
-        }
-        out.erase(std::remove_if(out.begin(), out.end(), [](const ExPolygonsTile &tile) { return tile.members.empty(); }), out.end());
-        return out;
-    }
-}
-
-static Slic3r::ExPolygons clipper_ex_by_piece(ClipperLib::ClipType clipType, const Slic3r::ExPolygons &subject, const Slic3r::Polygons &clip, ApplySafetyOffset do_safety_offset)
-{
-    // A few dozen subject ExPolygons to a tile, each tile one ClipperLib call with the clip cut to the tile's box.
-    const std::vector<ClipperUtils::ExPolygonsTile> tiles = ClipperUtils::tile_expolygons(subject, 32);
-    std::vector<BoundingBox> clip_bboxes;
-    clip_bboxes.reserve(clip.size());
-    for (const Polygon &polygon : clip)
-        clip_bboxes.emplace_back(get_extents(polygon));
-
-    std::vector<Slic3r::ExPolygons> out_tiles(tiles.size());
-    tbb::parallel_for(size_t(0), tiles.size(), [&](size_t tile_idx) {
-        const ClipperUtils::ExPolygonsTile &tile = tiles[tile_idx];
-        Slic3r::ExPolygons local_subject;
-        local_subject.reserve(tile.members.size());
-        for (size_t i : tile.members)
-            local_subject.emplace_back(subject[i]);
-        // Grown so that the cut edges of the clip stay clear of the subject, also after the safety offset.
-        const BoundingBox bbox = tile.bbox.inflated(SCALED_EPSILON);
-        Polygons local_clip;
-        for (size_t i = 0; i < clip.size(); ++i)
-            if (clip_bboxes[i].overlap(bbox))
-                if (Polygon clipped = ClipperUtils::clip_clipper_polygon_with_subject_bbox(clip[i], bbox); ! clipped.empty())
-                    local_clip.emplace_back(std::move(clipped));
-        out_tiles[tile_idx] = _clipper_ex(clipType, ClipperUtils::ExPolygonsProvider(local_subject), ClipperUtils::PolygonsProvider(local_clip), do_safety_offset);
-    });
-    Slic3r::ExPolygons out;
-    for (Slic3r::ExPolygons &out_tile : out_tiles)
-        append(out, std::move(out_tile));
-    return out;
-}
-Slic3r::ExPolygons diff_ex_by_piece(const Slic3r::ExPolygons &subject, const Slic3r::Polygons &clip, ApplySafetyOffset do_safety_offset)
-    { return clipper_ex_by_piece(ClipperLib::ctDifference, subject, clip, do_safety_offset); }
-Slic3r::ExPolygons intersection_ex_by_piece(const Slic3r::ExPolygons &subject, const Slic3r::Polygons &clip, ApplySafetyOffset do_safety_offset)
-    { return clipper_ex_by_piece(ClipperLib::ctIntersection, subject, clip, do_safety_offset); }
 // May be used to "heal" unusual models (3DLabPrints etc.) by providing fill_type (pftEvenOdd, pftNonZero, pftPositive, pftNegative).
 Slic3r::ExPolygons union_ex(const Slic3r::Polygons &subject, ClipperLib::PolyFillType fill_type)
     { return _clipper_ex(ClipperLib::ctUnion, ClipperUtils::PolygonsProvider(subject), ClipperUtils::EmptyPathsProvider(), ApplySafetyOffset::No, fill_type); }

src/libslic3r/ClipperUtils.hpp

@@ -2,7 +2,6 @@
 #define slic3r_ClipperUtils_hpp_
 
 #include "libslic3r.h"
-#include "BoundingBox.hpp"
 #include "clipper.hpp"
 #include "ExPolygon.hpp"
 #include "Polygon.hpp"
@@ -322,15 +321,6 @@ namespace ClipperUtils {
     [[nodiscard]] Polygons clip_clipper_polygons_with_subject_bbox(const ExPolygon &src, const BoundingBox &bbox, const bool get_entire_polygons = false);
     [[nodiscard]] Polygons clip_clipper_polygons_with_subject_bbox(const ExPolygons &src, const BoundingBox &bbox, const bool get_entire_polygons = false);
 
-    // Splits ExPolygons into tiles by the centres of their boxes, about `per_tile` of them to a tile, to run ClipperLib on a
-    // layer of many pieces tile by tile. Returns the non-empty tiles, each with the indices of its ExPolygons and their box.
-    struct ExPolygonsTile
-    {
-        BoundingBox         bbox;
-        std::vector<size_t> members;
-    };
-    [[nodiscard]] std::vector<ExPolygonsTile> tile_expolygons(const ExPolygons &expolygons, size_t per_tile);
-
     }
 
 // Perform union of input polygons using the non-zero rule, convert to ExPolygons.
@@ -528,11 +518,6 @@ Slic3r::ExPolygons intersection_ex(const Slic3r::Surfaces &subject, const Slic3r
 Slic3r::ExPolygons intersection_ex(const Slic3r::Surfaces &subject, const Slic3r::ExPolygons &clip, ApplySafetyOffset do_safety_offset = ApplySafetyOffset::No);
 Slic3r::ExPolygons intersection_ex(const Slic3r::Surfaces &subject, const Slic3r::Surfaces &clip, ApplySafetyOffset do_safety_offset = ApplySafetyOffset::No);
 Slic3r::ExPolygons intersection_ex(const Slic3r::SurfacesPtr &subject, const Slic3r::ExPolygons &clip, ApplySafetyOffset do_safety_offset = ApplySafetyOffset::No);
-// diff_ex() / intersection_ex() of the subject split into tiles, each against only the part of the clip near it, the tiles in
-// parallel. The same area as the operation on the whole subject when its ExPolygons do not overlap, and much faster for a
-// subject of thousands of pieces spread over a layer: ClipperLib slows down with the number of edges crossing a scan line.
-Slic3r::ExPolygons diff_ex_by_piece(const Slic3r::ExPolygons &subject, const Slic3r::Polygons &clip, ApplySafetyOffset do_safety_offset = ApplySafetyOffset::No);
-Slic3r::ExPolygons intersection_ex_by_piece(const Slic3r::ExPolygons &subject, const Slic3r::Polygons &clip, ApplySafetyOffset do_safety_offset = ApplySafetyOffset::No);
 Slic3r::Polylines  intersection_pl(const Slic3r::Polylines &subject, const Slic3r::Polygon &clip);
 Slic3r::Polylines  intersection_pl(const Slic3r::Polyline &subject, const Slic3r::ExPolygon &clip);
 Slic3r::Polylines  intersection_pl(const Slic3r::Polylines &subject, const Slic3r::ExPolygon &clip);

src/libslic3r/Fill/Fill.cpp

@@ -9,8 +9,6 @@
 #include "../PrintConfig.hpp"
 #include "../Surface.hpp"
 
-#include <tbb/parallel_for.h>
-
 #include "AABBTreeLines.hpp"
 #include "ExtrusionEntity.hpp"
 #include "Fill.hpp"
@@ -632,28 +630,24 @@ void split_solid_surface(size_t layer_id, const SurfaceFill &fill, ExPolygons &n
     if (!line_based_pattern) {
         const coord_t scaled_spacing = scaled<coord_t>(fill.params.spacing);
 
-        // Each expolygon is split on its own, so they run in parallel and are collected in their original order.
-        std::vector<std::pair<ExPolygons, ExPolygons>> split_parts(fill.expolygons.size()); // normal, narrow
-        tbb::parallel_for(size_t(0), fill.expolygons.size(), [&](size_t idx) {
-            const ExPolygon &expolygon = fill.expolygons[idx];
+        for (const ExPolygon &expolygon : fill.expolygons) {
             Polygons filled_area = to_polygons(expolygon);
 
             // "Core" area: open (erode+dilate) to drop thin features, then clamp back to the original polygon.
             Polygons inner_area  = intersection(filled_area, opening(filled_area, scaled_spacing, scaled_spacing));
 
             if (inner_area.empty()) {
-                split_parts[idx].second.emplace_back(expolygon);
-                return;
+                narrow_infill.emplace_back(expolygon);
+                continue;
             }
 
             ExPolygons inner_ex = union_ex(inner_area);
             ExPolygons expolys{expolygon};
-            split_parts[idx].second = diff_ex(expolys, inner_ex);         // narrow infill area
-            split_parts[idx].first  = intersection_ex(expolys, inner_ex); // normal infill area
-        });
-        for (auto &[normal_ex, narrow_ex] : split_parts) {
-            append(normal_infill, std::move(normal_ex));
-            append(narrow_infill, std::move(narrow_ex));
+            ExPolygons narrow_ex = diff_ex(expolys, inner_ex);
+            ExPolygons normal_ex = intersection_ex(expolys, inner_ex);
+
+            append(normal_infill, normal_ex); // normal infill area
+            append(narrow_infill, narrow_ex); // narrow infill area
         }
 
         return;
@@ -675,10 +669,7 @@ void split_solid_surface(size_t layer_id, const SurfaceFill &fill, ExPolygons &n
     }
     const double aligning_angle = -base_angle + PI;
 
-    // Each expolygon is reconstructed on its own, so they run in parallel and are collected in their original order.
-    std::vector<Polygons> split_reconstructed(fill.expolygons.size());
-    tbb::parallel_for(size_t(0), fill.expolygons.size(), [&](size_t expolygon_idx) {
-        const ExPolygon &expolygon = fill.expolygons[expolygon_idx];
+	for (const ExPolygon &expolygon : fill.expolygons) {
         Polygons filled_area = to_polygons(expolygon);
         polygons_rotate(filled_area, aligning_angle);
         BoundingBox bb = get_extents(filled_area);
@@ -809,10 +800,8 @@ void split_solid_surface(size_t layer_id, const SurfaceFill &fill, ExPolygons &n
             }
         }
 
-        split_reconstructed[expolygon_idx] = std::move(reconstructed_area);
-    });
-    for (Polygons &reconstructed_area : split_reconstructed)
-        polygons_append(normal_fill_areas, std::move(reconstructed_area));
+        polygons_append(normal_fill_areas, reconstructed_area);
+    }
 
     polygons_rotate(normal_fill_areas, -aligning_angle);
 
@@ -1420,15 +1409,7 @@ void Layer::make_fills(FillAdaptive::Octree* adaptive_fill_octree, FillAdaptive:
             // Orca: Reuse the body origin used for bridge anchoring, resetting it for each surface.
             f->set_bounding_box(infill_bounding_box(*this, surface_fill, expoly, bbox));
 
-            // Only the part of the layer-wide no-overlap area under this expolygon matters, so clip it to the
-            // expolygon's box first (padded past the safety offset, which grows the clip side). The result is
-            // identical; the cost is not: a layer split into many small fills, e.g. by colour painting,
-            // otherwise intersects every one of them with the whole layer.
-            BoundingBox no_overlap_bbox = get_extents(expoly);
-            no_overlap_bbox.offset(SCALED_EPSILON);
-            f->no_overlap_expolygons = intersection_ex(
-                ClipperUtils::clip_clipper_polygons_with_subject_bbox(surface_fill.no_overlap_expolygons, no_overlap_bbox),
-                ExPolygons() = {expoly}, ApplySafetyOffset::Yes);
+            f->no_overlap_expolygons = intersection_ex(surface_fill.no_overlap_expolygons, ExPolygons() = {expoly}, ApplySafetyOffset::Yes);
             if (params.symmetric_infill_y_axis) {
                 params.symmetric_y_axis = f->extended_object_bounding_box().center().x();
                 expoly.symmetric_y(params.symmetric_y_axis);

src/libslic3r/GCode/OrderingStrategies.cpp

@@ -10,7 +10,6 @@
 #include <limits>
 #include <numeric>
 #include <unordered_map>
-#include <unordered_set>
 #include <utility>
 #include <vector>
 
@@ -135,79 +134,15 @@ bool tsp_remove_crossings(std::vector<size_t>& path, const Points& centers)
         return {std::numeric_limits<size_t>::max(), std::numeric_limits<size_t>::max()};
     };
 
-    // For many islands, the same scan with the edges binned in a uniform grid over their boxes, so each edge is only tested against the edges sharing a
-    // cell with it - two edges whose boxes overlap always do. It returns the same crossing as the all-pairs scan
-    // (smallest i, then smallest j), so the result is unchanged; with thousands of islands on a layer the all-pairs
-    // scan, repeated after every reversal, never finished. Rebuilding the grid costs more than it saves on small inputs.
-    constexpr size_t grid_min_size = 500;
-    BoundingBox extent;
-    for (size_t idx : path)
-        extent.merge(centers[idx]);
-    const int     grid_n = std::clamp(int(std::sqrt(double(pn))), 1, 256);
-    const coord_t cell_w = std::max<coord_t>(1, (extent.max.x() - extent.min.x()) / grid_n + 1);
-    const coord_t cell_h = std::max<coord_t>(1, (extent.max.y() - extent.min.y()) / grid_n + 1);
-    const auto    for_cells = [&](const Point& a, const Point& b, auto&& fn) {
-        const int x0 = int((std::min(a.x(), b.x()) - extent.min.x()) / cell_w), x1 = int((std::max(a.x(), b.x()) - extent.min.x()) / cell_w);
-        const int y0 = int((std::min(a.y(), b.y()) - extent.min.y()) / cell_h), y1 = int((std::max(a.y(), b.y()) - extent.min.y()) / cell_h);
-        for (int y = y0; y <= y1; ++y)
-            for (int x = x0; x <= x1; ++x)
-                fn(y * grid_n + x);
-    };
-    std::vector<std::vector<size_t>> edge_cells(size_t(grid_n) * grid_n);
-
-    auto find_crossing_grid = [&]() -> std::pair<size_t, size_t> {
-        for (std::vector<size_t>& cell : edge_cells)
-            cell.clear();
-        for (size_t j = 0; j < n_edges; ++j)
-            for_cells(centers[path[j]], centers[path[(j + 1) % pn]], [&](int cell) { edge_cells[cell].emplace_back(j); });
-
-        for (size_t i = 0; i < n_edges; ++i) {
-            const Point& ai = centers[path[i]];
-            const Point& bi = centers[path[(i + 1) % pn]];
-
-            size_t first_j = std::numeric_limits<size_t>::max();
-            for_cells(ai, bi, [&](int cell) {
-                for (size_t j : edge_cells[cell]) {
-                    if (j < i + 2 || j >= first_j) continue;
-                    // Skip the (0, pn-1) pair: edges (0,1) and (pn-1,0) share node 0.
-                    if (i == 0 && j == pn - 1) continue;
-
-                    const Point& aj = centers[path[j]];
-                    const Point& bj = centers[path[(j + 1) % pn]];
-
-                    if (!bboxes_overlap(ai, bi, aj, bj)) continue;
-                    if (Geometry::segments_intersect(ai, bi, aj, bj))
-                        first_j = j;
-                }
-            });
-            if (first_j != std::numeric_limits<size_t>::max())
-                return {i, first_j};
-        }
-        return {std::numeric_limits<size_t>::max(), std::numeric_limits<size_t>::max()};
-    };
-
     // Process crossings one at a time: find first, reverse it, restart scan.
     // Cap iterations to prevent infinite loops on collinear/overlapping segments.
     int max_iters = static_cast<int>(pn * pn);
     bool improved = false;
-    // Reversing between two segments that only touch or overlap along a line need not remove the intersection, and on
-    // islands laid out on a regular grid (a tiled texture, an array of parts) the loop cycled through the same orderings
-    // until the pn * pn cap - effectively forever. Stop as soon as an ordering repeats: until then this is the same loop.
-    std::unordered_set<uint64_t> seen_paths;
-    const auto path_hash = [&path]() {
-        uint64_t h = 1469598103934665603ull; // FNV-1a
-        for (size_t idx : path)
-            h = (h ^ uint64_t(idx)) * 1099511628211ull;
-        return h;
-    };
-    seen_paths.insert(path_hash());
     while (max_iters-- > 0) {
-        auto [ci, cj] = pn >= grid_min_size ? find_crossing_grid() : find_crossing();
+        auto [ci, cj] = find_crossing();
         if (ci == std::numeric_limits<size_t>::max()) break;
         improved = true;
         std::reverse(path.begin() + ci + 1, path.begin() + cj + 1);
-        if (!seen_paths.insert(path_hash()).second)
-            break;
     }
     return improved;
 }

src/libslic3r/GCode/SeamPlacer.cpp

@@ -8,7 +8,6 @@
 #include <boost/log/trivial.hpp>
 #include <random>
 #include <algorithm>
-#include <limits>
 #include <queue>
 #include <unordered_map>
 
@@ -1179,21 +1178,21 @@ std::optional<std::pair<size_t, size_t>> SeamPlacer::find_next_seam_in_layer(
     const size_t layer_idx, const float max_distance,
     const SeamPlacerImpl::SeamComparator &comparator) const {
   using namespace SeamPlacerImpl;
-  // Find the best nearby point and the nearest one. A layer of a fine relief has tens of thousands of candidates within
-  // the radius, so they are looked at as the search finds them rather than collected into a vector first.
-  constexpr size_t none = std::numeric_limits<size_t>::max();
-  size_t best_nearby_point_index = none;
-  size_t nearest_point_index = none;
-  visit_nearby_points(*layers[layer_idx].points_tree, projected_position, max_distance,
-                      [&layers, &comparator, &projected_position, layer_idx, &best_nearby_point_index, &nearest_point_index]
-                      (size_t nearby_point_index) {
-    if (best_nearby_point_index == none) {
-      // The first point found starts both, as the first of the collected ones did.
-      best_nearby_point_index = nearest_point_index = nearby_point_index;
-    }
+  std::vector<size_t> nearby_points_indices = find_nearby_points(*layers[layer_idx].points_tree, projected_position,
+                                                                 max_distance);
+
+  if (nearby_points_indices.empty()) {
+    return {};
+  }
+
+  size_t best_nearby_point_index = nearby_points_indices[0];
+  size_t nearest_point_index = nearby_points_indices[0];
+
+  // Now find best nearby point, nearest point, and corresponding indices
+  for (const size_t &nearby_point_index : nearby_points_indices) {
     const SeamCandidate &point = layers[layer_idx].points[nearby_point_index];
     if (point.perimeter.finalized) {
-      return; // skip over finalized perimeters, try to find some that is not finalized
+      continue; // skip over finalized perimeters, try to find some that is not finalized
     }
     if (comparator.is_first_better(point, layers[layer_idx].points[best_nearby_point_index],
                                    projected_position.head<2>())
@@ -1205,10 +1204,6 @@ std::optional<std::pair<size_t, size_t>> SeamPlacer::find_next_seam_in_layer(
         || layers[layer_idx].points[nearest_point_index].perimeter.finalized) {
       nearest_point_index = nearby_point_index;
     }
-  });
-
-  if (best_nearby_point_index == none) {
-    return {};
   }
 
   const SeamCandidate &best_nearby_point = layers[layer_idx].points[best_nearby_point_index];

src/libslic3r/KDTreeIndirect.hpp

@@ -313,36 +313,6 @@ std::vector<size_t> find_nearby_points(const KDTreeIndirectType &kdtree, const P
     return visitor.result;
 }
 
-// Visits the points within max_distance of center, in the order find_nearby_points() would collect them, and hands
-// each of them to `visitor_fn` instead of returning them all: a search over a dense set spends more on collecting the
-// points into a vector than on the search itself, and its caller usually keeps only a few of them.
-template<typename KDTreeIndirectType, typename PointType, typename VisitorFn>
-void visit_nearby_points(const KDTreeIndirectType &kdtree, const PointType &center,
-                         const typename KDTreeIndirectType::CoordType &max_distance, VisitorFn visitor_fn)
-{
-    using CoordType = typename KDTreeIndirectType::CoordType;
-
-    struct Visitor {
-        const KDTreeIndirectType &kdtree;
-        const PointType           center;
-        const CoordType           max_distance_squared;
-        VisitorFn                 visitor_fn;
-
-        unsigned int operator()(size_t idx, size_t dimension) {
-            auto dist = CoordType(0);
-            for (size_t i = 0; i < KDTreeIndirectType::NumDimensions; ++i) {
-                CoordType d = center[i] - kdtree.coordinate(idx, i);
-                dist += d * d;
-            }
-            if (dist < max_distance_squared)
-                visitor_fn(idx);
-            return kdtree.descent_mask(center[dimension], max_distance_squared, idx, dimension);
-        }
-    } visitor { kdtree, center, max_distance * max_distance, visitor_fn };
-
-    kdtree.visit(visitor);
-}
-
 template<typename KDTreeIndirectType, typename PointType>
 std::vector<size_t> find_nearby_points(const KDTreeIndirectType &kdtree, const PointType &center,
                                        const typename KDTreeIndirectType::CoordType& max_distance)

src/libslic3r/LayerRegion.cpp

@@ -72,11 +72,10 @@ void LayerRegion::slices_to_fill_surfaces_clipped()
         by_surface[size_t(surface.surface_type)].emplace_back(&surface);
     // Trim surfaces by the fill_boundaries.
     this->fill_surfaces.surfaces.clear();
-    const Polygons fill_boundaries = to_polygons(this->fill_expolygons);
     for (size_t surface_type = 0; surface_type < size_t(stCount); ++ surface_type) {
         const SurfacesPtr &this_surfaces = by_surface[surface_type];
         if (! this_surfaces.empty())
-            this->fill_surfaces.append(intersection_ex_by_piece(to_expolygons(this_surfaces), fill_boundaries), SurfaceType(surface_type));
+            this->fill_surfaces.append(intersection_ex(this_surfaces, this->fill_expolygons), SurfaceType(surface_type));
     }
 }
 

src/libslic3r/MultiMaterialSegmentation.cpp

@@ -8,7 +8,6 @@
 #include "MutablePolygon.hpp"
 #include "format.hpp"
 
-#include <numeric>
 #include <utility>
 #include <unordered_set>
 
@@ -1312,15 +1311,10 @@ static inline std::vector<std::vector<ExPolygons>> segmentation_top_and_bottom_l
     }
 #endif // MM_SEGMENTATION_DEBUG_TOP_BOTTOM
 
-    // When the upper surface of an object is occluded, it should no longer be considered the upper surface.
-    // Every (colour, layer) pair is trimmed on its own, so they all run at once: the painted faces of a finely
-    // textured part project hundreds of thousands of triangles onto one layer, which used to be trimmed serially.
+    // When the upper surface of an object is occluded, it should no longer be considered the upper surface
     {
-        const size_t occluded_pairs = num_facets_states * layers.size();
-        tbb::parallel_for(tbb::blocked_range<size_t>(0, occluded_pairs), [&](const tbb::blocked_range<size_t> &range) {
-            for (size_t pair_idx = range.begin(); pair_idx < range.end(); ++pair_idx) {
-                const size_t extruder_idx = pair_idx / layers.size();
-                const size_t layer_idx    = pair_idx % layers.size();
+        for (size_t extruder_idx = 0; extruder_idx < num_facets_states; ++extruder_idx) {
+            for (size_t layer_idx = 0; layer_idx < layers.size(); ++layer_idx) {
                 if (!top_raw[extruder_idx].empty() && !top_raw[extruder_idx][layer_idx].empty() && layer_idx + 1 < layers.size()) {
                     top_raw[extruder_idx][layer_idx] = diff(top_raw[extruder_idx][layer_idx], input_expolygons[layer_idx + 1]);
                 }
@@ -1328,7 +1322,7 @@ static inline std::vector<std::vector<ExPolygons>> segmentation_top_and_bottom_l
                     bottom_raw[extruder_idx][layer_idx] = diff(bottom_raw[extruder_idx][layer_idx], input_expolygons[layer_idx - 1]);
                 }
             }
-        });
+        }
     }
 
     std::vector<std::vector<ExPolygons>> triangles_by_color_bottom(num_facets_states);
@@ -1384,62 +1378,13 @@ static inline std::vector<std::vector<ExPolygons>> segmentation_top_and_bottom_l
         return out;
     };
 
-    // Projects a painted top or bottom face `ex` of layer `layer_idx` onto the shell layers below or above it (in
-    // `shell_layers`, nearest first), one more perimeter in on each, stopping at the first layer where nothing is left.
-    // Only the slices within the deepest offset of `ex` (three times that with the miter joins) decide the result, so the
-    // work is done per tile of `ex`'s ExPolygons on the slices cut to the tile's box grown by that much: the same result, but
-    // each ClipperLib call stays the size of a tile rather than of a layer cut through a fine relief, and the tiles run in
-    // parallel.
-    const auto project_to_shells = [&input_expolygons](const ExPolygons &ex, size_t layer_idx, const std::vector<size_t> &shell_layers,
-                                                       const LayerColorStat &stat, std::vector<ExPolygons> &dst, size_t dst_offset) {
-        std::vector<float> offsets(shell_layers.size());
-        float              offset = 0.f;
-        for (size_t i = 0; i < shell_layers.size(); ++i) {
-            //BBS: offset width should be 2*spacing to avoid too narrow area which has overlap of wall line
-            offset -= (stat.extrusion_spacing + stat.extrusion_width);
-            offsets[i] = offset;
-        }
-        if (offsets.empty())
-            return;
-        const coord_t reach = coord_t(std::ceil(DefaultMiterLimit * std::abs(offsets.back()))) + 10 * SCALED_EPSILON;
-        const std::vector<ClipperUtils::ExPolygonsTile> tiles = ClipperUtils::tile_expolygons(ex, 16);
-        // [shell layer][tile]
-        std::vector<std::vector<ExPolygons>> shells(shell_layers.size(), std::vector<ExPolygons>(tiles.size()));
-        tbb::parallel_for(size_t(0), tiles.size(), [&](size_t tile_idx) {
-            const ClipperUtils::ExPolygonsTile &tile = tiles[tile_idx];
-            const BoundingBox                   bbox = tile.bbox.inflated(reach);
-            ExPolygons                          tile_ex;
-            tile_ex.reserve(tile.members.size());
-            for (size_t i : tile.members)
-                tile_ex.emplace_back(ex[i]);
-            Polygons layer_slices_trimmed = ClipperUtils::clip_clipper_polygons_with_subject_bbox(input_expolygons[layer_idx], bbox);
-            for (size_t i = 0; i < shell_layers.size() && ! layer_slices_trimmed.empty(); ++i) {
-                const ExPolygons trimmed = intersection_ex(layer_slices_trimmed, ClipperUtils::clip_clipper_polygons_with_subject_bbox(input_expolygons[shell_layers[i]], bbox));
-                shells[i][tile_idx]  = opening_ex(intersection_ex(tile_ex, offset_ex(trimmed, offsets[i])), stat.small_region_threshold);
-                layer_slices_trimmed = to_polygons(trimmed);
-            }
-        });
-        for (size_t i = 0; i < shell_layers.size(); ++i) {
-            bool empty = true;
-            for (ExPolygons &shell : shells[i])
-                if (! shell.empty()) {
-                    append(dst[shell_layers[i] + dst_offset], std::move(shell));
-                    empty = false;
-                }
-            if (empty)
-                break;
-        }
-    };
-
     tbb::parallel_for(tbb::blocked_range<size_t>(0, num_layers, granularity), [&granularity, &num_layers, &num_facets_states, &layer_color_stat, &top_raw, &triangles_by_color_top,
-                                                                               &throw_on_cancel_callback, &bottom_raw, &triangles_by_color_bottom, &project_to_shells,
+                                                                               &throw_on_cancel_callback, &input_expolygons, &bottom_raw, &triangles_by_color_bottom,
                                                                                &shell_triangles_by_color_top, &shell_triangles_by_color_bottom](const tbb::blocked_range<size_t> &range) {
         size_t group_idx   = range.begin() / granularity;
         size_t layer_idx_offset = (group_idx & 1) * num_layers;
         for (size_t layer_idx = range.begin(); layer_idx < range.end(); ++ layer_idx) {
-            // Each colour writes only its own vectors, so the colours run in parallel: a painted top or bottom face
-            // projects onto a single layer, which otherwise did all of its colours on one thread.
-            tbb::parallel_for(size_t(0), size_t(num_facets_states), [&](size_t color_idx) {
+            for (size_t color_idx = 0; color_idx < num_facets_states; ++color_idx) {
                 throw_on_cancel_callback();
                 LayerColorStat stat = layer_color_stat(layer_idx, color_idx);
                 if (std::vector<Polygons> &top = top_raw[color_idx]; ! top.empty() && ! top[layer_idx].empty())
@@ -1448,10 +1393,18 @@ static inline std::vector<std::vector<ExPolygons>> segmentation_top_and_bottom_l
                         top_ex = opening_ex(top_ex, stat.small_region_threshold);
                         if (! top_ex.empty()) {
                             append(triangles_by_color_top[color_idx][layer_idx + layer_idx_offset], top_ex);
-                            std::vector<size_t> shell_layers;
-                            for (int last_idx = int(layer_idx) - 1; last_idx > std::max(int(layer_idx - stat.top_shell_layers), int(0)); --last_idx)
-                                shell_layers.emplace_back(size_t(last_idx));
-                            project_to_shells(top_ex, layer_idx, shell_layers, stat, shell_triangles_by_color_top[color_idx], layer_idx_offset);
+                            float offset = 0.f;
+                            ExPolygons layer_slices_trimmed = input_expolygons[layer_idx];
+                            for (int last_idx = int(layer_idx) - 1; last_idx > std::max(int(layer_idx - stat.top_shell_layers), int(0)); --last_idx) {
+                                //BBS: offset width should be 2*spacing to avoid too narrow area which has overlap of wall line
+                                //offset -= stat.extrusion_width ;
+                                offset -= (stat.extrusion_spacing + stat.extrusion_width);
+                                layer_slices_trimmed = intersection_ex(layer_slices_trimmed, input_expolygons[last_idx]);
+                                ExPolygons last = opening_ex(intersection_ex(top_ex, offset_ex(layer_slices_trimmed, offset)), stat.small_region_threshold);
+                                if (last.empty())
+                                    break;
+                                append(shell_triangles_by_color_top[color_idx][last_idx + layer_idx_offset], std::move(last));
+                            }
                         }
                     }
                 if (std::vector<Polygons> &bottom = bottom_raw[color_idx]; ! bottom.empty() && ! bottom[layer_idx].empty())
@@ -1460,13 +1413,21 @@ static inline std::vector<std::vector<ExPolygons>> segmentation_top_and_bottom_l
                         bottom_ex = opening_ex(bottom_ex, stat.small_region_threshold);
                         if (! bottom_ex.empty()) {
                             append(triangles_by_color_bottom[color_idx][layer_idx + layer_idx_offset], bottom_ex);
-                            std::vector<size_t> shell_layers;
-                            for (size_t last_idx = layer_idx + 1; last_idx < std::min(layer_idx + stat.bottom_shell_layers, num_layers); ++last_idx)
-                                shell_layers.emplace_back(last_idx);
-                            project_to_shells(bottom_ex, layer_idx, shell_layers, stat, shell_triangles_by_color_bottom[color_idx], layer_idx_offset);
+                            float offset = 0.f;
+                            ExPolygons layer_slices_trimmed = input_expolygons[layer_idx];
+                            for (size_t last_idx = layer_idx + 1; last_idx < std::min(layer_idx + stat.bottom_shell_layers, num_layers); ++last_idx) {
+                                //BBS: offset width should be 2*spacing to avoid too narrow area which has overlap of wall line
+                                //offset -= stat.extrusion_width;
+                                offset -= (stat.extrusion_spacing + stat.extrusion_width);
+                                layer_slices_trimmed = intersection_ex(layer_slices_trimmed, input_expolygons[last_idx]);
+                                ExPolygons last = opening_ex(intersection_ex(bottom_ex, offset_ex(layer_slices_trimmed, offset)), stat.small_region_threshold);
+                                if (last.empty())
+                                    break;
+                                append(shell_triangles_by_color_bottom[color_idx][last_idx + layer_idx_offset], std::move(last));
+                            }
                         }
                     }
-            });
+            }
         }
     });
 
@@ -1476,25 +1437,22 @@ static inline std::vector<std::vector<ExPolygons>> segmentation_top_and_bottom_l
                                                                   &shell_triangles_by_color_top, &shell_triangles_by_color_bottom](const tbb::blocked_range<size_t> &range) {
         for (size_t layer_idx = range.begin(); layer_idx < range.end(); ++ layer_idx) {
             throw_on_cancel_callback();
-            // The per-colour unions below are independent of each other, so they run in parallel (a painted top or
-            // bottom face puts all of its colours on one layer); whatever combines the colours stays in colour order.
-            const auto merge_colour_union = [&](size_t color_idx) {
+            ExPolygons painted_exploys;
+            for (size_t color_idx = 0; color_idx < triangles_by_color_merged.size(); ++color_idx) {
                 auto &self = triangles_by_color_merged[color_idx][layer_idx];
                 append(self, std::move(triangles_by_color_bottom[color_idx][layer_idx]));
                 append(self, std::move(triangles_by_color_bottom[color_idx][layer_idx + num_layers]));
                 append(self, std::move(triangles_by_color_top[color_idx][layer_idx]));
                 append(self, std::move(triangles_by_color_top[color_idx][layer_idx + num_layers]));
                 self = union_ex(self);
-            };
-            tbb::parallel_for(size_t(0), triangles_by_color_merged.size(), merge_colour_union);
 
-            ExPolygons painted_exploys;
-            for (size_t color_idx = 0; color_idx < triangles_by_color_merged.size(); ++color_idx)
-                append(painted_exploys, triangles_by_color_merged[color_idx][layer_idx]);
+                append(painted_exploys, self);
+            }
+
             painted_exploys = union_ex(painted_exploys);
 
             //BBS: merge the top and bottom shell layers
-            tbb::parallel_for(size_t(0), triangles_by_color_merged.size(), [&](size_t color_idx) {
+            for (size_t color_idx = 0; color_idx < triangles_by_color_merged.size(); ++color_idx) {
                 auto &self = triangles_by_color_merged[color_idx][layer_idx];
 
                 auto top_area = diff_ex(union_ex(shell_triangles_by_color_top[color_idx][layer_idx],
@@ -1508,7 +1466,7 @@ static inline std::vector<std::vector<ExPolygons>> segmentation_top_and_bottom_l
                 append(self, top_area);
                 append(self, bottom_area);
                 self = union_ex(self);
-            });
+            }
             // Trim one region by the other if some of the regions overlap.
             ExPolygons painted_regions;
             for (size_t color_idx = 1; color_idx < triangles_by_color_merged.size(); ++color_idx) {
@@ -1875,69 +1833,7 @@ static void remove_multiple_edges_in_vertices(MMU_Graph &graph, const std::vecto
     }
 }
 
-
-// Finds the islands (layer ExPolygons) a region piece overlaps. A top or bottom region is projected from the neighbouring
-// layers and may reach past the island it belongs to, or over several islands.
-class IslandLocator
-{
-public:
-    explicit IslandLocator(const ExPolygons &islands) : m_islands(islands)
-    {
-        m_bboxes.reserve(islands.size());
-        for (const ExPolygon &island : islands) {
-            m_bboxes.emplace_back(get_extents(island));
-            m_extent.merge(m_bboxes.back());
-        }
-        if (!m_extent.defined)
-            return;
-        const Point size = m_extent.size();
-        m_cell_w = std::max<coord_t>(1, size.x() / GRID + 1);
-        m_cell_h = std::max<coord_t>(1, size.y() / GRID + 1);
-        m_grid.assign(GRID * GRID, {});
-        for (size_t i = 0; i < m_bboxes.size(); ++i)
-            for_cells(m_bboxes[i], [&](int cell) { m_grid[cell].emplace_back(i); });
-    }
-
-    void find(const ExPolygon &piece, std::vector<size_t> &out) const
-    {
-        out.clear();
-        const BoundingBox bbox = get_extents(piece);
-        if (!m_extent.defined || !m_extent.overlap(bbox))
-            return;
-        for_cells(bbox, [&](int cell) {
-            for (size_t i : m_grid[cell])
-                if (m_bboxes[i].overlap(bbox))
-                    out.emplace_back(i);
-        });
-        sort_remove_duplicates(out);
-        if (out.size() > 1)
-            out.erase(std::remove_if(out.begin(), out.end(), [&](size_t i) {
-                const BoundingBox common(m_bboxes[i].min.cwiseMax(bbox.min), m_bboxes[i].max.cwiseMin(bbox.max));
-                return intersection(ClipperUtils::clip_clipper_polygons_with_subject_bbox(piece, common.inflated(SCALED_EPSILON)),
-                                    ClipperUtils::clip_clipper_polygons_with_subject_bbox(m_islands[i], common.inflated(SCALED_EPSILON))).empty();
-            }), out.end());
-    }
-
-private:
-    static constexpr int GRID = 64;
-    template<typename Fn> void for_cells(const BoundingBox &bb, Fn &&fn) const
-    {
-        const int x0 = std::clamp(int((bb.min.x() - m_extent.min.x()) / m_cell_w), 0, GRID - 1), x1 = std::clamp(int((bb.max.x() - m_extent.min.x()) / m_cell_w), 0, GRID - 1);
-        const int y0 = std::clamp(int((bb.min.y() - m_extent.min.y()) / m_cell_h), 0, GRID - 1), y1 = std::clamp(int((bb.max.y() - m_extent.min.y()) / m_cell_h), 0, GRID - 1);
-        for (int y = y0; y <= y1; ++y)
-            for (int x = x0; x <= x1; ++x)
-                fn(y * GRID + x);
-    }
-
-    const ExPolygons                &m_islands;
-    std::vector<BoundingBox>         m_bboxes;
-    BoundingBox                      m_extent;
-    coord_t                          m_cell_w = 1, m_cell_h = 1;
-    std::vector<std::vector<size_t>> m_grid;
-};
-
-static std::vector<std::vector<ExPolygons>> merge_segmented_layers(const std::vector<ExPolygons>              &input_expolygons,
-                                                                   const std::vector<std::vector<ExPolygons>> &segmented_regions,
+static std::vector<std::vector<ExPolygons>> merge_segmented_layers(const std::vector<std::vector<ExPolygons>> &segmented_regions,
                                                                    std::vector<std::vector<ExPolygons>>      &&top_and_bottom_layers,
                                                                    const size_t                                num_facets_states,
                                                                    const std::function<void()>                &throw_on_cancel_callback)
@@ -1948,91 +1844,33 @@ static std::vector<std::vector<ExPolygons>> merge_segmented_layers(const std::ve
     assert(!top_and_bottom_layers.size() || num_facets_states == top_and_bottom_layers.size());
 
     BOOST_LOG_TRIVIAL(debug) << "Print object segmentation - Merging segmented layers in parallel - Begin";
-    // Every region of a layer is merged together with the regions of the islands it overlaps, and the islands are further
-    // apart than the dimple removal below reaches, so this gives the same result as merging the layer at once. On a layer
-    // cut through a fine relief every region shares thousands of hole contours with every other, and ClipperLib, splitting
-    // and re-linking one huge polygon over and over, took anything up to half an hour for a layer; per island each operation
-    // stays the size of the island, and the islands run in parallel.
-    tbb::parallel_for(tbb::blocked_range<size_t>(0, num_layers), [&](const tbb::blocked_range<size_t> &range) {
+    tbb::parallel_for(tbb::blocked_range<size_t>(0, num_layers), [&segmented_regions, &top_and_bottom_layers, &segmented_regions_merged, &num_facets_states, &throw_on_cancel_callback](const tbb::blocked_range<size_t> &range) {
         for (size_t layer_idx = range.begin(); layer_idx < range.end(); ++layer_idx) {
             assert(segmented_regions[layer_idx].size() == num_facets_states);
-            throw_on_cancel_callback();
-            // Group the islands joined by a region overlapping several of them; the last group takes the regions lying
-            // outside every island.
-            const ExPolygons   &islands = input_expolygons[layer_idx];
-            const IslandLocator locator(islands);
-            std::vector<size_t> parent(islands.size() + 1);
-            std::iota(parent.begin(), parent.end(), 0);
-            const auto root = [&parent](size_t i) {
-                while (parent[i] != i)
-                    i = parent[i] = parent[parent[i]];
-                return i;
-            };
-            // Islands of every piece: side regions of colours 1.., then top/bottom regions of colours 0..
-            std::vector<const ExPolygon *> pieces;
-            for (size_t extruder_id = 1; extruder_id < num_facets_states; ++extruder_id)
-                for (const ExPolygon &piece : segmented_regions[layer_idx][extruder_id])
-                    pieces.emplace_back(&piece);
-            if (!top_and_bottom_layers.empty())
-                for (size_t color_idx = 0; color_idx < num_facets_states; ++color_idx)
-                    for (const ExPolygon &piece : top_and_bottom_layers[color_idx][layer_idx])
-                        pieces.emplace_back(&piece);
-            std::vector<std::vector<size_t>> overlapped(pieces.size());
-            tbb::parallel_for(size_t(0), pieces.size(), [&](size_t i) { locator.find(*pieces[i], overlapped[i]); });
-            std::vector<size_t> piece_island(pieces.size());
-            for (size_t i = 0; i < pieces.size(); ++i) {
-                piece_island[i] = overlapped[i].empty() ? islands.size() : overlapped[i].front();
-                for (size_t island : overlapped[i])
-                    parent[root(island)] = root(piece_island[i]);
-            }
-            std::vector<size_t> bucket_of(parent.size(), size_t(-1));
-            size_t              num_buckets = 0;
-            for (size_t i = 0; i < parent.size(); ++i)
-                if (size_t &b = bucket_of[root(i)]; b == size_t(-1))
-                    b = num_buckets++;
-
-            // [bucket][colour]
-            std::vector<std::vector<ExPolygons>> sides(num_buckets, std::vector<ExPolygons>(num_facets_states));
-            std::vector<std::vector<ExPolygons>> tops(num_buckets, std::vector<ExPolygons>(num_facets_states));
-            size_t                               piece_idx = 0;
-            for (size_t extruder_id = 1; extruder_id < num_facets_states; ++extruder_id)
-                for (const ExPolygon &piece : segmented_regions[layer_idx][extruder_id])
-                    sides[bucket_of[root(piece_island[piece_idx++])]][extruder_id].emplace_back(piece);
-            if (!top_and_bottom_layers.empty())
-                for (size_t color_idx = 0; color_idx < num_facets_states; ++color_idx)
-                    for (const ExPolygon &piece : top_and_bottom_layers[color_idx][layer_idx])
-                        tops[bucket_of[root(piece_island[piece_idx++])]][color_idx].emplace_back(piece);
-
-            // Side regions minus the top/bottom regions of every colour.
-            std::vector<std::vector<ExPolygons>> merged(num_buckets, std::vector<ExPolygons>(num_facets_states));
-            tbb::parallel_for(size_t(0), num_buckets, [&](size_t bucket) {
-                Polygons tops_all;
-                for (const ExPolygons &t : tops[bucket])
-                    polygons_append(tops_all, t);
-                for (size_t extruder_id = 1; extruder_id < num_facets_states; ++extruder_id)
-                    if (!sides[bucket][extruder_id].empty())
-                        merged[bucket][extruder_id] = tops_all.empty() ? std::move(sides[bucket][extruder_id]) :
-                                                                         diff_ex_by_piece(sides[bucket][extruder_id], tops_all);
-            });
-
-            // Then this colour's top/bottom regions, with the dimples removed (#7235) when the layer has side regions left.
+            // Zero is skipped because it is the default color of the volume
             for (size_t extruder_id = 1; extruder_id < num_facets_states; ++extruder_id) {
-                if (top_and_bottom_layers.empty() || top_and_bottom_layers[extruder_id][layer_idx].empty()) {
-                    for (size_t bucket = 0; bucket < num_buckets; ++bucket)
-                        append(segmented_regions_merged[layer_idx][extruder_id - 1], std::move(merged[bucket][extruder_id]));
-                    continue;
+                throw_on_cancel_callback();
+                if (!segmented_regions[layer_idx][extruder_id].empty()) {
+                    ExPolygons segmented_regions_trimmed = segmented_regions[layer_idx][extruder_id];
+                    if (!top_and_bottom_layers.empty()) {
+                        for (const std::vector<ExPolygons> &top_and_bottom_by_extruder : top_and_bottom_layers) {
+                            if (!top_and_bottom_by_extruder[layer_idx].empty() && !segmented_regions_trimmed.empty()) {
+                                segmented_regions_trimmed = diff_ex(segmented_regions_trimmed, top_and_bottom_by_extruder[layer_idx]);
+                            }
+                        }
+                    }
+
+                    segmented_regions_merged[layer_idx][extruder_id - 1] = std::move(segmented_regions_trimmed);
+                }
+
+                if (!top_and_bottom_layers.empty() && !top_and_bottom_layers[extruder_id][layer_idx].empty()) {
+                    bool was_top_and_bottom_empty = segmented_regions_merged[layer_idx][extruder_id - 1].empty();
+                    append(segmented_regions_merged[layer_idx][extruder_id - 1], top_and_bottom_layers[extruder_id][layer_idx]);
+
+                    // Remove dimples (#7235) appearing after merging side segmentation of the model with tops and bottoms painted layers.
+                    if (!was_top_and_bottom_empty)
+                        segmented_regions_merged[layer_idx][extruder_id - 1] = offset2_ex(union_ex(segmented_regions_merged[layer_idx][extruder_id - 1]), float(SCALED_EPSILON), -float(SCALED_EPSILON));
                 }
-                bool was_top_and_bottom_empty = true;
-                for (size_t bucket = 0; bucket < num_buckets && was_top_and_bottom_empty; ++bucket)
-                    was_top_and_bottom_empty = merged[bucket][extruder_id].empty();
-                tbb::parallel_for(size_t(0), num_buckets, [&](size_t bucket) {
-                    ExPolygons &region = merged[bucket][extruder_id];
-                    append(region, tops[bucket][extruder_id]);
-                    if (!was_top_and_bottom_empty && !region.empty())
-                        region = offset2_ex(union_ex(region), float(SCALED_EPSILON), -float(SCALED_EPSILON));
-                });
-                for (size_t bucket = 0; bucket < num_buckets; ++bucket)
-                    append(segmented_regions_merged[layer_idx][extruder_id - 1], std::move(merged[bucket][extruder_id]));
             }
         }
     }); // end of parallel_for
@@ -2319,56 +2157,16 @@ std::vector<std::vector<ExPolygons>> segmentation_by_painting(const PrintObject
 
                 assert(!color_poly.empty());
                 assert(!color_poly.front().empty());
-                // Each island (an ExPolygon with its holes) is segmented on its own. Any point of an island is closer to
-                // that island's contours than to any other island's - the way out crosses its own boundary first - so its
-                // Voronoi cells, and with them its colour regions, depend on nothing else. A layer cut through a fine relief
-                // has thousands of islands, and one Voronoi diagram over all of them degenerated into overlapping regions
-                // that every boolean afterwards had to untangle. Per island the diagrams stay small and the islands run in
-                // parallel; an island in a single colour needs no diagram at all.
-                const ExPolygons                      &islands = input_expolygons[layer_idx];
-                std::vector<std::pair<size_t, size_t>> island_contours(islands.size()); // [first, last) into color_poly
-                {
-                    // The same order EdgeGrid::Grid::create() lists the contours in, and so colorize_contours().
-                    size_t idx = 0;
-                    for (size_t island_idx = 0; island_idx < islands.size(); ++island_idx) {
-                        const size_t first = idx;
-                        if (!islands[island_idx].contour.empty())
-                            ++idx;
-                        for (const Polygon &hole : islands[island_idx].holes)
-                            if (!hole.empty())
-                                ++idx;
-                        island_contours[island_idx] = {first, idx};
-                    }
-                    assert(idx == color_poly.size());
+                if (has_layer_only_one_color(color_poly)) {
+                    // If the whole layer is painted using the same color, it is not needed to construct a Voronoi diagram for the segmentation of this layer.
+                    segmented_regions[layer_idx][size_t(color_poly.front().front().color)] = input_expolygons[layer_idx];
+                } else {
+                    MMU_Graph graph = build_graph(layer_idx, color_poly);
+                    remove_multiple_edges_in_vertices(graph, color_poly);
+                    graph.remove_nodes_with_one_arc();
+                    segmented_regions[layer_idx] = extract_colored_segments(graph, num_facets_states);
+                    //segmented_regions[layer_idx] = extract_colored_segments(color_poly, num_extruders, layer_idx);
                 }
-                std::vector<std::vector<ExPolygons>> island_regions(islands.size());
-                tbb::parallel_for(size_t(0), islands.size(), [&](size_t island_idx) {
-                    const auto [first, last] = island_contours[island_idx];
-                    if (first == last)
-                        return;
-                    const std::vector<ColoredLines> island_poly(color_poly.begin() + first, color_poly.begin() + last);
-                    std::vector<ExPolygons>        &regions = island_regions[island_idx];
-                    if (has_layer_only_one_color(island_poly)) {
-                        regions.assign(num_facets_states, ExPolygons());
-                        regions[size_t(island_poly.front().front().color)].emplace_back(islands[island_idx]);
-                    } else {
-                        MMU_Graph graph = build_graph(layer_idx, island_poly);
-                        remove_multiple_edges_in_vertices(graph, island_poly);
-                        graph.remove_nodes_with_one_arc();
-                        regions = extract_colored_segments(graph, num_facets_states);
-                        // The faces of one colour tile it without overlapping; merged here, where an island is small,
-                        // every later boolean gets a few regions instead of thousands of faces sharing their edges. An
-                        // island with many holes keeps its faces: merged, each colour would be one region with thousands
-                        // of holes, and subtracting from that is far slower than from the faces one at a time.
-                        if (island_poly.size() <= 64)
-                            for (ExPolygons &faces : regions)
-                                if (faces.size() > 1)
-                                    faces = union_ex(faces);
-                    }
-                });
-                for (std::vector<ExPolygons> &regions : island_regions)
-                    for (size_t color_idx = 0; color_idx < regions.size(); ++color_idx)
-                        append(segmented_regions[layer_idx][color_idx], std::move(regions[color_idx]));
 
 #ifdef MM_SEGMENTATION_DEBUG_REGIONS
                 export_regions_to_svg(debug_out_path("3-mm-regions-sides-%d-%d.svg", layer_idx, iRun), segmented_regions[layer_idx], input_expolygons[layer_idx]);
@@ -2391,7 +2189,7 @@ std::vector<std::vector<ExPolygons>> segmentation_by_painting(const PrintObject
         throw_on_cancel_callback();
     }
 
-    std::vector<std::vector<ExPolygons>> segmented_regions_merged = merge_segmented_layers(input_expolygons, segmented_regions, std::move(top_and_bottom_layers), num_facets_states, throw_on_cancel_callback);
+    std::vector<std::vector<ExPolygons>> segmented_regions_merged = merge_segmented_layers(segmented_regions, std::move(top_and_bottom_layers), num_facets_states, throw_on_cancel_callback);
     throw_on_cancel_callback();
 
 #ifdef MM_SEGMENTATION_DEBUG_REGIONS

src/libslic3r/MultiPoint.hpp

@@ -19,13 +19,11 @@ public:
     
     MultiPoint() {}
     MultiPoint(const MultiPoint &other) : points(other.points) {}
-    MultiPoint(MultiPoint &&other) noexcept : points(std::move(other.points)) {}
+    MultiPoint(MultiPoint &&other) : points(std::move(other.points)) {}
     MultiPoint(std::initializer_list<Point> list) : points(list) {}
     explicit MultiPoint(const Points &_points) : points(_points) {}
-    // Without it, the derived classes' move constructors passing std::move(points) here copied them.
-    explicit MultiPoint(Points &&_points) noexcept : points(std::move(_points)) {}
     MultiPoint& operator=(const MultiPoint &other) { points = other.points; return *this; }
-    MultiPoint& operator=(MultiPoint &&other) noexcept { points = std::move(other.points); return *this; }
+    MultiPoint& operator=(MultiPoint &&other) { points = std::move(other.points); return *this; }
     virtual ~MultiPoint() = default;
     void scale(double factor);
     void scale(double factor_x, double factor_y);

src/libslic3r/PerimeterGenerator.cpp

@@ -17,8 +17,6 @@
 #include <cassert>
 #include <unordered_set>
 #include <thread>
-#include <tbb/blocked_range.h>
-#include <tbb/parallel_for.h>
 #include "libslic3r/AABBTreeLines.hpp"
 #include "Print.hpp"
 static const int overhang_sampling_number = 6;
@@ -2483,443 +2481,421 @@ void PerimeterGenerator::process_arachne()
     const bool only_one_wall_first_layer = this->config->only_one_wall_first_layer && has_bottom_shell_layers(*this->config);
     // we need to process each island separately because we might have different
     // extra perimeters for each one
-    // Each island is independent up to its outputs, so they are generated in parallel - a layer split into thousands
-    // of islands (e.g. by colour painting) otherwise ran on one thread - and the outputs are then committed in the
-    // original island order, which is what the extra overhang perimeters (applied to the last island's loops and to
-    // all fill surfaces so far) depend on.
-    struct ArachneSurfaceResult
-    {
-        ExtrusionEntityCollection loops;
-        bool                      has_loops = false;
-        ExPolygons                infill;
-        ExPolygons                no_overlap;
-    };
-    std::vector<ArachneSurfaceResult> results(all_surfaces.size());
-    tbb::parallel_for(tbb::blocked_range<size_t>(0, all_surfaces.size()), [&](const tbb::blocked_range<size_t> &range) {
-      for (size_t surface_idx = range.begin(); surface_idx < range.end(); ++surface_idx) {
-        const Surface        &surface = all_surfaces[surface_idx];
-        ArachneSurfaceResult &result  = results[surface_idx];
-            coord_t bead_width_0 = ext_perimeter_spacing;
-            // detect how many perimeters must be generated for this island
-            int loop_number = this->config->wall_loops + surface.extra_perimeters - 1; // 0-indexed loops
-            int sparse_infill_density = this->config->sparse_infill_density.value;
-            if (this->config->alternate_extra_wall && this->layer_id % 2 == 1 && !m_spiral_vase && sparse_infill_density > 0) // add alternating extra wall
-                loop_number++;
+    for (const Surface& surface : all_surfaces) {
+        coord_t bead_width_0 = ext_perimeter_spacing;
+        // detect how many perimeters must be generated for this island
+        int loop_number = this->config->wall_loops + surface.extra_perimeters - 1; // 0-indexed loops
+        int sparse_infill_density = this->config->sparse_infill_density.value;
+        if (this->config->alternate_extra_wall && this->layer_id % 2 == 1 && !m_spiral_vase && sparse_infill_density > 0) // add alternating extra wall
+            loop_number++;
 
-            // Set the bottommost layer to be one wall
-            const bool is_bottom_layer = (this->layer_id == object_config->raft_layers) ? true : false;
-            if (is_bottom_layer && only_one_wall_first_layer)
-                loop_number = 0;
+        // Set the bottommost layer to be one wall
+        const bool is_bottom_layer = (this->layer_id == object_config->raft_layers) ? true : false;
+        if (is_bottom_layer && only_one_wall_first_layer)
+            loop_number = 0;
 
-            // Orca: set the topmost layer to be one wall according to the config
-            const bool is_topmost_layer = (this->upper_slices == nullptr) ? true : false;
-            if (is_topmost_layer && loop_number > 0 && only_one_wall_top)
-                loop_number = 0;
+        // Orca: set the topmost layer to be one wall according to the config
+        const bool is_topmost_layer = (this->upper_slices == nullptr) ? true : false;
+        if (is_topmost_layer && loop_number > 0 && only_one_wall_top)
+            loop_number = 0;
         
-            auto apply_precise_outer_wall = config->precise_outer_wall && config->wall_sequence == WallSequence::InnerOuter;
-            // Orca: properly adjust offset for the outer wall if precise_outer_wall is enabled.
-            ExPolygons last = offset_ex(surface.expolygon.simplify_p(surface_simplify_resolution),
-                           apply_precise_outer_wall? -float(ext_perimeter_width - ext_perimeter_spacing )
-                                                     : -float(ext_perimeter_width / 2. - ext_perimeter_spacing / 2.));
+        auto apply_precise_outer_wall = config->precise_outer_wall && config->wall_sequence == WallSequence::InnerOuter;
+        // Orca: properly adjust offset for the outer wall if precise_outer_wall is enabled.
+        ExPolygons last = offset_ex(surface.expolygon.simplify_p(surface_simplify_resolution),
+                       apply_precise_outer_wall? -float(ext_perimeter_width - ext_perimeter_spacing )
+                                                 : -float(ext_perimeter_width / 2. - ext_perimeter_spacing / 2.));
         
-            Arachne::WallToolPathsParams input_params = Arachne::make_paths_params(this->layer_id, *object_config, *print_config);
-            // Set params is_top_or_bottom_layer for adjusting short-wall removal sensitivity.
-            input_params.is_top_or_bottom_layer = (is_bottom_layer || is_topmost_layer) ? true : false;
+        Arachne::WallToolPathsParams input_params = Arachne::make_paths_params(this->layer_id, *object_config, *print_config);
+        // Set params is_top_or_bottom_layer for adjusting short-wall removal sensitivity.
+        input_params.is_top_or_bottom_layer = (is_bottom_layer || is_topmost_layer) ? true : false;
 
-            coord_t wall_0_inset = 0;
-            if (apply_precise_outer_wall)
-               wall_0_inset = -coord_t(ext_perimeter_width / 2 - ext_perimeter_spacing / 2);
+        coord_t wall_0_inset = 0;
+        if (apply_precise_outer_wall)
+           wall_0_inset = -coord_t(ext_perimeter_width / 2 - ext_perimeter_spacing / 2);
 
-            //PS: One wall top surface for Arachne
-            ExPolygons top_expolygons;
-            // Calculate how many inner loops remain when TopSurfaces is selected.
-            const int inner_loop_number = (only_one_wall_top && upper_slices != nullptr) ? loop_number - 1 : -1;
+        //PS: One wall top surface for Arachne
+        ExPolygons top_expolygons;
+        // Calculate how many inner loops remain when TopSurfaces is selected.
+        const int inner_loop_number = (only_one_wall_top && upper_slices != nullptr) ? loop_number - 1 : -1;
 
-            // Set one perimeter when TopSurfaces is selected.
-            if (only_one_wall_top && loop_number > 0)
-                loop_number = 0;
+        // Set one perimeter when TopSurfaces is selected.
+        if (only_one_wall_top && loop_number > 0)
+            loop_number = 0;
 
-            Arachne::WallToolPathsParams input_params_tmp = input_params;
+        Arachne::WallToolPathsParams input_params_tmp = input_params;
         
-            Polygons   last_p = to_polygons(last);
-            Arachne::WallToolPaths wallToolPaths(last_p, bead_width_0, perimeter_spacing, coord_t(loop_number + 1),
-                                                   wall_0_inset, layer_height, input_params_tmp);
-            std::vector<Arachne::VariableWidthLines>   perimeters = wallToolPaths.getToolPaths();
-            ExPolygons  infill_contour = union_ex(wallToolPaths.getInnerContour());
+        Polygons   last_p = to_polygons(last);
+        Arachne::WallToolPaths wallToolPaths(last_p, bead_width_0, perimeter_spacing, coord_t(loop_number + 1),
+                                               wall_0_inset, layer_height, input_params_tmp);
+        std::vector<Arachne::VariableWidthLines>   perimeters = wallToolPaths.getToolPaths();
+        ExPolygons  infill_contour = union_ex(wallToolPaths.getInnerContour());
 
-            // Check if there are some remaining perimeters to generate (the number of perimeters
-            // is greater than one together with enabled the single perimeter on top surface feature).
-            if (inner_loop_number >= 0) {
-                assert(upper_slices != nullptr);
+        // Check if there are some remaining perimeters to generate (the number of perimeters
+        // is greater than one together with enabled the single perimeter on top surface feature).
+        if (inner_loop_number >= 0) {
+            assert(upper_slices != nullptr);
 
-                // Infill contour bounding box.
-                BoundingBox infill_contour_bbox = get_extents(infill_contour);
-                infill_contour_bbox.offset(SCALED_EPSILON);
+            // Infill contour bounding box.
+            BoundingBox infill_contour_bbox = get_extents(infill_contour);
+            infill_contour_bbox.offset(SCALED_EPSILON);
             
-                coord_t perimeter_width = this->perimeter_flow.scaled_width();
+            coord_t perimeter_width = this->perimeter_flow.scaled_width();
 
-                // Get top ExPolygons from current infill contour.
-                Polygons upper_slices_clipped;
-                if (object_config->interface_shells) {
-                    auto upper_slicer_same_region = to_expolygons(this->upper_slices_same_region->surfaces);
-                    upper_slices_clipped = ClipperUtils::clip_clipper_polygons_with_subject_bbox(upper_slicer_same_region, infill_contour_bbox);
-                } else
-                    upper_slices_clipped = ClipperUtils::clip_clipper_polygons_with_subject_bbox(*upper_slices, infill_contour_bbox);
+            // Get top ExPolygons from current infill contour.
+            Polygons upper_slices_clipped;
+            if (object_config->interface_shells) {
+                auto upper_slicer_same_region = to_expolygons(this->upper_slices_same_region->surfaces);
+                upper_slices_clipped = ClipperUtils::clip_clipper_polygons_with_subject_bbox(upper_slicer_same_region, infill_contour_bbox);
+            } else
+                upper_slices_clipped = ClipperUtils::clip_clipper_polygons_with_subject_bbox(*upper_slices, infill_contour_bbox);
 
-                top_expolygons = diff_ex(infill_contour, upper_slices_clipped);
+            top_expolygons = diff_ex(infill_contour, upper_slices_clipped);
 
-                if (!top_expolygons.empty()) {
-                    if (lower_slices != nullptr) {
-                        const float      bridge_offset          = float(std::max<coord_t>(ext_perimeter_spacing, perimeter_width));
-                        const Polygons   lower_slices_clipped   = ClipperUtils::clip_clipper_polygons_with_subject_bbox(*lower_slices, infill_contour_bbox);
-                        const ExPolygons current_slices_bridges = offset_ex(diff_ex(top_expolygons, lower_slices_clipped), bridge_offset);
+            if (!top_expolygons.empty()) {
+                if (lower_slices != nullptr) {
+                    const float      bridge_offset          = float(std::max<coord_t>(ext_perimeter_spacing, perimeter_width));
+                    const Polygons   lower_slices_clipped   = ClipperUtils::clip_clipper_polygons_with_subject_bbox(*lower_slices, infill_contour_bbox);
+                    const ExPolygons current_slices_bridges = offset_ex(diff_ex(top_expolygons, lower_slices_clipped), bridge_offset);
 
-                        // Remove bridges from top surface polygons.
-                        top_expolygons = diff_ex(top_expolygons, current_slices_bridges);
-                    }
-
-                    // Filter out areas that are too thin and expand top surface polygons a bit to hide the wall line.
-                    // ORCA: skip if the top surface area is smaller than "min_width_top_surface"
-                    const float top_surface_min_width = std::max<float>(float(ext_perimeter_spacing) / 4.f + scaled<float>(0.00001), float(scale_(config->min_width_top_surface.get_abs_value(unscale_(perimeter_width)))) / 4.f);
-                    // Shrink the polygon to remove the small areas, then expand it back out plus a maragin to hide the wall line a little.
-                    // ORCA: Expand the polygon with half the perimeter width in addition to the contracted amount,
-                    // not the full perimeter width as PS does, to enable thin lettering to print on the top surface without nozzle collisions
-                    // due to thin lines being generated
-                    top_expolygons = offset2_ex(top_expolygons, -top_surface_min_width, top_surface_min_width + float(perimeter_width * 0.85));
-
-                    // Get final top ExPolygons (bridges were excluded above, so they stay walled).
-                    top_expolygons = intersection_ex(top_expolygons, infill_contour);
-
-                    // ORCA: onion the real region (inside the outer wall) so the remaining walls follow the actual
-                    // geometry, then cut away the parts over the top surface. Re-onioning the non-top complement
-                    // instead - the fallback when there is no top fill - walls the top/non-top interface and rings
-                    // top-surface islands with inner walls that don't exist when the feature is disabled.
-                    const bool     clip_walls_over_top = top_fill_replaces_inner_walls(*this->config);
-                    const Polygons inner_region        = to_polygons(offset_ex(clip_walls_over_top ? infill_contour
-                                                                                                   : diff_ex(infill_contour, top_expolygons),
-                                                                              wall_0_inset));
-                    Arachne::WallToolPaths inner_wall_tool_paths(inner_region, perimeter_spacing, perimeter_spacing, coord_t(inner_loop_number + 1), 0, layer_height, input_params_tmp);
-                    std::vector<Arachne::VariableWidthLines> inner_perimeters = inner_wall_tool_paths.getToolPaths();
-
-                    if (clip_walls_over_top) {
-                        Polygons kept_over_top;
-                        clip_inner_walls_over_top(inner_perimeters, top_expolygons, perimeter_width, kept_over_top);
-                        // Route the top fill around the walls kept despite grazing the top.
-                        if (! kept_over_top.empty())
-                            top_expolygons = diff_ex(top_expolygons, kept_over_top);
-                    }
-
-                    // Recalculate indexes of inner perimeters before merging them: they come after the single outer wall.
-                    if (!perimeters.empty())
-                        for (Arachne::VariableWidthLines &inner_perimeter : inner_perimeters)
-                            for (Arachne::ExtrusionLine &el : inner_perimeter)
-                                ++el.inset_idx;
-
-                    perimeters.insert(perimeters.end(), inner_perimeters.begin(), inner_perimeters.end());
-                    infill_contour = union_ex(top_expolygons, inner_wall_tool_paths.getInnerContour());
-                } else {
-                    // There is no top surface ExPolygon, so we call Arachne again with parameters
-                    // like when the single perimeter feature is disabled.
-                    Arachne::WallToolPaths no_single_perimeter_tool_paths(last_p, bead_width_0, perimeter_spacing, coord_t(inner_loop_number + 2), wall_0_inset, layer_height, input_params_tmp);
-                    perimeters     = no_single_perimeter_tool_paths.getToolPaths();
-                    infill_contour = union_ex(no_single_perimeter_tool_paths.getInnerContour());
+                    // Remove bridges from top surface polygons.
+                    top_expolygons = diff_ex(top_expolygons, current_slices_bridges);
                 }
+
+                // Filter out areas that are too thin and expand top surface polygons a bit to hide the wall line.
+                // ORCA: skip if the top surface area is smaller than "min_width_top_surface"
+                const float top_surface_min_width = std::max<float>(float(ext_perimeter_spacing) / 4.f + scaled<float>(0.00001), float(scale_(config->min_width_top_surface.get_abs_value(unscale_(perimeter_width)))) / 4.f);
+                // Shrink the polygon to remove the small areas, then expand it back out plus a maragin to hide the wall line a little.
+                // ORCA: Expand the polygon with half the perimeter width in addition to the contracted amount,
+                // not the full perimeter width as PS does, to enable thin lettering to print on the top surface without nozzle collisions
+                // due to thin lines being generated
+                top_expolygons = offset2_ex(top_expolygons, -top_surface_min_width, top_surface_min_width + float(perimeter_width * 0.85));
+
+                // Get final top ExPolygons (bridges were excluded above, so they stay walled).
+                top_expolygons = intersection_ex(top_expolygons, infill_contour);
+
+                // ORCA: onion the real region (inside the outer wall) so the remaining walls follow the actual
+                // geometry, then cut away the parts over the top surface. Re-onioning the non-top complement
+                // instead - the fallback when there is no top fill - walls the top/non-top interface and rings
+                // top-surface islands with inner walls that don't exist when the feature is disabled.
+                const bool     clip_walls_over_top = top_fill_replaces_inner_walls(*this->config);
+                const Polygons inner_region        = to_polygons(offset_ex(clip_walls_over_top ? infill_contour
+                                                                                               : diff_ex(infill_contour, top_expolygons),
+                                                                          wall_0_inset));
+                Arachne::WallToolPaths inner_wall_tool_paths(inner_region, perimeter_spacing, perimeter_spacing, coord_t(inner_loop_number + 1), 0, layer_height, input_params_tmp);
+                std::vector<Arachne::VariableWidthLines> inner_perimeters = inner_wall_tool_paths.getToolPaths();
+
+                if (clip_walls_over_top) {
+                    Polygons kept_over_top;
+                    clip_inner_walls_over_top(inner_perimeters, top_expolygons, perimeter_width, kept_over_top);
+                    // Route the top fill around the walls kept despite grazing the top.
+                    if (! kept_over_top.empty())
+                        top_expolygons = diff_ex(top_expolygons, kept_over_top);
+                }
+
+                // Recalculate indexes of inner perimeters before merging them: they come after the single outer wall.
+                if (!perimeters.empty())
+                    for (Arachne::VariableWidthLines &inner_perimeter : inner_perimeters)
+                        for (Arachne::ExtrusionLine &el : inner_perimeter)
+                            ++el.inset_idx;
+
+                perimeters.insert(perimeters.end(), inner_perimeters.begin(), inner_perimeters.end());
+                infill_contour = union_ex(top_expolygons, inner_wall_tool_paths.getInnerContour());
+            } else {
+                // There is no top surface ExPolygon, so we call Arachne again with parameters
+                // like when the single perimeter feature is disabled.
+                Arachne::WallToolPaths no_single_perimeter_tool_paths(last_p, bead_width_0, perimeter_spacing, coord_t(inner_loop_number + 2), wall_0_inset, layer_height, input_params_tmp);
+                perimeters     = no_single_perimeter_tool_paths.getToolPaths();
+                infill_contour = union_ex(no_single_perimeter_tool_paths.getInnerContour());
             }
-            //PS
+        }
+        //PS
 
-            loop_number = int(perimeters.size()) - 1;
+        loop_number = int(perimeters.size()) - 1;
 
-            #ifdef ARACHNE_DEBUG
-            {
-                static int iRun = 0;
-                export_perimeters_to_svg(debug_out_path("arachne-perimeters-%d-%d.svg", layer_id, iRun++), to_polygons(last), perimeters, union_ex(wallToolPaths.getInnerContour()));
-            }
-    #endif
+        #ifdef ARACHNE_DEBUG
+        {
+            static int iRun = 0;
+            export_perimeters_to_svg(debug_out_path("arachne-perimeters-%d-%d.svg", layer_id, iRun++), to_polygons(last), perimeters, union_ex(wallToolPaths.getInnerContour()));
+        }
+#endif
 
-            // All closed ExtrusionLine should have the same the first and the last point.
-            // But in rare cases, Arachne produce ExtrusionLine marked as closed but without
-            // equal the first and the last point.
-            assert([&perimeters = std::as_const(perimeters)]() -> bool {
-                for (const Arachne::VariableWidthLines& perimeter : perimeters)
-                    for (const Arachne::ExtrusionLine& el : perimeter)
-                        if (el.is_closed && el.junctions.front().p != el.junctions.back().p)
-                            return false;
-                return true;
-            }());
+        // All closed ExtrusionLine should have the same the first and the last point.
+        // But in rare cases, Arachne produce ExtrusionLine marked as closed but without
+        // equal the first and the last point.
+        assert([&perimeters = std::as_const(perimeters)]() -> bool {
+            for (const Arachne::VariableWidthLines& perimeter : perimeters)
+                for (const Arachne::ExtrusionLine& el : perimeter)
+                    if (el.is_closed && el.junctions.front().p != el.junctions.back().p)
+                        return false;
+            return true;
+        }());
 
-            int start_perimeter = int(perimeters.size()) - 1;
-            int end_perimeter = -1;
-            int direction = -1;
+        int start_perimeter = int(perimeters.size()) - 1;
+        int end_perimeter = -1;
+        int direction = -1;
 
-    		bool is_outer_wall_first =
-                	this->config->wall_sequence == WallSequence::OuterInner ||
-                	this->config->wall_sequence == WallSequence::InnerOuterInner;
+		bool is_outer_wall_first =
+            	this->config->wall_sequence == WallSequence::OuterInner ||
+            	this->config->wall_sequence == WallSequence::InnerOuterInner;
         
-            if (layer_id == 0){ // disable inner outer inner algorithm after the first layer
-            	is_outer_wall_first =
-                	this->config->wall_sequence == WallSequence::OuterInner;
-            }
-            if (is_outer_wall_first) {
-                start_perimeter = 0;
-                end_perimeter = int(perimeters.size());
-                direction = 1;
+        if (layer_id == 0){ // disable inner outer inner algorithm after the first layer
+        	is_outer_wall_first =
+            	this->config->wall_sequence == WallSequence::OuterInner;
+        }
+        if (is_outer_wall_first) {
+            start_perimeter = 0;
+            end_perimeter = int(perimeters.size());
+            direction = 1;
+        }
+
+        std::vector<Arachne::ExtrusionLine*> all_extrusions;
+        for (int perimeter_idx = start_perimeter; perimeter_idx != end_perimeter; perimeter_idx += direction) {
+            if (perimeters[perimeter_idx].empty())
+                continue;
+            for (Arachne::ExtrusionLine& wall : perimeters[perimeter_idx])
+                all_extrusions.emplace_back(&wall);
+        }
+
+        // Find topological order with constraints from extrusions_constrains.
+        std::vector<size_t>              blocked(all_extrusions.size(), 0); // Value indicating how many extrusions it is blocking (preceding extrusions) an extrusion.
+        std::vector<std::vector<size_t>> blocking(all_extrusions.size());   // Each extrusion contains a vector of extrusions that are blocked by this extrusion.
+        std::unordered_map<const Arachne::ExtrusionLine*, size_t> map_extrusion_to_idx;
+        for (size_t idx = 0; idx < all_extrusions.size(); idx++)
+            map_extrusion_to_idx.emplace(all_extrusions[idx], idx);
+
+        auto extrusions_constrains = Arachne::WallToolPaths::getRegionOrder(all_extrusions, is_outer_wall_first);
+        for (auto [before, after] : extrusions_constrains) {
+            auto after_it = map_extrusion_to_idx.find(after);
+            ++blocked[after_it->second];
+            blocking[map_extrusion_to_idx.find(before)->second].emplace_back(after_it->second);
+        }
+
+        std::vector<bool> processed(all_extrusions.size(), false);          // Indicate that the extrusion was already processed.
+        Point             current_position = all_extrusions.empty() ? Point::Zero() : all_extrusions.front()->junctions.front().p; // Some starting position.
+        std::vector<PerimeterGeneratorArachneExtrusion> ordered_extrusions;         // To store our result in. At the end we'll std::swap.
+        ordered_extrusions.reserve(all_extrusions.size());
+
+        while (ordered_extrusions.size() < all_extrusions.size()) {
+            size_t best_candidate = 0;
+            double best_distance_sqr = std::numeric_limits<double>::max();
+            bool   is_best_closed = false;
+
+            std::vector<size_t> available_candidates;
+            for (size_t candidate = 0; candidate < all_extrusions.size(); ++candidate) {
+                if (processed[candidate] || blocked[candidate])
+                    continue; // Not a valid candidate.
+                available_candidates.push_back(candidate);
             }
 
-            std::vector<Arachne::ExtrusionLine*> all_extrusions;
-            for (int perimeter_idx = start_perimeter; perimeter_idx != end_perimeter; perimeter_idx += direction) {
-                if (perimeters[perimeter_idx].empty())
+            std::sort(available_candidates.begin(), available_candidates.end(), [&all_extrusions](const size_t a_idx, const size_t b_idx) -> bool {
+                return all_extrusions[a_idx]->is_closed < all_extrusions[b_idx]->is_closed;
+                });
+
+            for (const size_t candidate_path_idx : available_candidates) {
+                auto& path = all_extrusions[candidate_path_idx];
+
+                if (path->junctions.empty()) { // No vertices in the path. Can't find the start position then or really plan it in. Put that at the end.
+                    if (best_distance_sqr == std::numeric_limits<double>::max()) {
+                        best_candidate = candidate_path_idx;
+                        is_best_closed = path->is_closed;
+                    }
                     continue;
-                for (Arachne::ExtrusionLine& wall : perimeters[perimeter_idx])
-                    all_extrusions.emplace_back(&wall);
-            }
-
-            // Find topological order with constraints from extrusions_constrains.
-            std::vector<size_t>              blocked(all_extrusions.size(), 0); // Value indicating how many extrusions it is blocking (preceding extrusions) an extrusion.
-            std::vector<std::vector<size_t>> blocking(all_extrusions.size());   // Each extrusion contains a vector of extrusions that are blocked by this extrusion.
-            std::unordered_map<const Arachne::ExtrusionLine*, size_t> map_extrusion_to_idx;
-            for (size_t idx = 0; idx < all_extrusions.size(); idx++)
-                map_extrusion_to_idx.emplace(all_extrusions[idx], idx);
-
-            auto extrusions_constrains = Arachne::WallToolPaths::getRegionOrder(all_extrusions, is_outer_wall_first);
-            for (auto [before, after] : extrusions_constrains) {
-                auto after_it = map_extrusion_to_idx.find(after);
-                ++blocked[after_it->second];
-                blocking[map_extrusion_to_idx.find(before)->second].emplace_back(after_it->second);
-            }
-
-            std::vector<bool> processed(all_extrusions.size(), false);          // Indicate that the extrusion was already processed.
-            Point             current_position = all_extrusions.empty() ? Point::Zero() : all_extrusions.front()->junctions.front().p; // Some starting position.
-            std::vector<PerimeterGeneratorArachneExtrusion> ordered_extrusions;         // To store our result in. At the end we'll std::swap.
-            ordered_extrusions.reserve(all_extrusions.size());
-
-            while (ordered_extrusions.size() < all_extrusions.size()) {
-                size_t best_candidate = 0;
-                double best_distance_sqr = std::numeric_limits<double>::max();
-                bool   is_best_closed = false;
-
-                std::vector<size_t> available_candidates;
-                for (size_t candidate = 0; candidate < all_extrusions.size(); ++candidate) {
-                    if (processed[candidate] || blocked[candidate])
-                        continue; // Not a valid candidate.
-                    available_candidates.push_back(candidate);
                 }
 
-                std::sort(available_candidates.begin(), available_candidates.end(), [&all_extrusions](const size_t a_idx, const size_t b_idx) -> bool {
-                    return all_extrusions[a_idx]->is_closed < all_extrusions[b_idx]->is_closed;
-                    });
-
-                for (const size_t candidate_path_idx : available_candidates) {
-                    auto& path = all_extrusions[candidate_path_idx];
-
-                    if (path->junctions.empty()) { // No vertices in the path. Can't find the start position then or really plan it in. Put that at the end.
-                        if (best_distance_sqr == std::numeric_limits<double>::max()) {
-                            best_candidate = candidate_path_idx;
-                            is_best_closed = path->is_closed;
-                        }
-                        continue;
-                    }
-
-                    const Point candidate_position = path->junctions.front().p;
-                    double      distance_sqr = (current_position - candidate_position).cast<double>().norm();
-                    if (distance_sqr < best_distance_sqr) { // Closer than the best candidate so far.
-                        if (path->is_closed || (!path->is_closed && best_distance_sqr != std::numeric_limits<double>::max()) || (!path->is_closed && !is_best_closed)) {
-                            best_candidate = candidate_path_idx;
-                            best_distance_sqr = distance_sqr;
-                            is_best_closed = path->is_closed;
-                        }
+                const Point candidate_position = path->junctions.front().p;
+                double      distance_sqr = (current_position - candidate_position).cast<double>().norm();
+                if (distance_sqr < best_distance_sqr) { // Closer than the best candidate so far.
+                    if (path->is_closed || (!path->is_closed && best_distance_sqr != std::numeric_limits<double>::max()) || (!path->is_closed && !is_best_closed)) {
+                        best_candidate = candidate_path_idx;
+                        best_distance_sqr = distance_sqr;
+                        is_best_closed = path->is_closed;
                     }
                 }
-
-                auto& best_path = all_extrusions[best_candidate];
-                ordered_extrusions.push_back({ best_path, best_path->is_contour() });
-                processed[best_candidate] = true;
-                for (size_t unlocked_idx : blocking[best_candidate])
-                    blocked[unlocked_idx]--;
-
-                if (!best_path->junctions.empty()) { //If all paths were empty, the best path is still empty. We don't upate the current position then.
-                    if (best_path->is_closed)
-                        current_position = best_path->junctions[0].p; //We end where we started.
-                    else
-                        current_position = best_path->junctions.back().p; //Pick the other end from where we started.
-                }
             }
 
-           // printf("New Layer: Layer ID %d\n",layer_id); //debug - new layer
-            if (this->config->wall_sequence == WallSequence::InnerOuterInner && layer_id > 0) { // only enable inner outer inner algorithm after first layer
-                if (ordered_extrusions.size() > 2) { // 3 walls minimum needed to do inner outer inner ordering
-                    int position = 0; // index to run the re-ordering for multiple external perimeters in a single island.
-                    int arr_i, arr_j = 0;    // indexes to run through the walls in the for loops
-                    int outer, first_internal, second_internal, max_internal, current_perimeter; // allocate index values
-                
-                    // To address any remaining scenarios where the outer perimeter contour is not first on the list as arachne sometimes reorders the perimeters when clustering
-                    // for OI mode that is used the basis for IOI
-                    bringContoursToFront(ordered_extrusions);
-                    std::vector<PerimeterGeneratorArachneExtrusion> reordered_extrusions;
-                
-                    // Debug statement to print spacing values:
-                    //printf("External threshold - Ext perimeter: %d Ext spacing: %d Int perimeter: %d Int spacing: %d\n", this->ext_perimeter_flow.scaled_width(),this->ext_perimeter_flow.scaled_spacing(),this->perimeter_flow.scaled_width(), this->perimeter_flow.scaled_spacing());
+            auto& best_path = all_extrusions[best_candidate];
+            ordered_extrusions.push_back({ best_path, best_path->is_contour() });
+            processed[best_candidate] = true;
+            for (size_t unlocked_idx : blocking[best_candidate])
+                blocked[unlocked_idx]--;
 
-                    // Get searching thresholds. For an external perimeter we take the external perimeter spacing/2 plus the internal perimeter spacing/2 and expand by the factor
-                    // rounding errors. When precise wall is enabled, the external perimeter full spacing is used.
-                    coord_t threshold_external = (apply_precise_outer_wall)
-                        // Precise outer wall ⇒ use “full external spacing”
-                        ? ( this->ext_perimeter_flow.scaled_spacing()
-                            + this->perimeter_flow.scaled_spacing()/2.0 )
-                        // Normal ⇒ half ext spacing + half int spacing
-                        : ( this->ext_perimeter_flow.scaled_spacing()/2.0
-                            + this->perimeter_flow.scaled_spacing()/2.0 );
+            if (!best_path->junctions.empty()) { //If all paths were empty, the best path is still empty. We don't upate the current position then.
+                if (best_path->is_closed)
+                    current_position = best_path->junctions[0].p; //We end where we started.
+                else
+                    current_position = best_path->junctions.back().p; //Pick the other end from where we started.
+            }
+        }
+
+       // printf("New Layer: Layer ID %d\n",layer_id); //debug - new layer
+        if (this->config->wall_sequence == WallSequence::InnerOuterInner && layer_id > 0) { // only enable inner outer inner algorithm after first layer
+            if (ordered_extrusions.size() > 2) { // 3 walls minimum needed to do inner outer inner ordering
+                int position = 0; // index to run the re-ordering for multiple external perimeters in a single island.
+                int arr_i, arr_j = 0;    // indexes to run through the walls in the for loops
+                int outer, first_internal, second_internal, max_internal, current_perimeter; // allocate index values
                 
-                    // For the intenal perimeter threshold, the distance is the internal perimeter spacing expanded by the factor to cover rounding errors.
-                    coord_t threshold_internal = this->perimeter_flow.scaled_spacing();
+                // To address any remaining scenarios where the outer perimeter contour is not first on the list as arachne sometimes reorders the perimeters when clustering
+                // for OI mode that is used the basis for IOI
+                bringContoursToFront(ordered_extrusions);
+                std::vector<PerimeterGeneratorArachneExtrusion> reordered_extrusions;
                 
-                    // Re-order extrusions based on distance
-                    // Alorithm will aggresively optimise for the appearance of the outermost perimeter
-                    ordered_extrusions = reorderPerimetersByProximity(ordered_extrusions,threshold_external,threshold_internal );
-                    reordered_extrusions = ordered_extrusions; // copy them into the reordered extrusions vector to allow for IOI operations to be performed below without altering the base ordered extrusions list.
+                // Debug statement to print spacing values:
+                //printf("External threshold - Ext perimeter: %d Ext spacing: %d Int perimeter: %d Int spacing: %d\n", this->ext_perimeter_flow.scaled_width(),this->ext_perimeter_flow.scaled_spacing(),this->perimeter_flow.scaled_width(), this->perimeter_flow.scaled_spacing());
+
+                // Get searching thresholds. For an external perimeter we take the external perimeter spacing/2 plus the internal perimeter spacing/2 and expand by the factor
+                // rounding errors. When precise wall is enabled, the external perimeter full spacing is used.
+                coord_t threshold_external = (apply_precise_outer_wall)
+                    // Precise outer wall ⇒ use “full external spacing”
+                    ? ( this->ext_perimeter_flow.scaled_spacing()
+                        + this->perimeter_flow.scaled_spacing()/2.0 )
+                    // Normal ⇒ half ext spacing + half int spacing
+                    : ( this->ext_perimeter_flow.scaled_spacing()/2.0
+                        + this->perimeter_flow.scaled_spacing()/2.0 );
                 
-                    // Now start the sandwich mode wall re-ordering using the reordered_extrusions as the basis
-                    // scan to find the external perimeter, first internal, second internal and last perimeter in the island.
-                    // We then advance the position index to move to the second island and continue until there are no more
-                    // perimeters left.
-                    while (position < reordered_extrusions.size()) {
-                        outer = first_internal = second_internal = current_perimeter = -1; // initialise all index values to -1
-                        max_internal = reordered_extrusions.size()-1; // initialise the maximum internal perimeter to the last perimeter on the extrusion list
-                        // run through the walls to get the index values that need re-ordering until the first one for each
-                        // is found. Start at "position" index to enable the for loop to iterate for multiple external
-                        // perimeters in a single island
-                        // printf("Reorder Loop. Position %d, extrusion list size: %d, Outer index %d, inner index %d, second inner index %d\n", position, reordered_extrusions.size(),outer,first_internal,second_internal);
-                        for (arr_i = position; arr_i < reordered_extrusions.size(); ++arr_i) {
-                            // printf("Perimeter: extrusion inset index %d, ordered extrusions array position %d\n",reordered_extrusions[arr_i].extrusion->inset_idx, arr_i);
-                            switch (reordered_extrusions[arr_i].extrusion->inset_idx) {
-                                case 0: // external perimeter
-                                    if (outer == -1)
-                                        outer = arr_i;
-                                    break;
-                                case 1: // first internal wall
-                                    if (first_internal==-1 && arr_i>outer && outer!=-1){
-                                        first_internal = arr_i;
-                                    }
-                                    break;
-                                case 2: // second internal wall
-                                    if (second_internal == -1 && arr_i > first_internal && outer!=-1){
-                                        second_internal = arr_i;
-                                    }
-                                    break;
-                            }
-                            if(outer >-1 && first_internal>-1 && reordered_extrusions[arr_i].extrusion->inset_idx == 0){  // found a new external perimeter after we've found at least a first internal perimeter to re-order.
-                                                                                                                          // This means we entered a new island.
-                                arr_i=arr_i-1; //step back one perimeter
-                                max_internal = arr_i; // new maximum internal perimeter is now this as we have found a new external perimeter, hence a new island.
-                                break; // exit the for loop
-                            }
-                        }
-                    
-                        // printf("Layer ID %d, Outer index %d, inner index %d, second inner index %d, maximum internal perimeter %d \n",layer_id,outer,first_internal,second_internal, max_internal);
-                        if (outer > -1 && first_internal > -1 && second_internal > -1) { // found all three perimeters to re-order? If not the perimeters will be processed outside in.
-                            std::vector<PerimeterGeneratorArachneExtrusion> inner_outer_extrusions; // temporary array to hold extrusions for reordering
-                            inner_outer_extrusions.resize(max_internal - position + 1); // reserve array containing the number of perimeters before a new island. Variables are array indexes hence need to add +1 to convert to position allocations
-                            // printf("Allocated array size %d, max_internal index %d, start position index %d \n",max_internal-position+1,max_internal,position);
-                        
-                            for (arr_j = max_internal; arr_j >=position; --arr_j){ // go inside out towards the external perimeter (perimeters in reverse order) and store all internal perimeters until the first one identified with inset index 2
-                                if(arr_j >= second_internal){
-                                    //printf("Inside out loop: Mapped perimeter index %d to array position %d\n", arr_j, max_internal-arr_j);
-                                    inner_outer_extrusions[max_internal-arr_j] = reordered_extrusions[arr_j];
-                                    current_perimeter++; 
+                // For the intenal perimeter threshold, the distance is the internal perimeter spacing expanded by the factor to cover rounding errors.
+                coord_t threshold_internal = this->perimeter_flow.scaled_spacing();
+                
+                // Re-order extrusions based on distance
+                // Alorithm will aggresively optimise for the appearance of the outermost perimeter
+                ordered_extrusions = reorderPerimetersByProximity(ordered_extrusions,threshold_external,threshold_internal );
+                reordered_extrusions = ordered_extrusions; // copy them into the reordered extrusions vector to allow for IOI operations to be performed below without altering the base ordered extrusions list.
+                
+                // Now start the sandwich mode wall re-ordering using the reordered_extrusions as the basis
+                // scan to find the external perimeter, first internal, second internal and last perimeter in the island.
+                // We then advance the position index to move to the second island and continue until there are no more
+                // perimeters left.
+                while (position < reordered_extrusions.size()) {
+                    outer = first_internal = second_internal = current_perimeter = -1; // initialise all index values to -1
+                    max_internal = reordered_extrusions.size()-1; // initialise the maximum internal perimeter to the last perimeter on the extrusion list
+                    // run through the walls to get the index values that need re-ordering until the first one for each
+                    // is found. Start at "position" index to enable the for loop to iterate for multiple external
+                    // perimeters in a single island
+                    // printf("Reorder Loop. Position %d, extrusion list size: %d, Outer index %d, inner index %d, second inner index %d\n", position, reordered_extrusions.size(),outer,first_internal,second_internal);
+                    for (arr_i = position; arr_i < reordered_extrusions.size(); ++arr_i) {
+                        // printf("Perimeter: extrusion inset index %d, ordered extrusions array position %d\n",reordered_extrusions[arr_i].extrusion->inset_idx, arr_i);
+                        switch (reordered_extrusions[arr_i].extrusion->inset_idx) {
+                            case 0: // external perimeter
+                                if (outer == -1)
+                                    outer = arr_i;
+                                break;
+                            case 1: // first internal wall
+                                if (first_internal==-1 && arr_i>outer && outer!=-1){
+                                    first_internal = arr_i;
                                 }
-                            }
-                        
-                            for (arr_j = position; arr_j < second_internal; ++arr_j){ // go outside in and map the remaining perimeters (external and first internal wall(s)) using the outside in wall order
-                                // printf("Outside in loop: Mapped perimeter index %d to array position %d\n", arr_j, current_perimeter+1);
-                                inner_outer_extrusions[++current_perimeter] = reordered_extrusions[arr_j];
-                            }
-                        
-                            for(arr_j = position; arr_j <= max_internal; ++arr_j) // replace perimeter array with the new re-ordered array
-                                ordered_extrusions[arr_j] = inner_outer_extrusions[arr_j-position];
+                                break;
+                            case 2: // second internal wall
+                                if (second_internal == -1 && arr_i > first_internal && outer!=-1){
+                                    second_internal = arr_i;
+                                }
+                                break;
+                        }
+                        if(outer >-1 && first_internal>-1 && reordered_extrusions[arr_i].extrusion->inset_idx == 0){  // found a new external perimeter after we've found at least a first internal perimeter to re-order.
+                                                                                                                      // This means we entered a new island.
+                            arr_i=arr_i-1; //step back one perimeter
+                            max_internal = arr_i; // new maximum internal perimeter is now this as we have found a new external perimeter, hence a new island.
+                            break; // exit the for loop
                         }
-                        // go to the next perimeter from the current position to continue scanning for external walls in the same island
-                        position = arr_i + 1;
                     }
+                    
+                    // printf("Layer ID %d, Outer index %d, inner index %d, second inner index %d, maximum internal perimeter %d \n",layer_id,outer,first_internal,second_internal, max_internal);
+                    if (outer > -1 && first_internal > -1 && second_internal > -1) { // found all three perimeters to re-order? If not the perimeters will be processed outside in.
+                        std::vector<PerimeterGeneratorArachneExtrusion> inner_outer_extrusions; // temporary array to hold extrusions for reordering
+                        inner_outer_extrusions.resize(max_internal - position + 1); // reserve array containing the number of perimeters before a new island. Variables are array indexes hence need to add +1 to convert to position allocations
+                        // printf("Allocated array size %d, max_internal index %d, start position index %d \n",max_internal-position+1,max_internal,position);
+                        
+                        for (arr_j = max_internal; arr_j >=position; --arr_j){ // go inside out towards the external perimeter (perimeters in reverse order) and store all internal perimeters until the first one identified with inset index 2
+                            if(arr_j >= second_internal){
+                                //printf("Inside out loop: Mapped perimeter index %d to array position %d\n", arr_j, max_internal-arr_j);
+                                inner_outer_extrusions[max_internal-arr_j] = reordered_extrusions[arr_j];
+                                current_perimeter++; 
+                            }
+                        }
+                        
+                        for (arr_j = position; arr_j < second_internal; ++arr_j){ // go outside in and map the remaining perimeters (external and first internal wall(s)) using the outside in wall order
+                            // printf("Outside in loop: Mapped perimeter index %d to array position %d\n", arr_j, current_perimeter+1);
+                            inner_outer_extrusions[++current_perimeter] = reordered_extrusions[arr_j];
+                        }
+                        
+                        for(arr_j = position; arr_j <= max_internal; ++arr_j) // replace perimeter array with the new re-ordered array
+                            ordered_extrusions[arr_j] = inner_outer_extrusions[arr_j-position];
+                    }
+                    // go to the next perimeter from the current position to continue scanning for external walls in the same island
+                    position = arr_i + 1;
                 }
             }
+        }
         
-            bool steep_overhang_contour = false;
-            bool steep_overhang_hole    = false;
-            if (!config->overhang_reverse) {
-                // Skip steep overhang detection no reverse is specified
-                steep_overhang_contour = true;
-                steep_overhang_hole    = true;
-            }
-            if (ExtrusionEntityCollection extrusion_coll = traverse_extrusions(*this, ordered_extrusions, steep_overhang_contour, steep_overhang_hole); !extrusion_coll.empty()) {
-                if (config->overhang_reverse) {
-                    reorient_perimeters(extrusion_coll, steep_overhang_contour, steep_overhang_hole,
-                                        this->config->overhang_reverse_internal_only);
-                }
-                defer_unsupported_loops(*this, extrusion_coll);
-                result.loops     = std::move(extrusion_coll);
-                result.has_loops = true;
+        bool steep_overhang_contour = false;
+        bool steep_overhang_hole    = false;
+        if (!config->overhang_reverse) {
+            // Skip steep overhang detection no reverse is specified
+            steep_overhang_contour = true;
+            steep_overhang_hole    = true;
+        }
+        if (ExtrusionEntityCollection extrusion_coll = traverse_extrusions(*this, ordered_extrusions, steep_overhang_contour, steep_overhang_hole); !extrusion_coll.empty()) {
+            if (config->overhang_reverse) {
+                reorient_perimeters(extrusion_coll, steep_overhang_contour, steep_overhang_hole,
+                                    this->config->overhang_reverse_internal_only);
             }
+            defer_unsupported_loops(*this, extrusion_coll);
+            this->loops->append(extrusion_coll);
+        }
 
-            const coord_t spacing = (perimeters.size() == 1) ? ext_perimeter_spacing2 : perimeter_spacing;
+        const coord_t spacing = (perimeters.size() == 1) ? ext_perimeter_spacing2 : perimeter_spacing;
 
-            if (offset_ex(infill_contour, -float(spacing / 2.)).empty())
-                infill_contour.clear(); // Infill region is too small, so let's filter it out.
+        if (offset_ex(infill_contour, -float(spacing / 2.)).empty())
+            infill_contour.clear(); // Infill region is too small, so let's filter it out.
 
-            // create one more offset to be used as boundary for fill
-            // we offset by half the perimeter spacing (to get to the actual infill boundary)
-            // and then we offset back and forth by half the infill spacing to only consider the
-            // non-collapsing regions
-            coord_t inset =
-                (loop_number < 0) ? 0 :
-                (loop_number == 0) ?
-                // one loop
-                ext_perimeter_spacing :
-                // two or more loops?
-                perimeter_spacing;
-            coord_t top_inset = inset;
+        // create one more offset to be used as boundary for fill
+        // we offset by half the perimeter spacing (to get to the actual infill boundary)
+        // and then we offset back and forth by half the infill spacing to only consider the
+        // non-collapsing regions
+        coord_t inset =
+            (loop_number < 0) ? 0 :
+            (loop_number == 0) ?
+            // one loop
+            ext_perimeter_spacing :
+            // two or more loops?
+            perimeter_spacing;
+        coord_t top_inset = inset;
         
-            top_inset = coord_t(scale_(this->config->top_bottom_infill_wall_overlap.get_abs_value(unscale<double>(inset))));
-            if(is_topmost_layer || is_bottom_layer)
-                inset = coord_t(scale_(this->config->top_bottom_infill_wall_overlap.get_abs_value(unscale<double>(inset))));
-            else
-                inset = coord_t(scale_(this->config->infill_wall_overlap.get_abs_value(unscale<double>(inset))));
+        top_inset = coord_t(scale_(this->config->top_bottom_infill_wall_overlap.get_abs_value(unscale<double>(inset))));
+        if(is_topmost_layer || is_bottom_layer)
+            inset = coord_t(scale_(this->config->top_bottom_infill_wall_overlap.get_abs_value(unscale<double>(inset))));
+        else
+            inset = coord_t(scale_(this->config->infill_wall_overlap.get_abs_value(unscale<double>(inset))));
         
-            // simplify infill contours according to resolution
-            Polygons pp;
-            for (ExPolygon& ex : infill_contour)
-                ex.simplify_p(m_scaled_resolution, &pp);
-            ExPolygons not_filled_exp = union_ex(pp);
-            // collapse too narrow infill areas
-            const auto    min_perimeter_infill_spacing = coord_t(solid_infill_spacing * (1. - INSET_OVERLAP_TOLERANCE));
+        // simplify infill contours according to resolution
+        Polygons pp;
+        for (ExPolygon& ex : infill_contour)
+            ex.simplify_p(m_scaled_resolution, &pp);
+        ExPolygons not_filled_exp = union_ex(pp);
+        // collapse too narrow infill areas
+        const auto    min_perimeter_infill_spacing = coord_t(solid_infill_spacing * (1. - INSET_OVERLAP_TOLERANCE));
 
-            ExPolygons infill_exp = offset2_ex(
+        ExPolygons infill_exp = offset2_ex(
+            not_filled_exp,
+            float(-min_perimeter_infill_spacing / 2.),
+            float(inset + min_perimeter_infill_spacing / 2.));
+        // append infill areas to fill_surfaces
+        if (!top_expolygons.empty()) {
+            infill_exp = union_ex(infill_exp, offset_ex(top_expolygons, double(top_inset)));
+        }
+        this->fill_surfaces->append(infill_exp, stInternal);
+
+        apply_extra_perimeters(infill_exp);
+
+        // BBS: get the no-overlap infill expolygons
+        {
+            ExPolygons polyWithoutOverlap;
+            polyWithoutOverlap = offset2_ex(
                 not_filled_exp,
                 float(-min_perimeter_infill_spacing / 2.),
-                float(inset + min_perimeter_infill_spacing / 2.));
-            // append infill areas to fill_surfaces
-            if (!top_expolygons.empty()) {
-                infill_exp = union_ex(infill_exp, offset_ex(top_expolygons, double(top_inset)));
-            }
-            result.infill = std::move(infill_exp);
-
-            // BBS: get the no-overlap infill expolygons
-            {
-                ExPolygons polyWithoutOverlap;
-                polyWithoutOverlap = offset2_ex(
-                    not_filled_exp,
-                    float(-min_perimeter_infill_spacing / 2.),
-                    float(+min_perimeter_infill_spacing / 2.));
-                if (!top_expolygons.empty())
-                    polyWithoutOverlap = union_ex(polyWithoutOverlap, top_expolygons);
-                result.no_overlap = std::move(polyWithoutOverlap);
-            }
-      }
-    });
-    for (ArachneSurfaceResult &result : results) {
-        if (result.has_loops)
-            this->loops->append(result.loops);
-        this->fill_surfaces->append(result.infill, stInternal);
-        apply_extra_perimeters(result.infill);
-        this->fill_no_overlap->insert(this->fill_no_overlap->end(), result.no_overlap.begin(), result.no_overlap.end());
+                float(+min_perimeter_infill_spacing / 2.));
+            if (!top_expolygons.empty())
+                polyWithoutOverlap = union_ex(polyWithoutOverlap, top_expolygons);
+            this->fill_no_overlap->insert(this->fill_no_overlap->end(), polyWithoutOverlap.begin(), polyWithoutOverlap.end());
+        }
     }
 }
 

src/libslic3r/Polygon.hpp

@@ -27,7 +27,7 @@ public:
     explicit Polygon(const Points &points) : MultiPoint(points) {}
 	Polygon(std::initializer_list<Point> points) : MultiPoint(points) {}
     Polygon(const Polygon &other) : MultiPoint(other.points) {}
-    Polygon(Polygon &&other) noexcept : MultiPoint(std::move(other.points)) {}
+    Polygon(Polygon &&other) : MultiPoint(std::move(other.points)) {}
 	static Polygon new_scale(const std::vector<Vec2d> &points) { 
         Polygon pgn;
         pgn.points.reserve(points.size());
@@ -36,7 +36,7 @@ public:
 		return pgn;
 	}
     Polygon& operator=(const Polygon &other) { points = other.points; return *this; }
-    Polygon& operator=(Polygon &&other) noexcept { points = std::move(other.points); return *this; }
+    Polygon& operator=(Polygon &&other) { points = std::move(other.points); return *this; }
 
     Point& operator[](Points::size_type idx) { return this->points[idx]; }
     const Point& operator[](Points::size_type idx) const { return this->points[idx]; }

src/libslic3r/Polyline.hpp

@@ -20,7 +20,7 @@ class Polyline : public MultiPoint {
 public:
     Polyline() {};
     Polyline(const Polyline& other) : MultiPoint(other.points), fitting_result(other.fitting_result) {}
-    Polyline(Polyline &&other) noexcept : MultiPoint(std::move(other.points)), fitting_result(std::move(other.fitting_result))  {}
+    Polyline(Polyline &&other) : MultiPoint(std::move(other.points)), fitting_result(std::move(other.fitting_result))  {}
     Polyline(std::initializer_list<Point> list) : MultiPoint(list) { 
         fitting_result.clear();
     }
@@ -41,7 +41,7 @@ public:
         fitting_result = other.fitting_result;
         return *this;
     }
-    Polyline& operator=(Polyline&& other) noexcept {
+    Polyline& operator=(Polyline&& other) {
         points = std::move(other.points);
         fitting_result = std::move(other.fitting_result);
         return *this;

src/libslic3r/PrintObject.cpp

@@ -30,7 +30,6 @@
 
 #include <cstddef>
 #include <float.h>
-#include <array>
 #include <iterator>
 #include <mutex>
 #include <string>
@@ -43,7 +42,6 @@
 #include <boost/log/trivial.hpp>
 
 #include <tbb/parallel_for.h>
-#include <tbb/parallel_invoke.h>
 #include <tbb/spin_mutex.h>
 #include <tbb/concurrent_unordered_set.h>
 
@@ -1666,9 +1664,7 @@ void PrintObject::detect_surfaces_type()
     bool interface_shells = ! spiral_mode && m_config.interface_shells.value;
     size_t num_layers     = spiral_mode ? std::min(size_t(this->printing_region(0).config().bottom_shell_layers), m_layers.size()) : m_layers.size();
 
-    // The regions of a layer do not see each other here, and a layer cut through a fine relief takes far longer than the
-    // others, so the regions run next to each other instead of one after another, each still over all layers.
-    tbb::parallel_for(size_t(0), this->num_printing_regions(), [&](size_t region_id) {
+    for (size_t region_id = 0; region_id < this->num_printing_regions(); ++ region_id) {
         BOOST_LOG_TRIVIAL(debug) << "Detecting solid surfaces for region " << region_id << " in parallel - start";
 #ifdef SLIC3R_DEBUG_SLICE_PROCESSING
         for (Layer *layer : m_layers)
@@ -1726,7 +1722,7 @@ void PrintObject::detect_surfaces_type()
                     if (upper_layer) {
                         ExPolygons upper_slices = interface_shells ?
                             diff_ex(layerm_slices_surfaces, upper_layer->m_regions[region_id]->slices.surfaces, ApplySafetyOffset::Yes) :
-                            diff_ex_by_piece(layerm_slices_surfaces, to_polygons(upper_layer->lslices), ApplySafetyOffset::Yes);
+                            diff_ex(layerm_slices_surfaces, upper_layer->lslices, ApplySafetyOffset::Yes);
                         surfaces_append(top, opening_ex(upper_slices, offset), stTop);
                     } else {
                         // if no upper layer, all surfaces of this one are solid
@@ -1752,7 +1748,7 @@ void PrintObject::detect_surfaces_type()
                         surfaces_append(
                             bottom,
                             opening_ex(
-                                diff_ex_by_piece(layerm_slices_surfaces, to_polygons(lower_layer->lslices), ApplySafetyOffset::Yes),
+                                diff_ex(layerm_slices_surfaces, lower_layer->lslices, ApplySafetyOffset::Yes),
                                 offset),
                             surface_type_bottom_other);
                         // if user requested internal shells, we need to identify surfaces
@@ -1783,44 +1779,34 @@ void PrintObject::detect_surfaces_type()
                     // and top surfaces; let's do an intersection to discover them and consider them
                     // as bottom surfaces (to allow for bridge detection)
                     if (! top.empty() && ! bottom.empty()) {
-                        const auto cracks = intersection_ex_by_piece(to_expolygons(top), to_polygons(bottom));
+                        const auto cracks = intersection_ex(top, bottom);
                         if (!cracks.empty()) {
                             if (lower_layer) { // Only detect small cracks for non-first layer, because first layer should always be bottom
                                 const float small_crack_threshold = -layerm->flow(frExternalPerimeter).scaled_width() * 1.5;
                                 
-                                // Only the bottom surfaces near a crack can take part: one that contains it must contain its box,
-                                // and one whose box misses the grown crack is left unchanged by removing it. A layer cut through
-                                // a fine relief has thousands of both, which made this loop quadratic.
                                 for (const auto& crack : cracks) {
                                     if (offset_ex(crack, small_crack_threshold).empty()) {
                                         // For small cracks, if it's part of a large bottom surface, then it should be added to bottom as well
-                                        const BoundingBox crack_bbox = get_extents(crack);
-                                        if (std::any_of(bottom.begin(), bottom.end(), [&crack, &crack_bbox, small_crack_threshold](const Surface& s) {
+                                        if (std::any_of(bottom.begin(), bottom.end(), [&crack, small_crack_threshold](const Surface& s) {
                                                 const auto& se = s.expolygon;
-                                                return get_extents(se).inflated(SCALED_EPSILON).contains(crack_bbox)
-                                                    && diff_ex(crack, se, ApplySafetyOffset::Yes).empty()
+                                                return diff_ex(crack, se, ApplySafetyOffset::Yes).empty()
                                                     && se.area() > crack.area() * 2
                                                     && !offset_ex(diff_ex(se, crack), small_crack_threshold).empty();
                                         })) continue;
 
                                         // Crack too small, leave it as part of the top surface, remove it from bottom surfaces
-                                        const ExPolygons  grown_crack = offset_ex(crack, -small_crack_threshold);
-                                        const BoundingBox grown_bbox  = get_extents(grown_crack);
                                         Surfaces bot_tmp;
                                         for (auto& b : bottom) {
-                                            if (get_extents(b.expolygon).overlap(grown_bbox))
-                                                surfaces_append(bot_tmp, diff_ex(b.expolygon, grown_crack), b.surface_type);
-                                            else
-                                                bot_tmp.emplace_back(std::move(b));
+                                            surfaces_append(bot_tmp, diff_ex(b.expolygon, offset_ex(crack, -small_crack_threshold)), b.surface_type);
                                         }
                                         bottom = std::move(bot_tmp);
                                     }
                                 }
                             }
 
-                            ExPolygons top_expolygons = to_expolygons(std::move(top));
+                            Polygons top_polygons = to_polygons(std::move(top));
                             top.clear();
-                            surfaces_append(top, diff_ex_by_piece(top_expolygons, to_polygons(bottom)), stTop);
+                            surfaces_append(top, diff_ex(top_polygons, bottom), stTop);
                         }
                     }
 
@@ -1911,7 +1897,7 @@ void PrintObject::detect_surfaces_type()
                     {
                         Polygons topbottom = to_polygons(top);
                         polygons_append(topbottom, to_polygons(bottom));
-                        surfaces_append(surfaces_out, diff_ex_by_piece(surfaces_prev_expolys, topbottom), stInternal);
+                        surfaces_append(surfaces_out, diff_ex(surfaces_prev_expolys, topbottom), stInternal);
                     }
 
                     surfaces_append(surfaces_out, std::move(top));
@@ -2088,31 +2074,29 @@ void PrintObject::detect_surfaces_type()
                 }
             }
             );
+            // ==============================================================================================================
+            // === ORCA: Interim workaround - for now the new stInternalAfterExternalBridge surfaace is re-classified  ==============
+            // === back to a bottom bridge. As a starting point, this improves bridging reliability as it extrudes ==========
+            // === two external bridge layers. However, TODO: Implement a new surface type throughout the codebase ==========
+            // ==============================================================================================================
+            for (size_t region_id = 0; region_id < this->num_printing_regions(); ++region_id) {
+                tbb::parallel_for( tbb::blocked_range<size_t>(0, m_layers.size()), [this, region_id](const tbb::blocked_range<size_t> &range) {
+                    for (size_t idx_layer = range.begin(); idx_layer < range.end(); ++idx_layer) {
+                        Surfaces &surfs = m_layers[idx_layer]->m_regions[region_id]->slices.surfaces;
+                        for (Surface &s : surfs) {
+                            if (s.surface_type == stInternalAfterExternalBridge) {
+                                s.surface_type = stBottomBridge;
+                            }
+                        }
+                    }
+                }
+              );
+            }
         }
         // ==============================================================================================================
         // === ORCA: End of second external bridge layer changes  =======================================================
         // ==============================================================================================================
         
-    }); // for each this->print->region_count
-
-    // ==============================================================================================================
-    // === ORCA: Interim workaround - for now the new stInternalAfterExternalBridge surfaace is re-classified  ==============
-    // === back to a bottom bridge. As a starting point, this improves bridging reliability as it extrudes ==========
-    // === two external bridge layers. However, TODO: Implement a new surface type throughout the codebase ==========
-    // ==============================================================================================================
-    // Once all the regions have their second bridge layer, and before their slices are trimmed into fill surfaces below.
-    if ((this->config().enable_extra_bridge_layer.value == eblApplyToAll) || (this->config().enable_extra_bridge_layer.value == eblExternalBridgeOnly)) {
-        tbb::parallel_for(tbb::blocked_range<size_t>(0, m_layers.size()), [this](const tbb::blocked_range<size_t> &range) {
-            for (size_t idx_layer = range.begin(); idx_layer < range.end(); ++idx_layer)
-                for (LayerRegion *layerm : m_layers[idx_layer]->regions())
-                    for (Surface &s : layerm->slices.surfaces)
-                        if (s.surface_type == stInternalAfterExternalBridge)
-                            s.surface_type = stBottomBridge;
-        });
-        m_print->throw_if_canceled();
-    }
-
-    tbb::parallel_for(size_t(0), this->num_printing_regions(), [&](size_t region_id) {
         BOOST_LOG_TRIVIAL(debug) << "Detecting solid surfaces for region " << region_id << " - clipping in parallel - start";
         // Fill in layerm->fill_surfaces by trimming the layerm->slices by the cummulative layerm->fill_surfaces.
         tbb::parallel_for(
@@ -2129,7 +2113,7 @@ void PrintObject::detect_surfaces_type()
             });
         m_print->throw_if_canceled();
         BOOST_LOG_TRIVIAL(debug) << "Detecting solid surfaces for region " << region_id << " - clipping in parallel - end";
-    });
+    } // for each this->print->region_count
 
     // Mark the object to have the region slices classified (typed, which also means they are split based on whether they are supported, bridging, top layers etc.)
     m_typed_slices = true;
@@ -2196,10 +2180,8 @@ void PrintObject::process_external_surfaces()
 	    BOOST_LOG_TRIVIAL(debug) << "Collecting surfaces covered with extrusions in parallel - end";
 	}
 
-    BOOST_LOG_TRIVIAL(debug) << "Processing external surfaces in parallel - start";
-    // The regions of a layer do not see each other here, and a layer cut through a fine relief takes far longer than the
-    // others, so the regions run next to each other instead of one after another, each still over all layers.
-    tbb::parallel_for(size_t(0), this->num_printing_regions(), [this, &surfaces_covered](size_t region_id) {
+	for (size_t region_id = 0; region_id < this->num_printing_regions(); ++region_id) {
+        BOOST_LOG_TRIVIAL(debug) << "Processing external surfaces for region " << region_id << " in parallel - start";
         tbb::parallel_for(
             tbb::blocked_range<size_t>(0, m_layers.size()),
             [this, &surfaces_covered, region_id](const tbb::blocked_range<size_t>& range) {
@@ -2214,9 +2196,9 @@ void PrintObject::process_external_surfaces()
                 }
             }
         );
-    });
-    m_print->throw_if_canceled();
-    BOOST_LOG_TRIVIAL(debug) << "Processing external surfaces in parallel - end";
+        m_print->throw_if_canceled();
+        BOOST_LOG_TRIVIAL(debug) << "Processing external surfaces for region " << region_id << " in parallel - end";
+    }
 }
 
 void PrintObject::discover_vertical_shells()
@@ -2255,10 +2237,10 @@ void PrintObject::discover_vertical_shells()
             // The "ensure vertical wall thickness" feature is not applicable to any of the regions. Quit.
             return;
         BOOST_LOG_TRIVIAL(debug) << "Discovering vertical shells in parallel - start : cache top / bottom";
-        // One layer per task: on a layer cut through a fine relief the unions below take far longer than elsewhere, and a
-        // few such layers next to each other must not end up in one task.
+        //FIXME Improve the heuristics for a grain size.
+        size_t grain_size = std::max(num_layers / 16, size_t(1));
         tbb::parallel_for(
-            tbb::blocked_range<size_t>(0, num_layers, 1),
+            tbb::blocked_range<size_t>(0, num_layers, grain_size),
             [this, &cache_top_botom_regions](const tbb::blocked_range<size_t>& range) {
                 const std::initializer_list<SurfaceType> surfaces_bottom { stBottom, stBottomBridge };
                 const size_t num_regions = this->num_printing_regions();
@@ -2266,198 +2248,67 @@ void PrintObject::discover_vertical_shells()
                     m_print->throw_if_canceled();
                     const Layer                      &layer = *m_layers[idx_layer];
                     DiscoverVerticalShellsCacheEntry &cache = cache_top_botom_regions[idx_layer];
-                    const auto top_bottom_expansion = [&layer](size_t region_id) {
-                        return float(layer.m_regions[region_id]->flow(frSolidInfill).scaled_spacing()) * top_bottom_expansion_coeff;
-                    };
+                    // Simulate single set of perimeters over all merged regions.
+                    float                             perimeter_offset = 0.f;
+                    float                             perimeter_min_spacing = FLT_MAX;
 #ifdef SLIC3R_DEBUG_SLICE_PROCESSING
                     static size_t debug_idx = 0;
                     ++ debug_idx;
 #endif /* SLIC3R_DEBUG_SLICE_PROCESSING */
-                    // The top surfaces, the bottom surfaces and the holes are independent of each other.
-                    tbb::parallel_invoke(
-                        [&]() {
-                            for (size_t region_id = 0; region_id < num_regions; ++ region_id)
-                                append(cache.top_surfaces, offset(layer.m_regions[region_id]->slices.filter_by_type(stTop), top_bottom_expansion(region_id)));
-//                              append(cache.top_surfaces, offset(layerm.fill_surfaces.filter_by_type(stTop), top_bottom_expansion));
-                            // Save some computing time by reducing the number of polygons.
-                            cache.top_surfaces = union_(cache.top_surfaces);
-                        },
-                        [&]() {
-                            for (size_t region_id = 0; region_id < num_regions; ++ region_id)
-                                append(cache.bottom_surfaces, offset(layer.m_regions[region_id]->slices.filter_by_types(surfaces_bottom), top_bottom_expansion(region_id)));
-//                              append(cache.bottom_surfaces, offset(layerm.fill_surfaces.filter_by_types(surfaces_bottom), top_bottom_expansion));
-                            cache.bottom_surfaces = union_(cache.bottom_surfaces);
-                        },
-                        [&]() {
-                            // Simulate single set of perimeters over all merged regions.
-                            float perimeter_offset      = 0.f;
-                            float perimeter_min_spacing = FLT_MAX;
-                            for (size_t region_id = 0; region_id < num_regions; ++ region_id) {
-                                const LayerRegion &layerm = *layer.m_regions[region_id];
-                                // Calculate the maximum perimeter offset as if the slice was extruded with a single extruder only.
-                                // First find the maxium number of perimeters per region slice.
-                                unsigned int perimeters = 0;
-                                for (const Surface &s : layerm.slices.surfaces)
-                                    perimeters = std::max<unsigned int>(perimeters, s.extra_perimeters);
-                                perimeters += layerm.region().config().wall_loops.value;
-                                // Then calculate the infill offset.
-                                if (perimeters > 0) {
-                                    Flow extflow = layerm.flow(frExternalPerimeter);
-                                    Flow flow    = layerm.flow(frPerimeter);
-                                    perimeter_offset = std::max(perimeter_offset,
-                                        0.5f * float(extflow.scaled_width() + extflow.scaled_spacing()) + (float(perimeters) - 1.f) * flow.scaled_spacing());
-                                    perimeter_min_spacing = std::min(perimeter_min_spacing, float(std::min(extflow.scaled_spacing(), flow.scaled_spacing())));
-                                }
-                                polygons_append(cache.holes, to_polygons(layerm.fill_expolygons));
-                            }
-                            // For a multi-material print, simulate perimeter / infill split as if only a single extruder has been used for the whole print.
-                            if (perimeter_offset > 0.) {
-                                // The layer.lslices are forced to merge by expanding them first.
-                                polygons_append(cache.holes, offset2(layer.lslices, 0.3f * perimeter_min_spacing, - perimeter_offset - 0.3f * perimeter_min_spacing));
+                    for (size_t region_id = 0; region_id < num_regions; ++ region_id) {
+                        LayerRegion &layerm               = *layer.m_regions[region_id];
+                        float        top_bottom_expansion = float(layerm.flow(frSolidInfill).scaled_spacing()) * top_bottom_expansion_coeff;
+                        // Top surfaces.
+                        append(cache.top_surfaces, offset(layerm.slices.filter_by_type(stTop), top_bottom_expansion));
+//                        append(cache.top_surfaces, offset(layerm.fill_surfaces.filter_by_type(stTop), top_bottom_expansion));
+                        // Bottom surfaces.
+                        append(cache.bottom_surfaces, offset(layerm.slices.filter_by_types(surfaces_bottom), top_bottom_expansion));
+//                        append(cache.bottom_surfaces, offset(layerm.fill_surfaces.filter_by_types(surfaces_bottom), top_bottom_expansion));
+                        // Calculate the maximum perimeter offset as if the slice was extruded with a single extruder only.
+                        // First find the maxium number of perimeters per region slice.
+                        unsigned int perimeters = 0;
+                        for (Surface &s : layerm.slices.surfaces)
+                            perimeters = std::max<unsigned int>(perimeters, s.extra_perimeters);
+                        perimeters += layerm.region().config().wall_loops.value;
+                        // Then calculate the infill offset.
+                        if (perimeters > 0) {
+                            Flow extflow = layerm.flow(frExternalPerimeter);
+                            Flow flow    = layerm.flow(frPerimeter);
+                            perimeter_offset = std::max(perimeter_offset,
+                                0.5f * float(extflow.scaled_width() + extflow.scaled_spacing()) + (float(perimeters) - 1.f) * flow.scaled_spacing());
+                            perimeter_min_spacing = std::min(perimeter_min_spacing, float(std::min(extflow.scaled_spacing(), flow.scaled_spacing())));
+                        }
+                        polygons_append(cache.holes, to_polygons(layerm.fill_expolygons));
+                    }
+                    // Save some computing time by reducing the number of polygons.
+                    cache.top_surfaces    = union_(cache.top_surfaces);
+                    cache.bottom_surfaces = union_(cache.bottom_surfaces);
+                    // For a multi-material print, simulate perimeter / infill split as if only a single extruder has been used for the whole print.
+                    if (perimeter_offset > 0.) {
+                        // The layer.lslices are forced to merge by expanding them first.
+                        polygons_append(cache.holes, offset2(layer.lslices, 0.3f * perimeter_min_spacing, - perimeter_offset - 0.3f * perimeter_min_spacing));
 #ifdef SLIC3R_DEBUG_SLICE_PROCESSING
-                                {
-                                    Slic3r::SVG svg(debug_out_path("discover_vertical_shells-extra-holes-%d.svg", debug_idx), get_extents(layer.lslices));
-                                    svg.draw(layer.lslices, "blue");
-                                    svg.draw(union_ex(cache.holes), "red");
-                                    svg.draw_outline(union_ex(cache.holes), "black", "blue", scale_(0.05));
-                                    svg.Close();
-                                }
+                        {
+                            Slic3r::SVG svg(debug_out_path("discover_vertical_shells-extra-holes-%d.svg", debug_idx), get_extents(layer.lslices));
+                            svg.draw(layer.lslices, "blue");
+                            svg.draw(union_ex(cache.holes), "red");
+                            svg.draw_outline(union_ex(cache.holes), "black", "blue", scale_(0.05));
+                            svg.Close();
+                        }
 #endif /* SLIC3R_DEBUG_SLICE_PROCESSING */
-                            }
-                            cache.holes = union_(cache.holes);
-                        });
+                    }
+                    cache.holes = union_(cache.holes);
                 }
             });
         m_print->throw_if_canceled();
         BOOST_LOG_TRIVIAL(debug) << "Discovering vertical shells in parallel - end : cache top / bottom";
     }
 
-    // With one top/bottom cache for all regions, the shell and hole accumulation in the loop below depends on nothing
-    // region-specific but the shell settings and the external perimeter spacing, so a region sharing them with an earlier
-    // one reuses its result instead of repeating it: that accumulation is a union over several layers of top/bottom
-    // surfaces, and a multi-material print has a region per filament.
-    using AccumulationKey = std::array<double, 5>;
-    struct ShellAccumulation
-    {
-        AccumulationKey key;
-        Polygons        shell;
-        Polygons        holes;
-    };
-    const auto accumulation_key = [](const PrintRegionConfig &region_config, const LayerRegion *layerm) {
-        return AccumulationKey{ double(region_config.top_shell_layers.value), region_config.top_shell_thickness.value,
-                                double(region_config.bottom_shell_layers.value), region_config.bottom_shell_thickness.value,
-                                double(layerm->flow(frExternalPerimeter).scaled_spacing()) };
-    };
-    const auto accumulate_shell = [this, &cache_top_botom_regions](size_t idx_layer, const PrintRegionConfig &region_config,
-                                                                   const LayerRegion *layerm, Polygons &shell, Polygons &holes) {
-        const Layer *layer = m_layers[idx_layer];
-    			        polygons_append(holes, cache_top_botom_regions[idx_layer].holes);
-                        auto combine_holes = [&holes](const Polygons &holes2) {
-                            if (holes.empty() || holes2.empty())
-                                holes.clear();
-                            else
-                                holes = intersection(holes, holes2);
-                        };
-                        auto combine_shells = [&shell](const Polygons &shells2) {
-                            if (shell.empty())
-                                shell = std::move(shells2);
-                            else if (! shells2.empty()) {
-                                polygons_append(shell, shells2);
-                                // Running the union_ using the Clipper library piece by piece is cheaper
-                                // than running the union_ all at once.
-                                shell = union_(shell);
-                            }
-                        };
-                        static constexpr const bool one_more_layer_below_top_bottom_surfaces = false;
-    			        if (int n_top_layers = region_config.top_shell_layers.value; n_top_layers > 0) {
-                            // Gather top regions projected to this layer.
-                            coordf_t print_z = layer->print_z;
-                            int i = int(idx_layer) + 1;
-                            int itop = int(idx_layer) + n_top_layers;
-                            bool at_least_one_top_projected = false;
-    	                    for (; i < int(cache_top_botom_regions.size()) &&
-    	                         (i < itop || m_layers[i]->print_z - print_z < region_config.top_shell_thickness - EPSILON);
-    	                        ++ i) {
-                                at_least_one_top_projected = true;
-    	                        const DiscoverVerticalShellsCacheEntry &cache = cache_top_botom_regions[i];
-                                combine_holes(cache.holes);
-                                combine_shells(cache.top_surfaces);
-    	                    }
-                            if (!at_least_one_top_projected && i < int(cache_top_botom_regions.size())) {
-                                // Lets consider this a special case - with only 1 top solid and minimal shell thickness settings, the
-                                // boundaries of solid layers are not anchored over/under perimeters, so lets fix it by adding at least one
-                                // perimeter width of area
-                                Polygons anchor_area = intersection(expand(cache_top_botom_regions[idx_layer].top_surfaces,
-                                                                           layerm->flow(frExternalPerimeter).scaled_spacing()),
-                                                                    to_polygons(m_layers[i]->lslices));
-                                combine_shells(anchor_area);
-                            }
-
-                            if (one_more_layer_below_top_bottom_surfaces)
-                                if (i < int(cache_top_botom_regions.size()) &&
-                                    (i <= itop || m_layers[i]->bottom_z() - print_z < region_config.top_shell_thickness - EPSILON))
-                                    combine_holes(cache_top_botom_regions[i].holes);
-    	                }
-    	                if (int n_bottom_layers = region_config.bottom_shell_layers.value; n_bottom_layers > 0) {
-                            // Gather bottom regions projected to this layer.
-                            coordf_t bottom_z = layer->bottom_z();
-                            int i = int(idx_layer) - 1;
-                            int ibottom = int(idx_layer) - n_bottom_layers;
-                            bool at_least_one_bottom_projected = false;
-    	                    for (; i >= 0 &&
-    	                         (i > ibottom || bottom_z - m_layers[i]->bottom_z() < region_config.bottom_shell_thickness - EPSILON);
-    	                        -- i) {
-                                    at_least_one_bottom_projected = true;
-    	                        const DiscoverVerticalShellsCacheEntry &cache = cache_top_botom_regions[i];
-    							combine_holes(cache.holes);
-                                combine_shells(cache.bottom_surfaces);
-    	                    }
-
-                            if (!at_least_one_bottom_projected && i >= 0) {
-                                Polygons anchor_area = intersection(expand(cache_top_botom_regions[idx_layer].bottom_surfaces,
-                                                                           layerm->flow(frExternalPerimeter).scaled_spacing()),
-                                                                    to_polygons(m_layers[i]->lslices));
-                                combine_shells(anchor_area);
-                            }
-
-                            if (one_more_layer_below_top_bottom_surfaces)
-                                if (i >= 0 &&
-                                    (i > ibottom || bottom_z - m_layers[i]->print_z < region_config.bottom_shell_thickness - EPSILON))
-                                    combine_holes(cache_top_botom_regions[i].holes);
-    	                }
-    };
-    std::vector<std::vector<ShellAccumulation>> shell_accumulations(top_bottom_surfaces_all_regions ? num_layers : 0);
-    if (! shell_accumulations.empty()) {
-        // Every (layer, key) pair is accumulated once, before the regions, so that nothing in the loop below is shared
-        // between them and they can run next to each other.
-        std::vector<std::array<size_t, 3>> todo; // layer, its slot, a region holding the key
-        for (size_t idx_layer = 0; idx_layer < num_layers; ++ idx_layer) {
-            std::vector<ShellAccumulation> &accumulations = shell_accumulations[idx_layer];
-            for (size_t region_id = 0; region_id < this->num_printing_regions(); ++ region_id) {
-                if (this->printing_region(region_id).config().ensure_vertical_shell_thickness.value != evstAll)
-                    continue;
-                const LayerRegion    *layerm = m_layers[idx_layer]->m_regions[region_id];
-                const AccumulationKey key    = accumulation_key(layerm->region().config(), layerm);
-                if (std::none_of(accumulations.begin(), accumulations.end(), [&key](const ShellAccumulation &a) { return a.key == key; })) {
-                    todo.push_back({ idx_layer, accumulations.size(), region_id });
-                    accumulations.push_back({ key, {}, {} });
-                }
-            }
-        }
-        tbb::parallel_for(size_t(0), todo.size(), [this, &todo, &shell_accumulations, &accumulate_shell](size_t i) {
-            m_print->throw_if_canceled();
-            const LayerRegion *layerm = m_layers[todo[i][0]]->m_regions[todo[i][2]];
-            ShellAccumulation &out    = shell_accumulations[todo[i][0]][todo[i][1]];
-            accumulate_shell(todo[i][0], layerm->region().config(), layerm, out.shell, out.holes);
-        });
-        m_print->throw_if_canceled();
-    }
-
-    const auto process_region = [&](size_t region_id) {
+    for (size_t region_id = 0; region_id < this->num_printing_regions(); ++ region_id) {
         const PrintRegion &region = this->printing_region(region_id);
         if (region.config().ensure_vertical_shell_thickness.value != evstAll )
             // This region will be handled by discover_horizontal_shells().
-            return;
+            continue;
 
         //FIXME Improve the heuristics for a grain size.
         size_t grain_size = std::max(num_layers / 16, size_t(1));
@@ -2497,7 +2348,7 @@ void PrintObject::discover_vertical_shells()
         grain_size = 1;
         tbb::parallel_for(
             tbb::blocked_range<size_t>(0, num_layers, grain_size),
-            [this, region_id, &shell_accumulations, &accumulation_key, &accumulate_shell]
+            [this, region_id, &cache_top_botom_regions]
             (const tbb::blocked_range<size_t>& range) {
                 // printf("discover_vertical_shells from %d to %d\n", range.begin(), range.end());
                 for (size_t idx_layer = range.begin(); idx_layer < range.end(); ++ idx_layer) {
@@ -2547,19 +2398,80 @@ void PrintObject::discover_vertical_shells()
                         }
                     }
 #endif /* SLIC3R_DEBUG_SLICE_PROCESSING */
-                    const AccumulationKey key = accumulation_key(region_config, layerm);
-                    const ShellAccumulation *reused = shell_accumulations.empty() ? nullptr :
-                        [&]() -> const ShellAccumulation * {
-                            for (const ShellAccumulation &a : shell_accumulations[idx_layer])
-                                if (a.key == key)
-                                    return &a;
-                            return nullptr;
-                        }();
-                    if (reused != nullptr) {
-                        shell = reused->shell;
-                        holes = reused->holes;
-                    } else
-                        accumulate_shell(idx_layer, region_config, layerm, shell, holes);
+			        polygons_append(holes, cache_top_botom_regions[idx_layer].holes);
+                    auto combine_holes = [&holes](const Polygons &holes2) {
+                        if (holes.empty() || holes2.empty())
+                            holes.clear();
+                        else
+                            holes = intersection(holes, holes2);
+                    };
+                    auto combine_shells = [&shell](const Polygons &shells2) {
+                        if (shell.empty())
+                            shell = std::move(shells2);
+                        else if (! shells2.empty()) {
+                            polygons_append(shell, shells2);
+                            // Running the union_ using the Clipper library piece by piece is cheaper
+                            // than running the union_ all at once.
+                            shell = union_(shell);
+                        }
+                    };
+                    static constexpr const bool one_more_layer_below_top_bottom_surfaces = false;
+			        if (int n_top_layers = region_config.top_shell_layers.value; n_top_layers > 0) {
+                        // Gather top regions projected to this layer.
+                        coordf_t print_z = layer->print_z;
+                        int i = int(idx_layer) + 1;
+                        int itop = int(idx_layer) + n_top_layers;
+                        bool at_least_one_top_projected = false;
+	                    for (; i < int(cache_top_botom_regions.size()) &&
+	                         (i < itop || m_layers[i]->print_z - print_z < region_config.top_shell_thickness - EPSILON);
+	                        ++ i) {
+                            at_least_one_top_projected = true;
+	                        const DiscoverVerticalShellsCacheEntry &cache = cache_top_botom_regions[i];
+                            combine_holes(cache.holes);
+                            combine_shells(cache.top_surfaces);
+	                    }
+                        if (!at_least_one_top_projected && i < int(cache_top_botom_regions.size())) {
+                            // Lets consider this a special case - with only 1 top solid and minimal shell thickness settings, the
+                            // boundaries of solid layers are not anchored over/under perimeters, so lets fix it by adding at least one
+                            // perimeter width of area
+                            Polygons anchor_area = intersection(expand(cache_top_botom_regions[idx_layer].top_surfaces,
+                                                                       layerm->flow(frExternalPerimeter).scaled_spacing()),
+                                                                to_polygons(m_layers[i]->lslices));
+                            combine_shells(anchor_area);
+                        }
+
+                        if (one_more_layer_below_top_bottom_surfaces)
+                            if (i < int(cache_top_botom_regions.size()) &&
+                                (i <= itop || m_layers[i]->bottom_z() - print_z < region_config.top_shell_thickness - EPSILON))
+                                combine_holes(cache_top_botom_regions[i].holes);
+	                }
+	                if (int n_bottom_layers = region_config.bottom_shell_layers.value; n_bottom_layers > 0) {
+                        // Gather bottom regions projected to this layer.
+                        coordf_t bottom_z = layer->bottom_z();
+                        int i = int(idx_layer) - 1;
+                        int ibottom = int(idx_layer) - n_bottom_layers;
+                        bool at_least_one_bottom_projected = false;
+	                    for (; i >= 0 &&
+	                         (i > ibottom || bottom_z - m_layers[i]->bottom_z() < region_config.bottom_shell_thickness - EPSILON);
+	                        -- i) {
+                                at_least_one_bottom_projected = true;
+	                        const DiscoverVerticalShellsCacheEntry &cache = cache_top_botom_regions[i];
+							combine_holes(cache.holes);
+                            combine_shells(cache.bottom_surfaces);
+	                    }
+
+                        if (!at_least_one_bottom_projected && i >= 0) {
+                            Polygons anchor_area = intersection(expand(cache_top_botom_regions[idx_layer].bottom_surfaces,
+                                                                       layerm->flow(frExternalPerimeter).scaled_spacing()),
+                                                                to_polygons(m_layers[i]->lslices));
+                            combine_shells(anchor_area);
+                        }
+
+                        if (one_more_layer_below_top_bottom_surfaces)
+                            if (i >= 0 &&
+                                (i > ibottom || bottom_z - m_layers[i]->print_z < region_config.bottom_shell_thickness - EPSILON))
+                                combine_holes(cache_top_botom_regions[i].holes);
+	                }
 #ifdef SLIC3R_DEBUG_SLICE_PROCESSING
                     {
         				Slic3r::SVG svg(debug_out_path("discover_vertical_shells-perimeters-before-union-%d.svg", debug_idx), get_extents(shell));
@@ -2653,8 +2565,11 @@ void PrintObject::discover_vertical_shells()
                         Polygons object_volume;
                         Polygons internal_volume;
                         {
-                            if (idx_layer > 0 && idx_layer + 1 < m_layers.size())
-                                object_volume = to_polygons(intersection_ex_by_piece(m_layers[idx_layer - 1]->lslices, to_polygons(m_layers[idx_layer + 1]->lslices)));
+                            Polygons shrinked_bottom_slice = idx_layer > 0 ? to_polygons(m_layers[idx_layer - 1]->lslices) : Polygons{};
+                            Polygons shrinked_upper_slice  = (idx_layer + 1) < m_layers.size() ?
+                                                                 to_polygons(m_layers[idx_layer + 1]->lslices) :
+                                                                 Polygons{};
+                            object_volume = intersection(shrinked_bottom_slice, shrinked_upper_slice);
                             internal_volume = closing(polygonsInternal, SCALED_EPSILON);
                         }
 
@@ -2665,34 +2580,15 @@ void PrintObject::discover_vertical_shells()
                         //      the in-model condition is there due to small sloping surfaces, e.g. top of the hull of the benchy
                         //   2. the area does not fully cover an internal polygon
                         //         This is there mainly for a very thin parts, where the solid layers would be missing if the part area is quite small
-                        // Both tests below compare a small piece against the whole layer. Done literally, that is
-                        // quadratic in the number of pieces, which is what a layer split up by colour painting has,
-                        // so each is restricted to the part of the layer near the piece with an identical result:
-                        // object_volume is clipped to the piece's box, and only the internal polygons whose box meets
-                        // the expanded piece take part in the count, since the others pass through the difference
-                        // unchanged and add the same number to both sides of it.
-                        std::vector<BoundingBox> internal_bboxes;
-                        internal_bboxes.reserve(internal_volume.size());
-                        for (const Polygon &poly : internal_volume)
-                            internal_bboxes.emplace_back(get_extents(poly));
                         regularized_shell.erase(std::remove_if(regularized_shell.begin(), regularized_shell.end(),
-                                                               [&internal_volume, &internal_bboxes, &min_perimeter_infill_spacing,
+                                                               [&internal_volume, &min_perimeter_infill_spacing,
                                                                 &object_volume](const ExPolygon &p) {
-                                                                   const bool small = p.area() < min_perimeter_infill_spacing * scaled(1.5) ||
-                                                                                      (p.area() < min_perimeter_infill_spacing * scaled(8.0) &&
-                                                                                       diff(to_polygons(p),
-                                                                                            ClipperUtils::clip_clipper_polygons_with_subject_bbox(
-                                                                                                object_volume, get_extents(p).inflated(SCALED_EPSILON)))
-                                                                                           .empty());
-                                                                   if (!small)
-                                                                       return false;
-                                                                   const Polygons    expanded = expand(to_polygons(p), min_perimeter_infill_spacing);
-                                                                   const BoundingBox bbox     = get_extents(expanded);
-                                                                   Polygons          nearby;
-                                                                   for (size_t i = 0; i < internal_volume.size(); ++i)
-                                                                       if (internal_bboxes[i].overlap(bbox))
-                                                                           nearby.emplace_back(internal_volume[i]);
-                                                                   return diff(nearby, expanded).size() >= nearby.size();
+                                                                   return (p.area() < min_perimeter_infill_spacing * scaled(1.5) ||
+                                                                           (p.area() < min_perimeter_infill_spacing * scaled(8.0) &&
+                                                                            diff(to_polygons(p), object_volume).empty())) &&
+                                                                          diff(internal_volume,
+                                                                               expand(to_polygons(p), min_perimeter_infill_spacing))
+                                                                                  .size() >= internal_volume.size();
                                                                }),
                                                 regularized_shell.end());
                     }
@@ -2714,9 +2610,8 @@ void PrintObject::discover_vertical_shells()
 #endif /* SLIC3R_DEBUG_SLICE_PROCESSING */
 
                     // Trim the internal & internalvoid by the shell.
-                    const Polygons     regularized_shell_polygons = to_polygons(regularized_shell);
-                    Slic3r::ExPolygons new_internal = diff_ex_by_piece(to_expolygons(layerm->fill_surfaces.filter_by_type(stInternal)), regularized_shell_polygons);
-                    Slic3r::ExPolygons new_internal_void = diff_ex_by_piece(to_expolygons(layerm->fill_surfaces.filter_by_type(stInternalVoid)), regularized_shell_polygons);
+                    Slic3r::ExPolygons new_internal = diff_ex(layerm->fill_surfaces.filter_by_type(stInternal), regularized_shell);
+                    Slic3r::ExPolygons new_internal_void = diff_ex(layerm->fill_surfaces.filter_by_type(stInternalVoid), regularized_shell);
 
 #ifdef SLIC3R_DEBUG_SLICE_PROCESSING
                     {
@@ -2743,15 +2638,7 @@ void PrintObject::discover_vertical_shells()
 			layerm->export_region_fill_surfaces_to_svg_debug("3_discover_vertical_shells-final");
 		}
 #endif /* SLIC3R_DEBUG_SLICE_PROCESSING */
-    }; // for each region
-    if (top_bottom_surfaces_all_regions)
-        // Nothing is shared between the regions, and a layer cut through a fine relief takes far longer than the others,
-        // so they run next to each other instead of one after another.
-        tbb::parallel_for(size_t(0), this->num_printing_regions(), process_region);
-    else
-        // Here every region fills the one top/bottom cache with its own surfaces first.
-        for (size_t region_id = 0; region_id < this->num_printing_regions(); ++ region_id)
-            process_region(region_id);
+    } // for each region
 } // void PrintObject::discover_vertical_shells()
 
 // #define DEBUG_BRIDGE_OVER_INFILL
@@ -3272,16 +3159,6 @@ void PrintObject::bridge_over_infill()
                 vertical_lines[i].b = Point{x, y_max};
             }
 
-            // The vertical lines only span the bridged area's x range, so anchors entirely outside it can never be
-            // hit. Leaving them out gives the same intersections without building a tree over the whole layer's
-            // boundary for every bridge.
-            const coord_t scan_x_min = bb_x.min.x();
-            const coord_t scan_x_max = bb_x.min.x() + coord_t(n_vlines) * scan_spacing;
-            anchors.erase(std::remove_if(anchors.begin(), anchors.end(),
-                                         [scan_x_min, scan_x_max](const Line &l) {
-                                             return std::max(l.a.x(), l.b.x()) < scan_x_min || std::min(l.a.x(), l.b.x()) > scan_x_max;
-                                         }),
-                          anchors.end());
             auto anchors_and_walls_tree = AABBTreeLines::LinesDistancer<Line>{std::move(anchors)};
             auto bridged_area_tree      = AABBTreeLines::LinesDistancer<Line>{to_lines(bridged_area)};
 
@@ -3526,63 +3403,28 @@ void PrintObject::bridge_over_infill()
 
                 std::vector<CandidateSurface> expanded_surfaces;
                 expanded_surfaces.reserve(surfaces_by_layer[lidx].size());
-                // The expanded fill boundary depends only on the bridging flow, and total_fill_area is not
-                // modified below, so build it once per spacing rather than once per candidate. A layer split
-                // into many candidates (e.g. by colour painting) otherwise repeats a layer-wide offset for each.
-                std::map<coord_t, Polylines> boundary_by_spacing;
-                // expansion_area is a clean, non-overlapping set, so uniting it with a bridge or cutting a bridge
-                // out of it only changes the polygons near that bridge. The rest are passed through untouched
-                // instead of being fed to ClipperLib with the whole layer again for every candidate.
-                // Not `near`/`far`: the Windows headers still define those as macros, and they expand to
-                // nothing, which turns the declaration below into an empty one.
-                const auto split_near = [](const Polygons &polys, const BoundingBox &bbox, Polygons &rest) {
-                    Polygons nearby;
-                    for (const Polygon &p : polys)
-                        (get_extents(p).overlap(bbox) ? nearby : rest).emplace_back(p);
-                    return nearby;
-                };
                 for (const CandidateSurface &candidate : surfaces_by_layer[lidx]) {
                     const auto &region_config = candidate.region->region().config();
                     const bool turning_pattern = region_config.sparse_infill_pattern == ipHilbertCurve ||
                                                  region_config.sparse_infill_pattern == ipOctagramSpiral;
                     const Flow &flow              = candidate.region->bridging_flow(frSolidInfill, true);
                     Polygons    area_to_be_bridge = expand(candidate.new_polys, flow.scaled_spacing());
-                    // deep_infill_area and internal_unsupported_area cover the whole layer; only their part under
-                    // this candidate can change the results, so they are clipped to its box first.
-                    if (!area_to_be_bridge.empty())
-                        area_to_be_bridge = intersection(area_to_be_bridge,
-                                                         ClipperUtils::clip_clipper_polygons_with_subject_bbox(
-                                                             deep_infill_area, get_extents(area_to_be_bridge).inflated(SCALED_EPSILON)));
+                    area_to_be_bridge             = intersection(area_to_be_bridge, deep_infill_area);
 
                     area_to_be_bridge.erase(std::remove_if(area_to_be_bridge.begin(), area_to_be_bridge.end(),
-                                                           [&internal_unsupported_area](const Polygon &p) {
-                                                               return intersection({p}, ClipperUtils::clip_clipper_polygons_with_subject_bbox(
-                                                                                            internal_unsupported_area,
-                                                                                            get_extents(p).inflated(SCALED_EPSILON)))
-                                                                   .empty();
+                                                           [internal_unsupported_area](const Polygon &p) {
+                                                               return intersection({p}, internal_unsupported_area).empty();
                                                            }),
                                             area_to_be_bridge.end());
 
+                    Polygons limiting_area = union_(area_to_be_bridge, expansion_area);
+
                     if (area_to_be_bridge.empty())
                         continue;
 
-                    Polygons       limiting_area;
-                    const Polygons near_expansion = split_near(expansion_area, get_extents(area_to_be_bridge).inflated(SCALED_EPSILON),
-                                                               limiting_area);
-                    append(limiting_area, union_(area_to_be_bridge, near_expansion));
-
-                    auto boundary_it = boundary_by_spacing.find(flow.scaled_spacing());
-                    if (boundary_it == boundary_by_spacing.end())
-                        boundary_it = boundary_by_spacing
-                                          .emplace(flow.scaled_spacing(), to_polylines(expand(total_fill_area, 1.3 * flow.scaled_spacing())))
-                                          .first;
-                    Polylines boundary_plines = boundary_it->second;
+                    Polylines boundary_plines = to_polylines(expand(total_fill_area, 1.3 * flow.scaled_spacing()));
                     {
-                        // No offset here: flow.spacing() is in mm, so the expand(limiting_area, 0.3 * flow.spacing())
-                        // this used to be moved the outline by 0.135 scaled units - nothing beyond rounding - while
-                        // costing a whole-layer ClipperLib pass for every candidate. limiting_area is already a clean
-                        // union, so its own outline is the same boundary.
-                        Polylines limiting_plines = to_polylines(limiting_area);
+                        Polylines limiting_plines = to_polylines(expand(limiting_area, 0.3*flow.spacing()));
                         boundary_plines.insert(boundary_plines.end(), limiting_plines.begin(), limiting_plines.end());
                     }
 
@@ -3656,12 +3498,9 @@ void PrintObject::bridge_over_infill()
                     // Check collision with other expanded surfaces
                     {
                         bool     reconstruct       = false;
-                        Polygons          tmp_expanded_area = expand(bridging_area, 3.0 * flow.scaled_spacing());
-                        const BoundingBox tmp_expanded_bbox = get_extents(tmp_expanded_area);
+                        Polygons tmp_expanded_area = expand(bridging_area, 3.0 * flow.scaled_spacing());
                         for (const CandidateSurface &s : expanded_surfaces) {
-                            // Surfaces whose boxes miss each other cannot intersect, which is most pairs on a busy layer.
-                            if (get_extents(s.new_polys).overlap(tmp_expanded_bbox) &&
-                                !intersection(s.new_polys, tmp_expanded_area).empty()) {
+                            if (!intersection(s.new_polys, tmp_expanded_area).empty()) {
                                 bridging_angle = s.bridge_angle;
                                 reconstruct    = true;
                                 break;
@@ -3685,20 +3524,10 @@ void PrintObject::bridge_over_infill()
                         bridging_area = union_(bridging_area, construct_anchored_polygon(bridging_area, to_lines(boundary_plines), flow,
                                                                                        bridging_angle, scan_spacing, true));
                     }
-                    // Each of these meets one bridge with the whole layer, so the layer side is first cut down to the
-                    // bridge's box (and expansion_area split as above); the result is the same.
-                    if (!bridging_area.empty()) {
-                        const BoundingBox bridging_bbox = get_extents(bridging_area).inflated(SCALED_EPSILON);
-                        bridging_area = intersection(bridging_area, ClipperUtils::clip_clipper_polygons_with_subject_bbox(limiting_area, bridging_bbox));
-                        bridging_area = intersection(bridging_area, ClipperUtils::clip_clipper_polygons_with_subject_bbox(total_fill_area, bridging_bbox));
-                        bridging_area = diff(bridging_area, ClipperUtils::clip_clipper_polygons_with_subject_bbox(total_top_area, bridging_bbox));
-                    }
-                    if (!bridging_area.empty()) {
-                        Polygons       kept;
-                        const Polygons cut = split_near(expansion_area, get_extents(bridging_area).inflated(SCALED_EPSILON), kept);
-                        append(kept, diff(cut, bridging_area));
-                        expansion_area = std::move(kept);
-                    }
+                    bridging_area          = intersection(bridging_area, limiting_area);
+                    bridging_area          = intersection(bridging_area, total_fill_area);
+                    bridging_area          = diff(bridging_area, total_top_area);
+                    expansion_area         = diff(expansion_area, bridging_area);
 
 #ifdef DEBUG_BRIDGE_OVER_INFILL
                     debug_draw(std::to_string(lidx) + "_" + std::to_string(cluster_idx) + "_" + std::to_string(job_idx) + "_" + "_expanded_bridging" +  std::to_string(r),

src/libslic3r/Support/SupportMaterial.cpp

@@ -929,9 +929,9 @@ public:
                 ::fread(&y, sizeof(coord_t), 1, file);
                 poly.points.emplace_back(Point(x * scale, y * scale));
             }
-            printf("Polygon %d, area: %lf\n", i, area(poly.points));
             if (which == -1 || which == i)
                 m_support_polygons_deserialized.emplace_back(std::move(poly));
+            printf("Polygon %d, area: %lf\n", i, area(poly.points));
         }
         ::fread(&n_polygons, 4, 1, file);
         m_trimming_polygons_deserialized.reserve(n_polygons);

src/libslic3r/Support/TreeSupport.cpp

@@ -854,41 +854,11 @@ void TreeSupport::detect_overhangs(bool check_support_necessity/* = false*/)
                 if (is_auto(stype) && config_detect_sharp_tails)
                 {
                     // BBS detect sharp tail
-                    // Each island is tested only against the lower islands whose box meets its own: overlaps() tries every
-                    // pair, which on a layer cut through a fine relief (thousands of islands above thousands) never ends.
-                    std::vector<BoundingBox> lower_bboxes;
-                    lower_bboxes.reserve(lower_polys.size());
-                    for (const ExPolygon &lower : lower_polys)
-                        lower_bboxes.emplace_back(get_extents(lower));
                     for (const ExPolygon& expoly : curr_polys) {
                         bool  is_sharp_tail = false;
                         // 1. nothing below
                         // this is a sharp tail region if it's floating and non-ignorable
-                        const ExPolygons  expanded = offset_ex(expoly, 0.1 * extrusion_width_scaled);
-                        const BoundingBox bbox     = get_extents(expanded);
-                        ExPolygons        lower_nearby;
-                        for (size_t i = 0; i < lower_polys.size(); ++i)
-                            if (lower_bboxes[i].overlap(bbox))
-                                lower_nearby.emplace_back(lower_polys[i]);
-                        // As overlaps(expanded, lower_nearby), with each lower island cut to the island's box first: below
-                        // a fine relief the lower layer is a few islands with thousands of holes, whose whole boundary
-                        // was otherwise intersected again for every island above.
-                        const auto overlaps_nearby = [&]() {
-                            for (const ExPolygon &a : expanded) {
-                                if (a.empty())
-                                    continue;
-                                const BoundingBox a_bbox = get_extents(a);
-                                for (const ExPolygon &b : lower_nearby) {
-                                    if (b.empty() || !get_extents(b).overlap(a_bbox))
-                                        continue;
-                                    const Polygons b_near = ClipperUtils::clip_clipper_polygons_with_subject_bbox(b, a_bbox.inflated(SCALED_EPSILON));
-                                    if (!intersection_pl(to_polylines(b_near), a).empty() || b.contains(a.contour.points.front()))
-                                        return true;
-                                }
-                            }
-                            return false;
-                        };
-                        if (!overlaps_nearby()) {
+                        if (!overlaps(offset_ex(expoly, 0.1 * extrusion_width_scaled), lower_polys)) {
                             is_sharp_tail = !offset_ex(expoly, -0.1 * extrusion_width_scaled).empty();
                         }
 

src/libslic3r/Surface.hpp

@@ -61,7 +61,7 @@ public:
             thickness(other.thickness), thickness_layers(other.thickness_layers),
             bridge_angle(other.bridge_angle), extra_perimeters(other.extra_perimeters)
         {};
-    Surface(Surface &&rhs) noexcept
+    Surface(Surface &&rhs)
         : surface_type(rhs.surface_type), expolygon(std::move(rhs.expolygon)),
             thickness(rhs.thickness), thickness_layers(rhs.thickness_layers),
             bridge_angle(rhs.bridge_angle), extra_perimeters(rhs.extra_perimeters)
@@ -87,7 +87,7 @@ public:
         return *this;
     }
 
-    Surface& operator=(Surface &&rhs) noexcept
+    Surface& operator=(Surface &&rhs)
     {
         surface_type     = rhs.surface_type;
         expolygon        = std::move(rhs.expolygon);

src/libslic3r/libslic3r.h

@@ -162,10 +162,10 @@ inline void append(std::vector<T, Alloc> &dest, std::vector<T, Alloc> &&src)
 {
     if (dest.empty())
         dest = std::move(src);
-    else
-        // insert() grows the capacity geometrically; reserving exactly the new size reallocated on every call, which
-        // made appending piece by piece quadratic.
-        dest.insert(dest.end(), std::make_move_iterator(src.begin()), std::make_move_iterator(src.end()));
+    else {
+        dest.reserve(dest.size() + src.size());
+        std::move(std::begin(src), std::end(src), std::back_inserter(dest));
+    }
     src.clear();
     src.shrink_to_fit();
 }

src/libvgcode/include/Types.hpp

@@ -158,6 +158,21 @@ enum class EGCodeExtrusionRole : uint8_t
 
 static constexpr std::size_t GCODE_EXTRUSION_ROLES_COUNT = static_cast<std::size_t>(EGCodeExtrusionRole::COUNT);
 
+//
+// What the reduced set, drawn while the user is dragging, holds in place of the full toolpaths
+//
+enum class EReducedDetailMode : uint8_t
+{
+    // nothing: no reduced set is built
+    Off,
+    // only the bottom and top layers of the visible range, for a caller that draws the print
+    // itself some other way
+    EndLayersOnly,
+    // every role but the infill under the skins and the gap fill: what can be seen from outside
+    ShellOnly,
+    COUNT
+};
+
 //
 // Option types
 //

src/libvgcode/include/Viewer.hpp

@@ -114,6 +114,19 @@ public:
     //
     bool is_dim_previous_layers() const;
     void set_dim_previous_layers(bool value);
+    //
+    // The reduced set drawn while the user drags: what the mode keeps, and always the bottom and
+    // top layers of the visible range. While a mode is set it is built alongside the full set, so
+    // set_reduced_detail() rebuilds nothing. Ignored on the OpenGL ES path.
+    //
+    EReducedDetailMode get_reduced_detail_mode() const;
+    void set_reduced_detail_mode(EReducedDetailMode mode);
+    // Whether the shell mode hides the infill roles; off for a profile that leaves them on the
+    // surface, with no top or bottom shell or no walls.
+    bool get_reduced_detail_hide_infill() const;
+    void set_reduced_detail_hide_infill(bool value);
+    void set_reduced_detail(bool value);
+    bool is_reduced_detail() const;
     float get_dim_previous_layers_brightness() const;
     void set_dim_previous_layers_brightness(float value);
     //

src/libvgcode/src/Settings.hpp

@@ -25,6 +25,11 @@ struct Settings
 		// ORCA: how bright those darkened layers are rendered, 1.0 = unchanged, 0.0 = black
 		float dim_previous_layers_brightness{ 0.4f };
 		bool spiral_vase_mode{ false };
+		// what the reduced set holds and whether it is drawn. Ignored on the OpenGL ES path.
+		EReducedDetailMode reduced_detail_mode{ EReducedDetailMode::Off };
+		// whether the shell mode may hide the infill roles: false when the profile leaves them on the surface
+		bool reduced_detail_hide_infill{ true };
+		bool reduced_detail{ false };
 		//
 		// Required update flags
 		//

src/libvgcode/src/Viewer.cpp

@@ -92,6 +92,36 @@ bool Viewer::is_dim_previous_layers() const
     return m_impl->is_dim_previous_layers();
 }
 
+void Viewer::set_reduced_detail(bool value)
+{
+    m_impl->set_reduced_detail(value);
+}
+
+bool Viewer::is_reduced_detail() const
+{
+    return m_impl->is_reduced_detail();
+}
+
+EReducedDetailMode Viewer::get_reduced_detail_mode() const
+{
+    return m_impl->get_reduced_detail_mode();
+}
+
+void Viewer::set_reduced_detail_mode(EReducedDetailMode mode)
+{
+    m_impl->set_reduced_detail_mode(mode);
+}
+
+bool Viewer::get_reduced_detail_hide_infill() const
+{
+    return m_impl->get_reduced_detail_hide_infill();
+}
+
+void Viewer::set_reduced_detail_hide_infill(bool value)
+{
+    m_impl->set_reduced_detail_hide_infill(value);
+}
+
 void Viewer::set_dim_previous_layers(bool value)
 {
     m_impl->set_dim_previous_layers(value);

src/libvgcode/src/ViewerImpl.cpp

@@ -17,6 +17,7 @@
 #include <algorithm>
 #include <cmath>
 #include <numeric>
+#include <cfloat>
 
 namespace libvgcode {
 
@@ -901,9 +902,17 @@ void ViewerImpl::reset()
 #else
     m_enabled_segments_count = 0;
     m_enabled_options_count = 0;
+    m_enabled_segments_reduced_count = 0;
+    m_enabled_options_reduced_count = 0;
+    m_enabled_segments_reduced_tex_size = 0;
+    m_enabled_options_reduced_tex_size = 0;
 
     m_settings_used_for_ranges = std::nullopt;
 
+    delete_textures(m_enabled_options_reduced_tex_id);
+    delete_buffers(m_enabled_options_reduced_buf_id);
+    delete_textures(m_enabled_segments_reduced_tex_id);
+    delete_buffers(m_enabled_segments_reduced_buf_id);
     delete_textures(m_enabled_options_tex_id);
     delete_buffers(m_enabled_options_buf_id);
     delete_textures(m_enabled_segments_tex_id);
@@ -1163,6 +1172,17 @@ void ViewerImpl::load(GCodeInputData&& gcode_data)
         glsafe(glGenTextures(1, &m_enabled_options_tex_id));
         glsafe(glBindTexture(GL_TEXTURE_BUFFER, m_enabled_options_tex_id));
 
+        // create (but do not fill) the reduced counterparts of the two buffers above
+        glsafe(glGenBuffers(1, &m_enabled_segments_reduced_buf_id));
+        glsafe(glBindBuffer(GL_TEXTURE_BUFFER, m_enabled_segments_reduced_buf_id));
+        glsafe(glGenTextures(1, &m_enabled_segments_reduced_tex_id));
+        glsafe(glBindTexture(GL_TEXTURE_BUFFER, m_enabled_segments_reduced_tex_id));
+
+        glsafe(glGenBuffers(1, &m_enabled_options_reduced_buf_id));
+        glsafe(glBindBuffer(GL_TEXTURE_BUFFER, m_enabled_options_reduced_buf_id));
+        glsafe(glGenTextures(1, &m_enabled_options_reduced_tex_id));
+        glsafe(glBindTexture(GL_TEXTURE_BUFFER, m_enabled_options_reduced_tex_id));
+
         glsafe(glBindBuffer(GL_TEXTURE_BUFFER, 0));
         glsafe(glBindTexture(GL_TEXTURE_BUFFER, old_bound_texture));
 #endif // ENABLE_OPENGL_ES
@@ -1174,6 +1194,18 @@ void ViewerImpl::load(GCodeInputData&& gcode_data)
     update_colors();
 }
 
+#ifndef ENABLE_OPENGL_ES
+// the roles that lie under a skin or between walls, never seen from outside the print
+static bool is_hidden_in_shell(EGCodeExtrusionRole role)
+{
+    return role == EGCodeExtrusionRole::InternalInfill ||
+           role == EGCodeExtrusionRole::SolidInfill ||
+           role == EGCodeExtrusionRole::InternalBridgeInfill ||
+           role == EGCodeExtrusionRole::GapFill;
+}
+#endif // ENABLE_OPENGL_ES
+
+
 void ViewerImpl::update_enabled_entities()
 {
     if (m_vertices.empty())
@@ -1181,6 +1213,17 @@ void ViewerImpl::update_enabled_entities()
 
     std::vector<uint32_t> enabled_segments;
     std::vector<uint32_t> enabled_options;
+#ifndef ENABLE_OPENGL_ES
+    // the reduced set is filled by the same walk, so switching to it costs no rebuild. Whatever the
+    // mode leaves out, the bottom and top layers of the visible range are kept whole: they are the
+    // surfaces the range cuts open
+    const EReducedDetailMode reduced_mode = m_settings.reduced_detail_mode;
+    const bool build_reduced = reduced_mode != EReducedDetailMode::Off;
+    const bool hide_infill = m_settings.reduced_detail_hide_infill;
+    std::vector<uint32_t> enabled_segments_reduced;
+    std::vector<uint32_t> enabled_options_reduced;
+    const Interval& layers_range = m_layers.get_view_range();
+#endif // ENABLE_OPENGL_ES
     Interval range = m_view_range.get_visible();
 
     // when top layer only visualization is enabled, we need to render
@@ -1228,6 +1271,20 @@ void ViewerImpl::update_enabled_entities()
             enabled_options.push_back(static_cast<uint32_t>(i));
         else
             enabled_segments.push_back(static_cast<uint32_t>(i));
+
+#ifndef ENABLE_OPENGL_ES
+        if (build_reduced) {
+            const bool end_layer = v.layer_id == layers_range[0] || v.layer_id == layers_range[1];
+            if (end_layer)
+                (v.is_option() ? enabled_options_reduced : enabled_segments_reduced).push_back(static_cast<uint32_t>(i));
+            else if (reduced_mode == EReducedDetailMode::ShellOnly) {
+                if (v.is_option())
+                    enabled_options_reduced.push_back(static_cast<uint32_t>(i));
+                else if (!v.is_extrusion() || !hide_infill || !is_hidden_in_shell(v.role))
+                    enabled_segments_reduced.push_back(static_cast<uint32_t>(i));
+            }
+        }
+#endif // ENABLE_OPENGL_ES
     }
 
 #ifdef ENABLE_OPENGL_ES
@@ -1256,6 +1313,23 @@ void ViewerImpl::update_enabled_entities()
     else
         glsafe(glBufferData(GL_TEXTURE_BUFFER, 0, nullptr, GL_STATIC_DRAW));
 
+    m_enabled_segments_reduced_count = enabled_segments_reduced.size();
+    m_enabled_options_reduced_count = enabled_options_reduced.size();
+    m_enabled_segments_reduced_tex_size = enabled_segments_reduced.size() * sizeof(uint32_t);
+    m_enabled_options_reduced_tex_size = enabled_options_reduced.size() * sizeof(uint32_t);
+
+    // uploaded even when nothing was built, so that the last reduced set is released as soon as
+    // the preference is switched off
+    assert(m_enabled_segments_reduced_buf_id > 0);
+    glsafe(glBindBuffer(GL_TEXTURE_BUFFER, m_enabled_segments_reduced_buf_id));
+    glsafe(glBufferData(GL_TEXTURE_BUFFER, m_enabled_segments_reduced_tex_size,
+                        enabled_segments_reduced.empty() ? nullptr : enabled_segments_reduced.data(), GL_STATIC_DRAW));
+
+    assert(m_enabled_options_reduced_buf_id > 0);
+    glsafe(glBindBuffer(GL_TEXTURE_BUFFER, m_enabled_options_reduced_buf_id));
+    glsafe(glBufferData(GL_TEXTURE_BUFFER, m_enabled_options_reduced_tex_size,
+                        enabled_options_reduced.empty() ? nullptr : enabled_options_reduced.data(), GL_STATIC_DRAW));
+
     glsafe(glBindBuffer(GL_TEXTURE_BUFFER, 0));
 #endif // ENABLE_OPENGL_ES
 
@@ -1463,6 +1537,23 @@ void ViewerImpl::toggle_top_layer_only_view_range()
     update_colors_texture();
 }
 
+// Both decide which vertices land in the reduced set, so the sets are rebuilt.
+void ViewerImpl::set_reduced_detail_mode(EReducedDetailMode mode)
+{
+    if (m_settings.reduced_detail_mode == mode)
+        return;
+    m_settings.reduced_detail_mode = mode;
+    m_settings.update_enabled_entities = true;
+}
+
+void ViewerImpl::set_reduced_detail_hide_infill(bool value)
+{
+    if (m_settings.reduced_detail_hide_infill == value)
+        return;
+    m_settings.reduced_detail_hide_infill = value;
+    m_settings.update_enabled_entities = true;
+}
+
 // ORCA: enable/disable darkening of the layers the layer slider is not scrubbed to
 void ViewerImpl::set_dim_previous_layers(bool value)
 {
@@ -1856,6 +1947,8 @@ size_t ViewerImpl::get_used_gpu_memory() const
     ret += m_colors_tex_size;
     ret += m_enabled_segments_tex_size;
     ret += m_enabled_options_tex_size;
+    ret += m_enabled_segments_reduced_tex_size;
+    ret += m_enabled_options_reduced_tex_size;
 #endif // ENABLE_OPENGL_ES
     return ret;
 }
@@ -2072,7 +2165,8 @@ void ViewerImpl::render_segments(const Mat4x4& view_matrix, const Mat4x4& projec
 #ifdef ENABLE_OPENGL_ES
     if (m_texture_data.get_enabled_segments_count() == 0)
 #else
-    if (m_enabled_segments_count == 0)
+    const ActiveSet segments = active_segments();
+    if (segments.count == 0)
 #endif // ENABLE_OPENGL_ES
         return;
 
@@ -2099,7 +2193,7 @@ void ViewerImpl::render_segments(const Mat4x4& view_matrix, const Mat4x4& projec
     const bool top_down = !m_rendering_shadow_casters && view_matrix[10] > 0.0f;
     glsafe(glUniform1i(m_uni_segments_reverse_order_id, top_down ? 1 : 0));
 #ifndef ENABLE_OPENGL_ES
-    glsafe(glUniform1i(m_uni_segments_instance_count_id, static_cast<int>(m_enabled_segments_count)));
+    glsafe(glUniform1i(m_uni_segments_instance_count_id, static_cast<int>(segments.count)));
 #endif // ENABLE_OPENGL_ES
     // ORCA: realistic view. The depth pass writes the map it would otherwise read, so it shades
     // with the lookup off.
@@ -2149,10 +2243,10 @@ void ViewerImpl::render_segments(const Mat4x4& view_matrix, const Mat4x4& projec
     glsafe(glBindTexture(GL_TEXTURE_BUFFER, m_colors_tex_id));
     glsafe(glTexBuffer(GL_TEXTURE_BUFFER, GL_R32F, m_colors_buf_id));
     glsafe(glActiveTexture(GL_TEXTURE3));
-    glsafe(glBindTexture(GL_TEXTURE_BUFFER, m_enabled_segments_tex_id));
-    glsafe(glTexBuffer(GL_TEXTURE_BUFFER, GL_R32UI, m_enabled_segments_buf_id));
+    glsafe(glBindTexture(GL_TEXTURE_BUFFER, segments.tex_id));
+    glsafe(glTexBuffer(GL_TEXTURE_BUFFER, GL_R32UI, segments.buf_id));
 
-    m_segment_template.render(m_enabled_segments_count);
+    m_segment_template.render(segments.count);
 #endif // ENABLE_OPENGL_ES
 
     if (curr_cull_face)
@@ -2178,7 +2272,8 @@ void ViewerImpl::render_options(const Mat4x4& view_matrix, const Mat4x4& project
 #ifdef ENABLE_OPENGL_ES
     if (m_texture_data.get_enabled_options_count() == 0)
 #else
-    if (m_enabled_options_count == 0)
+    const ActiveSet options = active_options();
+    if (options.count == 0)
 #endif // ENABLE_OPENGL_ES
         return;
 
@@ -2236,10 +2331,10 @@ void ViewerImpl::render_options(const Mat4x4& view_matrix, const Mat4x4& project
     glsafe(glBindTexture(GL_TEXTURE_BUFFER, m_colors_tex_id));
     glsafe(glTexBuffer(GL_TEXTURE_BUFFER, GL_R32F, m_colors_buf_id));
     glsafe(glActiveTexture(GL_TEXTURE3));
-    glsafe(glBindTexture(GL_TEXTURE_BUFFER, m_enabled_options_tex_id));
-    glsafe(glTexBuffer(GL_TEXTURE_BUFFER, GL_R32UI, m_enabled_options_buf_id));
+    glsafe(glBindTexture(GL_TEXTURE_BUFFER, options.tex_id));
+    glsafe(glTexBuffer(GL_TEXTURE_BUFFER, GL_R32UI, options.buf_id));
 
-    m_option_template.render(m_enabled_options_count);
+    m_option_template.render(options.count);
 #endif // ENABLE_OPENGL_ES
 
     if (!curr_cull_face)

src/libvgcode/src/ViewerImpl.hpp

@@ -109,6 +109,21 @@ public:
     // 0.0 = black
     bool is_dim_previous_layers() const { return m_settings.dim_previous_layers; }
     void set_dim_previous_layers(bool value);
+    //
+    // Draw from the reduced set; it is already built, so this is just a buffer binding.
+    //
+    void set_reduced_detail(bool value) {
+#ifdef ENABLE_OPENGL_ES
+        // no reduced set is built on OpenGL ES
+        value = false;
+#endif // ENABLE_OPENGL_ES
+        m_settings.reduced_detail = value;
+    }
+    bool is_reduced_detail() const { return m_settings.reduced_detail; }
+    EReducedDetailMode get_reduced_detail_mode() const { return m_settings.reduced_detail_mode; }
+    void set_reduced_detail_mode(EReducedDetailMode mode);
+    bool get_reduced_detail_hide_infill() const { return m_settings.reduced_detail_hide_infill; }
+    void set_reduced_detail_hide_infill(bool value);
     float get_dim_previous_layers_brightness() const { return m_settings.dim_previous_layers_brightness; }
     void set_dim_previous_layers_brightness(float value);
 
@@ -501,6 +516,15 @@ private:
     unsigned int m_enabled_options_tex_id{ 0 };
     size_t m_enabled_options_count{ 0 };
     //
+    // OpenGL buffers to store the reduced set drawn while Settings::reduced_detail is set
+    //
+    unsigned int m_enabled_segments_reduced_buf_id{ 0 };
+    unsigned int m_enabled_segments_reduced_tex_id{ 0 };
+    size_t m_enabled_segments_reduced_count{ 0 };
+    unsigned int m_enabled_options_reduced_buf_id{ 0 };
+    unsigned int m_enabled_options_reduced_tex_id{ 0 };
+    size_t m_enabled_options_reduced_count{ 0 };
+    //
     // Caches for size of data sent to gpu, in bytes
     //
     size_t m_positions_tex_size{ 0 };
@@ -508,6 +532,27 @@ private:
     size_t m_colors_tex_size{ 0 };
     size_t m_enabled_segments_tex_size{ 0 };
     size_t m_enabled_options_tex_size{ 0 };
+    size_t m_enabled_segments_reduced_tex_size{ 0 };
+    size_t m_enabled_options_reduced_tex_size{ 0 };
+
+    // The set the next draw reads from: the reduced one while dragging, if one is built.
+    bool use_reduced_set() const { return m_settings.reduced_detail && m_settings.reduced_detail_mode != EReducedDetailMode::Off; }
+    struct ActiveSet
+    {
+        size_t count{ 0 };
+        unsigned int buf_id{ 0 };
+        unsigned int tex_id{ 0 };
+    };
+    ActiveSet active_segments() const {
+        if (use_reduced_set())
+            return { m_enabled_segments_reduced_count, m_enabled_segments_reduced_buf_id, m_enabled_segments_reduced_tex_id };
+        return { m_enabled_segments_count, m_enabled_segments_buf_id, m_enabled_segments_tex_id };
+    }
+    ActiveSet active_options() const {
+        if (use_reduced_set())
+            return { m_enabled_options_reduced_count, m_enabled_options_reduced_buf_id, m_enabled_options_reduced_tex_id };
+        return { m_enabled_options_count, m_enabled_options_buf_id, m_enabled_options_tex_id };
+    }
 #endif // ENABLE_OPENGL_ES
 
     //

src/slic3r/GUI/GCodeViewer.cpp

@@ -1267,6 +1267,8 @@ void GCodeViewer::load_as_gcode(const GCodeProcessorResult& gcode_result, const
     if (current_top_layer_only != required_top_layer_only)
         m_viewer.toggle_top_layer_only_view_range();
 
+    read_reduced_detail_preferences();
+
     // ORCA: darken the layers the preview layer slider is not scrubbed to
     m_viewer.set_dim_previous_layers(get_app_config()->get_bool("preview_dim_previous_layers"));
     m_viewer.set_dim_previous_layers_brightness(0.01f * std::stoi(get_app_config()->get("preview_dim_previous_layers_brightness")));
@@ -1491,6 +1493,10 @@ void GCodeViewer::load_as_gcode(const GCodeProcessorResult& gcode_result, const
 
     // BBS: data for rendering color arrangement recommendation
     m_nozzle_nums = print.config().option<ConfigOptionFloats>("nozzle_diameter")->values.size();
+    // the shell drag mode hides the infill only where the profile covers it: with no top or bottom
+    // shell, or no walls, the infill is the surface. Per-object settings are not consulted.
+    const PrintRegionConfig& region = print.default_region_config();
+    m_viewer.set_reduced_detail_hide_infill(region.top_shell_layers.value > 0 && region.bottom_shell_layers.value > 0 && region.wall_loops.value > 0);
     // Orca hack: Hide filament group for non-bbl printers
     if (!print.is_BBL_printer()) m_nozzle_nums = 1;
     std::vector<int>         filament_maps = print.get_filament_maps();
@@ -1661,6 +1667,7 @@ void GCodeViewer::reset_shell()
 {
     m_shells.volumes.clear();
     m_shells.print_id = -1;
+    m_shells.with_wipe_tower = false;
     m_shell_bounding_box = BoundingBoxf3();
 }
 
@@ -1697,7 +1704,12 @@ void GCodeViewer::reset()
 void GCodeViewer::render_scene(int canvas_width, int canvas_height)
 {
     glsafe(::glEnable(GL_DEPTH_TEST));
-    render_shells(canvas_width, canvas_height);
+    // while dragging in the solid model mode, the objects stand in for their toolpaths, cut to the
+    // visible layer range; the toolpath set then holds only the range's bottom and top layers
+    if (m_viewer.is_reduced_detail() && solid_model_enabled())
+        render_solid_model(canvas_width, canvas_height);
+    else
+        render_shells(canvas_width, canvas_height);
 
     if (m_viewer.get_extrusion_roles_count() == 0)
         return;
@@ -2020,6 +2032,56 @@ void GCodeViewer::update_layers_slider_mode()
     // TODO m_layers_slider->SetModeAndOnlyExtruder(one_extruder_printed_model, only_extruder);
 }
 
+void GCodeViewer::set_interacting(bool interacting)
+{
+    // with no shells to stand in for the toolpaths, the solid model would leave only the end layers
+    const bool usable = !solid_model_enabled() || !m_shells.volumes.empty();
+    m_viewer.set_reduced_detail(interacting && usable);
+}
+
+void GCodeViewer::read_reduced_detail_preferences()
+{
+    m_reduced_detail_mode = reduced_detail_mode_from_string(get_app_config()->get("preview_reduced_detail_mode"));
+    apply_reduced_detail_settings();
+}
+
+void GCodeViewer::apply_reduced_detail_settings()
+{
+    m_viewer.set_reduced_detail_mode(m_reduced_detail_mode);
+}
+
+void GCodeViewer::set_reduced_detail_mode(const std::string& mode)
+{
+    const bool was_solid = solid_model_enabled();
+    m_reduced_detail_mode = reduced_detail_mode_from_string(mode);
+    apply_reduced_detail_settings();
+    reload_shells_if_solid_model_changed(was_solid);
+}
+
+libvgcode::EReducedDetailMode GCodeViewer::reduced_detail_mode_from_string(const std::string& mode)
+{
+    if (mode == "solid")
+        return libvgcode::EReducedDetailMode::EndLayersOnly;
+    if (mode == "shell")
+        return libvgcode::EReducedDetailMode::ShellOnly;
+    return libvgcode::EReducedDetailMode::Off;
+}
+
+void GCodeViewer::reload_shells_if_solid_model_changed(bool was_enabled)
+{
+    if (was_enabled == solid_model_enabled() || m_shells.print_id == -1)
+        return;
+    // only the prime tower comes and goes with the mode: a full reload would drop the shells
+    // whenever the print has moved on since they were loaded, leaving the solid model nothing to draw
+    if (wxGetApp().plater() == nullptr)
+        return;
+    // the shells are loaded from the current plate's print, which is not the plater's own
+    const Print& print = wxGetApp().plater()->get_partplate_list().get_current_fff_print();
+    if (static_cast<int>(print.id().id) != m_shells.print_id)
+        return;
+    update_shell_wipe_tower(print, m_gl_data_initialized);
+}
+
 void GCodeViewer::set_layers_z_range(const std::array<unsigned int, 2>& layers_z_range)
 {
     m_viewer.set_layers_view_range(static_cast<uint32_t>(layers_z_range[0]), static_cast<uint32_t>(layers_z_range[1]));
@@ -2344,7 +2406,11 @@ void GCodeViewer::export_toolpaths_to_obj(const char* filename) const
 void GCodeViewer::load_shells(const Print& print, bool initialized, bool force_previewing)
 {
     BOOST_LOG_TRIVIAL(info) << __FUNCTION__ << boost::format(": initialized=%1%, force_previewing=%2%")%initialized %force_previewing;
+    // the shells can load before the first G-code does, so the preferences are read here as well
+    read_reduced_detail_preferences();
     if ((print.id().id == m_shells.print_id)&&(print.get_modified_count() == m_shells.print_modify_count)) {
+        // the prime tower comes and goes on its own, without reloading the objects
+        update_shell_wipe_tower(print, initialized);
         //BBS: update force previewing logic
         if (force_previewing)
             m_shells.previewing = force_previewing;
@@ -2453,10 +2519,45 @@ void GCodeViewer::load_shells(const Print& print, bool initialized, bool force_p
     m_shells.print_id = print.id().id;
     m_shells.print_modify_count = print.get_modified_count();
     m_shells.previewing = true;
+    update_shell_wipe_tower(print, initialized);
     BOOST_LOG_TRIVIAL(debug) << __FUNCTION__ << boost::format(": shell loaded, id change to %1%, modify_count %2%, object count %3%, glvolume count %4%")
         % m_shells.print_id % m_shells.print_modify_count % object_count %m_shells.volumes.volumes.size();
 }
 
+// The prime tower as it was sliced, so that the solid model shows what the print shows. It keeps its
+// opaque colour, so it never appears among the translucent shells, and stays out of their bounding box.
+void GCodeViewer::update_shell_wipe_tower(const Print& print, bool initialized)
+{
+    const bool with_wipe_tower = solid_model_enabled() && print.is_step_done(psWipeTower) && print.wipe_tower_data().wipe_tower_mesh_data;
+    if (with_wipe_tower == m_shells.with_wipe_tower)
+        return;
+    m_shells.with_wipe_tower = with_wipe_tower;
+    GLVolumePtrs& volumes = m_shells.volumes.volumes;
+    if (!with_wipe_tower) {
+        volumes.erase(std::remove_if(volumes.begin(), volumes.end(), [](GLVolume* volume) {
+            if (!volume->is_wipe_tower)
+                return false;
+            delete volume;
+            return true;
+        }), volumes.end());
+        return;
+    }
+    const PrintConfig& config = print.config();
+    const int plate_idx = print.get_plate_index();
+    const Vec3d plate_origin = print.get_plate_origin();
+    const float x = static_cast<float>(config.wipe_tower_x.get_at(plate_idx) + plate_origin.x());
+    const float y = static_cast<float>(config.wipe_tower_y.get_at(plate_idx) + plate_origin.y());
+    const size_t first_new = volumes.size();
+    m_shells.volumes.load_real_wipe_tower_preview(1000 + plate_idx, x, y, print.wipe_tower_data().wipe_tower_mesh_data->real_wipe_tower_mesh,
+                                                  print.wipe_tower_data().wipe_tower_mesh_data->real_brim_mesh, true,
+                                                  static_cast<float>(config.wipe_tower_rotation_angle), false, initialized);
+    for (size_t i = first_new; i < volumes.size(); ++i) {
+        volumes[i]->zoom_to_volumes = false;
+        volumes[i]->force_native_color = true;
+        volumes[i]->set_render_color();
+    }
+}
+
 void GCodeViewer::render_toolpaths()
 {
     const Camera& camera = wxGetApp().plater()->get_camera();
@@ -2657,6 +2758,50 @@ void GCodeViewer::render_shells(int canvas_width, int canvas_height)
     glsafe(::glDepthMask(GL_TRUE));
 }
 
+// The sliced objects and the prime tower drawn opaque, in their filament colours, cut to the
+// visible layer range by the shader's z range. The toolpaths of the range's bottom and top layers
+// are drawn afterwards and cap the cut.
+void GCodeViewer::render_solid_model(int canvas_width, int canvas_height)
+{
+    if (m_shells.volumes.empty())
+        return;
+    // gouraud_light has no z range, so it could not cut the model
+    GLShaderProgram* shader = wxGetApp().get_shader("gouraud");
+    if (shader == nullptr)
+        return;
+
+    const libvgcode::Interval& layers = m_viewer.get_layers_view_range();
+    const float z_top = m_viewer.get_layer_z(layers[1]) - m_z_offset + 0.001f;
+    const float z_bottom = (layers[0] > 0) ? m_viewer.get_layer_z(layers[0] - 1) - m_z_offset - 0.001f : -FLT_MAX;
+
+    std::vector<float> alphas;
+    alphas.reserve(m_shells.volumes.volumes.size());
+    for (GLVolume* volume : m_shells.volumes.volumes) {
+        alphas.push_back(volume->color.a());
+        volume->color.a(1.0f);
+        volume->set_render_color();
+    }
+    m_shells.volumes.set_z_range(z_bottom, z_top);
+    // gouraud also clips by this plane, which nothing else sets on the shells
+    m_shells.volumes.set_clipping_plane(ClippingPlane::ClipsNothing().get_data());
+
+    shader->start_using();
+    // the 3D view leaves its shadow settings on the shared program
+    shader->set_uniform("shadow_intensity", 0.0f);
+    const Camera& camera = wxGetApp().plater()->get_camera();
+    shader->set_uniform("z_far", camera.get_far_z());
+    shader->set_uniform("z_near", camera.get_near_z());
+    m_shells.volumes.render(GLVolumeCollection::ERenderType::Opaque, false, camera.get_view_matrix(), camera.get_projection_matrix(), {canvas_width, canvas_height});
+    shader->stop_using();
+
+    m_shells.volumes.set_z_range(-FLT_MAX, FLT_MAX);
+    size_t k = 0;
+    for (GLVolume* volume : m_shells.volumes.volumes) {
+        volume->color.a(alphas[k++]);
+        volume->set_render_color();
+    }
+}
+
 //BBS
 void GCodeViewer::render_all_plates_stats(const std::vector<const GCodeProcessorResult*>& gcode_result_list, bool show /*= true*/) const {
     if (!show)

src/slic3r/GUI/GCodeViewer.hpp

@@ -174,6 +174,8 @@ public:
         int  print_id{-1};
         int  print_modify_count{-1};
         bool previewing{false};
+        // the prime tower was loaded with the objects, for the solid model
+        bool with_wipe_tower{false};
     };
     //BBS
     ConflictResultOpt m_conflict_result;
@@ -234,6 +236,17 @@ private:
 
     bool m_legend_visible{ true };
     bool m_legend_enabled{ true };
+    // the reduced-detail preferences, pushed to libvgcode by apply_reduced_detail_settings()
+    libvgcode::EReducedDetailMode m_reduced_detail_mode{ libvgcode::EReducedDetailMode::Off };
+    void read_reduced_detail_preferences();
+    void apply_reduced_detail_settings();
+    static libvgcode::EReducedDetailMode reduced_detail_mode_from_string(const std::string& mode);
+    // in the solid model mode, the sliced objects are drawn as solid shapes instead of toolpaths
+    bool solid_model_enabled() const { return m_reduced_detail_mode == libvgcode::EReducedDetailMode::EndLayersOnly; }
+    void render_solid_model(int canvas_width, int canvas_height);
+    // the prime tower is only among the shells for the solid model, so it is added or removed when that changes
+    void reload_shells_if_solid_model_changed(bool was_enabled);
+    void update_shell_wipe_tower(const Print& print, bool initialized);
 
     float m_legend_height;
     PrintEstimatedStatistics m_print_statistics;
@@ -291,7 +304,7 @@ public:
     // void _render_calibration_thumbnail_internal(ThumbnailData& thumbnail_data, const ThumbnailsParams& thumbnail_params, PartPlateList& partplate_list, OpenGLManager& opengl_manager);
     // void _render_calibration_thumbnail_framebuffer(ThumbnailData& thumbnail_data, unsigned int w, unsigned int h, const ThumbnailsParams& thumbnail_params, PartPlateList& partplate_list, OpenGLManager& opengl_manager);
     // void render_calibration_thumbnail(ThumbnailData& thumbnail_data, unsigned int w, unsigned int h, const ThumbnailsParams& thumbnail_params, PartPlateList& partplate_list, OpenGLManager& opengl_manager);
-    bool has_data() const { return !m_viewer.get_extrusion_roles().empty(); }
+    bool has_data() const { return m_viewer.get_extrusion_roles_count() != 0; }
 
     bool can_export_toolpaths() const;
     std::vector<int> get_plater_extruder();
@@ -363,6 +376,14 @@ public:
     void set_dim_previous_layers_brightness(float value) { m_viewer.set_dim_previous_layers_brightness(value); }
     float get_dim_previous_layers_brightness() const { return m_viewer.get_dim_previous_layers_brightness(); }
 
+    // whether the mouse is holding either slider's handle
+    bool is_slider_dragging() const { return m_layers_slider->is_dragging() || m_moves_slider->is_dragging(); }
+    // while the user drags the camera or a slider, draw the reduced set, if the preference asks for one
+    void set_interacting(bool interacting);
+    bool is_reduced_detail() const { return m_viewer.is_reduced_detail(); }
+    // the preference's string value: "off", "solid" or "shell"
+    void set_reduced_detail_mode(const std::string& mode);
+
     void set_layers_z_range(const std::array<unsigned int, 2>& layers_z_range);
 
     bool is_legend_shown() const { return m_legend_visible && m_legend_enabled; }

src/slic3r/GUI/GLCanvas3D.cpp

@@ -2084,6 +2084,11 @@ void GLCanvas3D::_render_frame(bool scene_dirty, bool only_init)
     const bool overlay_tick = m_fps_overlay_tick;
     m_fps_overlay_tick = false;
 
+    // Whether the preview draws its reduced set is decided before the cached scene is consulted,
+    // since switching changes what the scene pass draws.
+    if (m_canvas_type == ECanvasType::CanvasPreview && m_render_preview && m_gcode_viewer.has_data() && _update_preview_interaction())
+        scene_dirty = true;
+
     // An overlay-only frame reuses the last scene pass. The overlay is rebuilt either way, and drawn
     // below once it is known whether the frame differs from the one on screen.
     const bool reuse_scene = !scene_dirty && _can_reuse_cached_scene(camera);
@@ -3277,9 +3282,16 @@ void GLCanvas3D::bind_event_handlers()
                 if (m_selection_edit.kind != SelectionEdit::None)
                     finish_selection_edit();
                 ImGui::SetWindowFocus(nullptr);
+                // a drag cut short never sees its button release, which would leave the reduced set drawn
+                if (m_canvas_type == CanvasPreview && m_mouse.dragging && m_gcode_viewer.is_reduced_detail())
+                    mouse_up_cleanup();
                 render();
                 evt.Skip();
             });
+        m_canvas->Bind(wxEVT_MOUSE_CAPTURE_LOST, [this](wxMouseCaptureLostEvent&) {
+                if (m_canvas_type == CanvasPreview && m_mouse.dragging && m_gcode_viewer.is_reduced_detail())
+                    mouse_up_cleanup();
+            });
         m_event_handlers_bound = true;
 
         m_canvas->Bind(wxEVT_GESTURE_PAN, &GLCanvas3D::on_gesture, this);
@@ -3355,6 +3367,17 @@ void GLCanvas3D::on_idle(wxIdleEvent& evt)
     m_overlay_dirty |= imgui_requires_extra_frame;
 #endif // ENABLE_ENHANCED_IMGUI_SLIDER_FLOAT
     m_dirty |= GLTexture::Compressor::has_compressed_texture_to_refresh();
+    // the render timer only wakes the idle loop; the frame that puts the preview's toolpaths back
+    // after a wheel burst has to be asked for here, once the settle time is really up
+    if (m_preview_settle_pending) {
+        const auto now = std::chrono::steady_clock::now();
+        if (now >= m_preview_interaction_until) {
+            m_preview_settle_pending = false;
+            m_dirty = true;
+        }
+        else // the timer fired early
+            schedule_extra_frame(static_cast<int>(std::chrono::duration_cast<std::chrono::milliseconds>(m_preview_interaction_until - now).count()) + 1);
+    }
 
     if (!m_dirty && !m_overlay_dirty)
         return;
@@ -3885,6 +3908,10 @@ void GLCanvas3D::on_mouse_wheel(wxMouseEvent& evt)
         return;
     }
 
+    // only a wheel the panels did not take moves the camera
+    if (m_canvas_type == CanvasPreview)
+        note_preview_interaction();
+
 #ifdef __WXMSW__
 	// For some reason the Idle event is not being generated after the mouse scroll event in case of scrolling with the two fingers on the touch pad,
 	// if the event is not allowed to be passed further.
@@ -3985,6 +4012,11 @@ void GLCanvas3D::on_fps_overlay_timer(wxTimerEvent& evt)
     wxWakeUpIdle();
 }
 
+void GLCanvas3D::note_preview_interaction()
+{
+    m_preview_interaction_until = std::chrono::steady_clock::now() + std::chrono::milliseconds(150);
+}
+
 void GLCanvas3D::schedule_extra_frame(int milliseconds)
 {
     // Schedule idle event right now
@@ -5631,6 +5663,9 @@ void GLCanvas3D::mouse_up_cleanup()
     m_mouse.ignore_left_up = false;
     m_mouse.ignore_right_up = false;
     m_dirty = true;
+    // the frame that follows a release puts the preview's toolpaths back, and on some platforms
+    // no idle event follows a button release until the next input
+    wxWakeUpIdle();
 
     if (m_canvas->HasCapture())
         m_canvas->ReleaseMouse();
@@ -7779,13 +7814,20 @@ bool GLCanvas3D::_is_scene_cacheable() const
         return false;
 #endif
 
-    // The scene follows the cursor during a drag, under a gizmo that draws at the cursor, and while
-    // the cursor is on the layer height bar, where the object shader draws a band at its height.
+    // The scene follows the cursor while the user drags, under a gizmo that draws at the cursor, and
+    // while the cursor is on the layer height bar, where the object shader draws a band at its height.
     const GLGizmoBase* gizmo = m_gizmos.get_current();
     const bool cursor_on_layers_bar = is_layers_editing_enabled() &&
         m_layers_editing.bar_rect_contains(*this, (float)m_mouse.position.x(), (float)m_mouse.position.y());
-    return !m_mouse.dragging && !m_gizmos.is_dragging() && !m_rectangle_selection.is_dragging() &&
-           (gizmo == nullptr || !gizmo->render_follows_cursor()) && !cursor_on_layers_bar;
+    return !is_user_interacting() && (gizmo == nullptr || !gizmo->render_follows_cursor()) && !cursor_on_layers_bar;
+}
+
+// Whether the user is holding something that moves the scene: the camera, the navigator, a gizmo,
+// the rectangle selection or a preview slider.
+bool GLCanvas3D::is_user_interacting() const
+{
+    return m_mouse.dragging || m_navigator_dragging || m_gizmos.is_dragging() || m_rectangle_selection.is_dragging() ||
+           m_gcode_viewer.is_slider_dragging();
 }
 
 bool GLCanvas3D::_is_frame_skipping_enabled() const
@@ -8776,6 +8818,24 @@ void GLCanvas3D::_render_wireframe_overlay()
     shader->stop_using();
 }
 
+// The reduced set is drawn while the camera, the navigator or either slider is dragged. A wheel
+// step has no duration, so it holds the reduced set for a settle time instead, and the frame that
+// restores the full toolpaths is scheduled for when that time runs out. Returns whether what the scene
+// pass draws changed, since a frame that reuses the cached scene would hide the change.
+bool GLCanvas3D::_update_preview_interaction()
+{
+    const auto now = std::chrono::steady_clock::now();
+    const bool settling = now < m_preview_interaction_until;
+    const bool dragging = is_user_interacting();
+    const bool was_reduced = m_gcode_viewer.is_reduced_detail();
+    m_gcode_viewer.set_interacting(dragging || settling);
+    if (settling && !dragging && m_gcode_viewer.is_reduced_detail()) {
+        m_preview_settle_pending = true;
+        schedule_extra_frame(static_cast<int>(std::chrono::duration_cast<std::chrono::milliseconds>(m_preview_interaction_until - now).count()) + 1);
+    }
+    return m_gcode_viewer.is_reduced_detail() != was_reduced;
+}
+
 //BBS: GUI refactor: add canvas size as parameters
 void GLCanvas3D::_render_gcode(int canvas_width, int canvas_height)
 {

src/slic3r/GUI/GLCanvas3D.hpp

@@ -656,6 +656,10 @@ private:
     ECursorType m_cursor_type;
     GLSelectionRectangle m_rectangle_selection;
     bool m_navigator_dragging{ false };
+    // until when a wheel step keeps the preview's reduced set drawn
+    std::chrono::time_point<std::chrono::steady_clock> m_preview_interaction_until{};
+    // whether the frame that restores the toolpaths once that time is up is still owed
+    bool m_preview_settle_pending{ false };
 
     //BBS:add plate related logic
     mutable std::vector<int> m_hover_volume_idxs;
@@ -1222,6 +1226,10 @@ public:
     void msw_rescale() { m_gcode_viewer.invalidate_legend(); }
 
     void request_extra_frame() { m_extra_frame_requested = true; }
+    // whether the user is holding the camera, the navigator, a gizmo, the rectangle selection or a preview slider
+    bool is_user_interacting() const;
+    // a wheel step is over before the next frame, so it holds the preview's reduced set for a settle time
+    void note_preview_interaction();
 
     void schedule_extra_frame(int milliseconds);
 
@@ -1372,6 +1380,9 @@ private:
     //BBS: GUI refactor: add canvas size as parameters
     void _render_gcode(int canvas_width, int canvas_height);
     void _render_gcode_overlay(int canvas_width, int canvas_height);
+    // decides whether the preview draws its reduced set this frame and returns whether what the scene
+    // pass draws changed; runs before the cached scene is consulted
+    bool _update_preview_interaction();
     //BBS: render a plane for assemble
     void _render_plane() const;
     void _render_selection();

src/slic3r/GUI/IMSlider.cpp

@@ -483,6 +483,11 @@ void IMSlider::draw_background_and_groove(const ImRect& bg_rect, const ImRect& g
     ImGui::RenderFrame(groove.Min, groove.Max, groove_col, false, 0.5 * groove.GetWidth());
 }
 
+bool IMSlider::is_dragging() const
+{
+    return GImGui != nullptr && m_imgui_id != 0 && GImGui->ActiveId == m_imgui_id && GImGui->IO.MouseDown[0];
+}
+
 bool IMSlider::horizontal_slider(const char* str_id, int* value, int v_min, int v_max, const ImVec2& size, float scale)
 {
     ImGuiWindow* window = ImGui::GetCurrentWindow();
@@ -491,6 +496,7 @@ bool IMSlider::horizontal_slider(const char* str_id, int* value, int v_min, int
 
     ImGuiContext& context = *GImGui;
     const ImGuiID id = window->GetID(str_id);
+    m_imgui_id = id;
 
     const ImVec2 pos = window->DC.CursorPos;
     const ImRect draw_region(pos, pos + size);
@@ -883,6 +889,7 @@ bool IMSlider::vertical_slider(const char* str_id, int* higher_value, int* lower
 
     ImGuiContext& context = *GImGui;
     const ImGuiID id = window->GetID(str_id);
+    m_imgui_id = id;
 
     const ImVec2 pos = window->DC.CursorPos;
     const ImRect draw_region(pos, pos + size);

src/slic3r/GUI/IMSlider.hpp

@@ -118,6 +118,9 @@ public:
 
     //BBS update scroll value changed
     bool is_dirty() { return m_dirty; }
+    // whether the mouse is holding this slider's handle, read from ImGui's active id rather than
+    // from the dirty flag, which is raised and consumed inside a single frame
+    bool is_dragging() const;
     void set_as_dirty(bool dirty = true) { m_dirty = dirty; }
     bool is_need_post_tick_event() { return m_is_need_post_tick_changed_event; }
     void reset_post_tick_event(bool val = false) {
@@ -182,6 +185,8 @@ private:
     int  m_higher_value;
     int  m_one_layer_value; // ORCA
     bool m_dirty = false;
+    // the ImGui id of the slider widget, as of its last render
+    unsigned int m_imgui_id = 0;
 
     bool m_render_as_disabled{ false };
 

src/slic3r/GUI/Preferences.cpp

@@ -322,7 +322,7 @@ wxBoxSizer* PreferencesDialog::create_item_combobox(wxString title, wxString too
     return sizer;
 }
 
-wxBoxSizer *PreferencesDialog::create_item_combobox(wxString title, wxString tooltip, std::string param, std::vector<wxString> vlist, std::vector<std::string> config_name_index, const wxString wiki_url)
+wxBoxSizer *PreferencesDialog::create_item_combobox(wxString title, wxString tooltip, std::string param, std::vector<wxString> vlist, std::vector<std::string> config_name_index, std::function<void(std::string)> onchange, const wxString wiki_url)
 {
     assert(vlist.size() == config_name_index.size());
     unsigned int current_index = 0;
@@ -338,8 +338,9 @@ wxBoxSizer *PreferencesDialog::create_item_combobox(wxString title, wxString too
     auto [sizer, combobox] = create_item_combobox_base(title, tooltip, param, vlist, current_index);
 
     //// save config
-    combobox->GetDropDown().Bind(wxEVT_COMBOBOX, [this, param, config_name_index](wxCommandEvent& e) {
+    combobox->GetDropDown().Bind(wxEVT_COMBOBOX, [this, param, config_name_index, onchange](wxCommandEvent& e) {
         app_config->set(param, config_name_index[e.GetSelection()]);
+        if (onchange != nullptr) onchange(config_name_index[e.GetSelection()]);
         e.Skip();
     });
 
@@ -2044,6 +2045,32 @@ void PreferencesDialog::create_items()
         "preview_default_view_type", PreviewViewTypeLabels, PreviewViewTypeValues);
     g_sizer->Add(item_preview_view_type);
 
+    auto item_reduced_detail_mode = create_item_combobox(
+        _L("Simplify preview while dragging"),
+        _L("What the sliced preview draws while you drag the camera or a preview slider, or zoom with the mouse wheel, so that large prints stay responsive. "
+           "The full toolpaths are restored as soon as you let go.\n"
+           "Off: the full toolpaths.\n"
+           "Solid model: the sliced objects and the prime tower as solid shapes in their filament colors, cut to the visible layer range, "
+           "with its bottom and top layers drawn as toolpaths. Supports are not shown, and negative volumes are not cut out.\n"
+           "Shell only: every layer without its sparse infill, internal solid infill and gap fill, which lie under the walls and skins. "
+           "Walls, top and bottom surfaces, bridges, supports and the prime tower are drawn whole, so the print looks the same from outside.\n"
+           "The bottom and top of the visible layer range are always drawn whole."),
+        "preview_reduced_detail_mode",
+        {_L("Off"), _L("Solid model"), _L("Shell only")},
+        {"off", "solid", "shell"},
+        // apply the new mode immediately to the currently loaded preview
+        [](std::string value) {
+            if (Plater* plater = wxGetApp().plater()) {
+                if (GLCanvas3D* canvas = plater->get_preview_canvas3D()) {
+                    canvas->get_gcode_viewer().set_reduced_detail_mode(value);
+                    canvas->set_as_dirty();
+                    canvas->request_extra_frame();
+                }
+            }
+        }
+    );
+    g_sizer->Add(item_reduced_detail_mode);
+
     auto item_dim_previous_layers = create_item_checkbox(
         _L("Dim lower layers"),
         _L("When scrubbing the layer slider in the sliced preview, render the layers below the current one darkened so that only the layer being viewed is shown at full brightness."),

src/slic3r/GUI/Preferences.hpp

@@ -93,7 +93,7 @@ public:
     wxBoxSizer *create_item_title(wxString title);
     wxBoxSizer *create_item_label(wxString label, const wxString tooltip = "", const wxString wiki_url = "");
     wxBoxSizer *create_item_combobox(wxString title, wxString tooltip, std::string param, std::vector<wxString> vlist, std::function<void(wxString)> onchange = {}, const wxString wiki_url = "");
-    wxBoxSizer *create_item_combobox(wxString title, wxString tooltip, std::string param, std::vector<wxString> vlist, std::vector<std::string> config_name_index, const wxString wiki_url = "");
+    wxBoxSizer *create_item_combobox(wxString title, wxString tooltip, std::string param, std::vector<wxString> vlist, std::vector<std::string> config_name_index, std::function<void(std::string)> onchange = {}, const wxString wiki_url = "");
     wxBoxSizer *create_item_region_combobox(wxString title, wxString tooltip);
     wxBoxSizer *create_item_language_combobox(wxString title, wxString tooltip);
     wxBoxSizer *create_item_loglevel_combobox(wxString title, wxString tooltip, std::vector<wxString> vlist);

tests/libslic3r/CMakeLists.txt

@@ -28,7 +28,6 @@ add_executable(${_TEST_NAME}_tests
     test_filament_mixer.cpp
     test_fill_plane_path.cpp
     test_geometry.cpp
-    test_kdtree.cpp
     test_multimaterial_segmentation.cpp
     test_placeholder_parser.cpp
     test_polygon.cpp

tests/libslic3r/test_clipper_utils.cpp

@@ -299,47 +299,3 @@ TEST_CASE("Traversing Clipper PolyTree", "[ClipperUtils]") {
         REQUIRE(count_polys(output) == reference.size());
     }
 }
-
-TEST_CASE("Tiled diff and intersection cover the same area as the plain calls", "[ClipperUtils]") {
-    // A grid of disjoint framed squares, enough of them to be split into several tiles.
-    const int  n    = 40;
-    const coord_t cell = scaled<coord_t>(2.), side = scaled<coord_t>(1.5), frame = scaled<coord_t>(0.3);
-    ExPolygons subject;
-    for (int y = 0; y < n; ++ y)
-        for (int x = 0; x < n; ++ x) {
-            const Point o(x * cell, y * cell);
-            ExPolygon square(Polygon({ o, o + Point(side, 0), o + Point(side, side), o + Point(0, side) }));
-            Polygon hole({ o + Point(frame, frame), o + Point(frame, side - frame), o + Point(side - frame, side - frame), o + Point(side - frame, frame) });
-            square.holes.emplace_back(std::move(hole));
-            subject.emplace_back(std::move(square));
-        }
-    // Clip polygons crossing many squares, one of them large with holes of its own.
-    Polygons clip;
-    const coord_t span = n * cell;
-    for (int i = 0; i < 8; ++ i) {
-        const coord_t y0 = coord_t(i) * span / 8, y1 = y0 + scaled<coord_t>(0.9);
-        clip.emplace_back(Polygon({ Point(- cell, y0), Point(span, y0 + cell * 3), Point(span, y1 + cell * 3), Point(- cell, y1) }));
-    }
-    ExPolygon big(Polygon({ Point(span / 4, span / 4), Point(3 * span / 4, span / 4), Point(3 * span / 4, 3 * span / 4), Point(span / 4, 3 * span / 4) }));
-    for (int i = 0; i < 4; ++ i) {
-        const Point o(span / 4 + scaled<coord_t>(3.1) + i * scaled<coord_t>(9.7), span / 4 + scaled<coord_t>(5.3));
-        big.holes.emplace_back(Polygon({ o, o + Point(0, scaled<coord_t>(20.)), o + Point(scaled<coord_t>(5.), scaled<coord_t>(20.)), o + Point(scaled<coord_t>(5.), 0) }));
-    }
-    polygons_append(clip, to_polygons(big));
-
-    const auto xor_area = [](const ExPolygons &a, const ExPolygons &b) { return area(diff_ex(a, b)) + area(diff_ex(b, a)); };
-    const ApplySafetyOffset safety = GENERATE(ApplySafetyOffset::No, ApplySafetyOffset::Yes);
-    const double tolerance = double(scaled<coord_t>(0.001)) * double(span);
-
-    const ExPolygons diff_plain = diff_ex(subject, clip, safety);
-    const ExPolygons diff_tiled = diff_ex_by_piece(subject, clip, safety);
-    REQUIRE(area(diff_plain) > 0.);
-    CHECK_THAT(area(diff_tiled), Catch::Matchers::WithinRel(area(diff_plain), 1e-9));
-    CHECK(xor_area(diff_tiled, diff_plain) < tolerance);
-
-    const ExPolygons intersection_plain = intersection_ex(subject, clip, safety);
-    const ExPolygons intersection_tiled = intersection_ex_by_piece(subject, clip, safety);
-    REQUIRE(area(intersection_plain) > 0.);
-    CHECK_THAT(area(intersection_tiled), Catch::Matchers::WithinRel(area(intersection_plain), 1e-9));
-    CHECK(xor_area(intersection_tiled, intersection_plain) < tolerance);
-}

tests/libslic3r/test_kdtree.cpp

@@ -1,67 +0,0 @@
-#include <catch2/catch_all.hpp>
-
-#include <numeric>
-#include <random>
-#include <vector>
-
-#include "libslic3r/KDTreeIndirect.hpp"
-#include "libslic3r/Point.hpp"
-
-using namespace Slic3r;
-
-TEST_CASE("Visiting the nearby points gives what collecting them gives", "[KDTree]") {
-    std::mt19937                          rng(19937);
-    std::uniform_real_distribution<float> coord(-50.f, 50.f);
-    // Points in a box, so that a radius search returns anything from none of them to all of them.
-    std::vector<Vec3f> points(2000);
-    for (Vec3f &p : points)
-        p = Vec3f(coord(rng), coord(rng), coord(rng));
-
-    auto coordinate = [&points](size_t idx, size_t dimension) { return points[idx](int(dimension)); };
-    KDTreeIndirect<3, float, decltype(coordinate)> tree(coordinate);
-    std::vector<size_t> indices(points.size());
-    std::iota(indices.begin(), indices.end(), 0);
-    tree.build(indices);
-
-    const float radius = GENERATE(0.5f, 5.f, 25.f, 200.f);
-    for (int i = 0; i < 20; ++ i) {
-        const Vec3f center(coord(rng), coord(rng), coord(rng));
-
-        const std::vector<size_t> collected = find_nearby_points(tree, center, radius);
-        std::vector<size_t>       visited;
-        visit_nearby_points(tree, center, radius, [&visited](size_t idx) { visited.emplace_back(idx); });
-
-        // Same points, and in the same order: a caller that keeps the first of several equally good ones
-        // must get the same answer either way.
-        REQUIRE(visited == collected);
-    }
-}
-
-TEST_CASE("A radius search returns every point within the radius and no other", "[KDTree]") {
-    std::mt19937                          rng(2024);
-    std::uniform_real_distribution<float> coord(-20.f, 20.f);
-    std::vector<Vec3f> points(500);
-    for (Vec3f &p : points)
-        p = Vec3f(coord(rng), coord(rng), coord(rng));
-
-    auto coordinate = [&points](size_t idx, size_t dimension) { return points[idx](int(dimension)); };
-    KDTreeIndirect<3, float, decltype(coordinate)> tree(coordinate);
-    std::vector<size_t> indices(points.size());
-    std::iota(indices.begin(), indices.end(), 0);
-    tree.build(indices);
-
-    const Vec3f center(1.f, -2.f, 3.f);
-    const float radius = 7.f;
-
-    std::vector<size_t> expected;
-    for (size_t i = 0; i < points.size(); ++ i)
-        if ((points[i] - center).squaredNorm() < radius * radius)
-            expected.emplace_back(i);
-
-    std::vector<size_t> visited;
-    visit_nearby_points(tree, center, radius, [&visited](size_t idx) { visited.emplace_back(idx); });
-    std::sort(visited.begin(), visited.end());
-
-    REQUIRE(! expected.empty());
-    REQUIRE(visited == expected);
-}