Fix internal bridges over Hilbert Curve/Octagram Spiral sparse infill (#15206)

* Fix internal bridges over Hilbert Curve/Octagram Spiral sparse infill

For patterns with curved/turning anchor lines (Hilbert Curve, Octagram
Spiral), the bridge_over_infill algorithm produced incorrect results:

1. determine_bridging_angle: sampling curved anchor orientations
   produced noise across all turning directions (0/90/180/270°)
   instead of a single dominant one, yielding unstable bridge angles
   with 180° spread. Fix: use the configured infill_direction + 90°
   directly, bypassing the noisy sampling. The old blind +0.25*PI
   (Hilbert) and +1/16*PI (Octagram) offsets are removed.

2. construct_anchored_polygon: curved Hilbert/Octagram anchors
   intersected each vertical scan line many times at wildly different
   Y positions, producing chaotic polygon sections — holes in random
   places, bridges over air, rotated bridges. Fix: replace the curved
   infill polylines with synthetic straight lines parallel to
   infill_direction, spaced at the real infill line spacing
   (flow_spacing / density). Lines are centered on the limiting_area
   bbox center so that after rotation they span the full bridged_area.
   Anchors are left at full bbox length (not clipped) to guarantee
   every scan line finds an anchor.

Rectilinear and other straight-line patterns are unaffected.

Known limitation: some bridge edges may still terminate over air in
edge cases where the nearest synthetic anchor line is more than one
infill spacing away from the bridge boundary. This will be addressed
in a follow-up.

* fix: anchor internal bridges to actual sparse infill

Preserve real anchors across regions and align plane-path anchor origins with printed infill. Respect lower-layer rotation templates and model alignment, and sample curved bridge boundaries more finely.

Add regression coverage for anchor alignment, bridge angles and region isolation, with Orca comments explaining the geometry constraints. Verified 175 FFF tests before the comment-only follow-up; preserve CRLF in modified files.

* Fix internal bridge support contacts and separated infill origins

Restore anchor contact after bridge smoothing and share per-body pattern origins between anchors and printed infill. Recompute origins when preparation settings change.

Cover multiline counts 1, 2 and 3 and add regressions for printed bridge support, separated infill alignment and reslicing.

* Add explicit standard headers to PrintObject tests

* test: cover surface centering when infill settings change

Verify top and bottom Archimedean Chords and Octagram Spiral paths after switching centering modes or toggling separated infills. Compare reslicing against fresh slicing and document dependent infill invalidation.

* test: preserve directional surface infill when settings change

* perf: index layer islands for connected-body detection

* test: use public print pipeline for body centering checks

tests/fff_print/test_fill.cpp

@@ -9,6 +9,7 @@
 #include <vector>
 
 #include "libslic3r/ClipperUtils.hpp"
+#include "libslic3r/AABBTreeLines.hpp"
 #include "libslic3r/Fill/Fill.hpp"
 #include "libslic3r/Flow.hpp"
 #include "libslic3r/Geometry.hpp"
@@ -1229,3 +1230,68 @@ TEST_CASE("Smoothing multiline lightning infill keeps its outlines connected", "
     REQUIRE(smooth.point_count > sharp.point_count);
     REQUIRE(smooth.sharp_turns < sharp.sharp_turns);
 }
+
+TEST_CASE("Sparse plane-path anchors match the printed infill", "[Fill][InternalBridge][Regression]")
+{
+    // Orca: Compare generated anchors with actual extrusion across plane-path patterns,
+    // smoothing, multiline and rotations; an origin shift must not pass as valid support.
+    const std::string pattern = GENERATE("hilbertcurve", "octagramspiral", "archimedeanchords");
+    const std::string smoothing = GENERATE("0%", "100%");
+    const int multiline = GENERATE(1, 2);
+    const bool rotated = GENERATE(false, true);
+    const bool separated = GENERATE(false, true);
+    CAPTURE(pattern, smoothing, multiline, rotated, separated);
+
+    auto config = DynamicPrintConfig::full_print_config();
+    config.set_deserialize_strict({{"sparse_infill_pattern", pattern},
+                                   {"sparse_infill_density", "15%"},
+                                   {"sparse_infill_smooth_factor", smoothing},
+                                   {"fill_multiline", multiline},
+                                   {"infill_direction", 45},
+                                   {"sparse_infill_rotate_template", rotated ? "0,25,50" : ""},
+                                   {"align_infill_direction_to_model", rotated},
+                                   {"separated_infills", separated},
+                                   {"top_shell_layers", 0},
+                                   {"bottom_shell_layers", 0},
+                                   {"top_shell_thickness", 0},
+                                   {"bottom_shell_thickness", 0},
+                                   {"layer_height", 0.2},
+                                   {"initial_layer_print_height", 0.2},
+                                   {"resolution", 0.012}});
+    Print print;
+    Model model;
+    TriangleMesh mesh = make_cube(30, 24, 1);
+    if (separated) {
+        // Orca: Two disconnected bodies in one object must each use their own infill origin.
+        TriangleMesh second = make_cube(30, 24, 1);
+        second.translate(50, 0, 0);
+        mesh.merge(second);
+    }
+    Slic3r::Test::init_print({mesh}, print, model, config, nullptr, false);
+    if (rotated) {
+        model.objects.front()->instances.front()->set_rotation(Vec3d(0., 0., Geometry::deg2rad(23.)));
+        print.apply(model, config);
+    }
+    print.process();
+
+    const Layer &layer = *print.objects().front()->get_layer(4);
+    Polylines printed;
+    for (const LayerRegion *region : layer.regions())
+        for (const ExtrusionEntity *entity : region->fills.flatten().entities)
+            if (entity->role() == erInternalInfill)
+                entity->collect_polylines(printed);
+    REQUIRE_FALSE(printed.empty());
+    const AABBTreeLines::LinesDistancer<Line> printed_tree(to_lines(printed));
+
+    // Orca: Exclude perimeter connections: anchoring and extrusion can trim those differently.
+    const Polylines anchors = intersection_pl(layer.generate_sparse_infill_polylines_for_anchoring(nullptr, nullptr, nullptr),
+                                              shrink(to_polygons(layer.lslices), scale_(3.)));
+    REQUIRE_FALSE(anchors.empty());
+    double max_distance = 0.;
+    for (const Polyline &path : anchors)
+        for (const Point &point : path.equally_spaced_points(scale_(0.25)))
+            max_distance = std::max(max_distance, printed_tree.distance_from_lines<false>(point));
+    // Orca: Allow only the configured simplification tolerance; infill-scale offsets
+    // would hide anchors that no longer coincide with printed lines.
+    CHECK(unscale<double>(max_distance) <= config.opt_float("resolution"));
+}

tests/fff_print/test_printobject.cpp

@@ -4,11 +4,18 @@
 #include "libslic3r/Print.hpp"
 #include "libslic3r/Layer.hpp"
 #include "libslic3r/GCodeReader.hpp"
+#include "libslic3r/ClipperUtils.hpp"
+#include "libslic3r/AABBTreeLines.hpp"
 
 #include "test_helpers.hpp"
 
+#include <cmath>
 #include <iterator>
+#include <map>
 #include <set>
+#include <string>
+#include <utility>
+#include <vector>
 
 using namespace Slic3r;
 using namespace Slic3r::Test;
@@ -130,3 +137,438 @@ TEST_CASE("Initial layer height is honored", "[PrintObject]")
     REQUIRE_THAT(*layer_zs.begin(),            Catch::Matchers::WithinAbs(0.3, 1e-4));
     REQUIRE_THAT(*std::next(layer_zs.begin()), Catch::Matchers::WithinAbs(0.5, 1e-4));
 }
+
+static TriangleMesh internal_bridge_step()
+{
+    // Orca: The smaller tower leaves a shoulder whose solid skin needs internal bridges
+    // over the sparse infill in the base, without relying on an external model file.
+    TriangleMesh mesh = make_cube(30, 24, 3);
+    TriangleMesh tower = make_cube(14, 10, 1);
+    tower.translate(8, 7, 3);
+    mesh.merge(tower);
+    return mesh;
+}
+
+static DynamicPrintConfig internal_bridge_config(const std::string &pattern, int multiline)
+{
+    auto config = DynamicPrintConfig::full_print_config();
+    config.set_deserialize_strict({{"sparse_infill_pattern", pattern},
+                                   {"fill_multiline", multiline},
+                                   {"sparse_infill_density", "15%"},
+                                   {"sparse_infill_smooth_factor", "100%"},
+                                   {"infill_direction", 45},
+                                   {"internal_bridge_angle", 0},
+                                   {"thick_internal_bridges", true},
+                                   {"top_shell_layers", 3},
+                                   {"bottom_shell_layers", 2},
+                                   {"top_shell_thickness", 0},
+                                   {"bottom_shell_thickness", 0},
+                                   {"layer_height", 0.2},
+                                   {"initial_layer_print_height", 0.2}});
+    return config;
+}
+
+TEST_CASE("Internal bridge angles follow the lower infill layer and model rotation", "[PrintObject][InternalBridge][Regression]")
+{
+    const std::string pattern = GENERATE("hilbertcurve", "octagramspiral");
+    // Orca: Cover both a central line (odd counts) and offset pairs (even counts).
+    const int multiline = GENERATE(1, 2, 3);
+    CAPTURE(multiline);
+    const double rotation = GENERATE(23., -123.);
+    const std::vector<double> cycle{10., 30., 70.};
+    auto config = internal_bridge_config(pattern, multiline);
+    config.set_deserialize_strict({{"sparse_infill_rotate_template", "10,30,70"},
+                                   {"align_infill_direction_to_model", true},
+                                   {"separated_infills", false}});
+    Print print;
+    Model model;
+    init_print({internal_bridge_step()}, print, model, config, nullptr, false);
+    model.objects.front()->instances.front()->set_rotation(Vec3d(0., 0., Geometry::deg2rad(rotation)));
+    print.apply(model, config);
+    print.process();
+    const PrintObject &object = *print.objects().front();
+    size_t bridges = 0;
+    for (size_t i = 1; i < object.layer_count(); ++i) {
+        // Orca: The support is one layer below the bridge. Check the template and model
+        // rotation together, including normalization when the resulting angle is negative.
+        double expected = std::fmod(cycle[(i - 1) % cycle.size()] + 90. + rotation, 180.);
+        if (expected < 0.) expected += 180.;
+        for (const LayerRegion *region : object.get_layer(i)->regions())
+            for (const Surface *surface : region->fill_surfaces.filter_by_type(stInternalBridge)) {
+                CAPTURE(pattern, rotation, i);
+                CHECK_THAT(Geometry::rad2deg(surface->bridge_angle), Catch::Matchers::WithinAbs(expected, 0.001));
+                ++bridges;
+            }
+    }
+    REQUIRE(bridges > 0);
+}
+
+TEST_CASE("Turning infill does not replace the anchors of another region", "[PrintObject][InternalBridge][Regression]")
+{
+    // Orca: Keep the right-hand region fixed while changing the left-hand pattern in the
+    // same object. Its bridge areas must be independent of a previous candidate's anchors.
+    const int multiline = GENERATE(1, 2, 3);
+    CAPTURE(multiline);
+    auto right_bridges = [multiline](const std::string &left_pattern) {
+        auto config = internal_bridge_config(left_pattern, multiline);
+        Print print;
+        Model model;
+        init_print({internal_bridge_step()}, print, model, config, nullptr, false);
+        TriangleMesh right = internal_bridge_step();
+        right.translate(50, 0, 0);
+        ModelVolume *volume = model.objects.front()->add_volume(std::move(right));
+        volume->config.set_key_value("sparse_infill_pattern", new ConfigOptionEnum<InfillPattern>(ipRectilinear));
+        volume->config.set_key_value("infill_direction", new ConfigOptionFloat(17.));
+        print.apply(model, config);
+        print.process();
+        std::map<size_t, Polygons> result;
+        const PrintObject &object = *print.objects().front();
+        for (size_t i = 0; i < object.layer_count(); ++i)
+            for (const LayerRegion *region : object.get_layer(i)->regions())
+                if (region->region().config().infill_direction == 17.)
+                    polygons_append(result[i], to_polygons(region->fill_surfaces.filter_by_type(stInternalBridge)));
+        return result;
+    };
+    const auto baseline = right_bridges("rectilinear");
+    const auto actual = right_bridges(GENERATE("hilbertcurve", "octagramspiral"));
+    REQUIRE(actual.size() == baseline.size());
+    double total_area = 0.;
+    for (const auto &[layer, expected] : baseline) {
+        CAPTURE(layer);
+        const auto &polys = actual.at(layer);
+        CHECK(area(diff(expected, polys)) < scaled<double>(1.) * scaled<double>(1.) * 1e-6);
+        CHECK(area(diff(polys, expected)) < scaled<double>(1.) * scaled<double>(1.) * 1e-6);
+        total_area += area(expected);
+    }
+    REQUIRE(total_area > 0.);
+}
+
+TEST_CASE("Rounded internal bridges end on printed support", "[PrintObject][InternalBridge][Regression]")
+{
+    const std::string pattern = GENERATE("hilbertcurve", "octagramspiral");
+    const bool separated = GENERATE(false, true);
+    CAPTURE(pattern, separated);
+    auto config = internal_bridge_config(pattern, 1);
+    config.set_deserialize_strict({{"infill_wall_overlap", "0%"}, {"separated_infills", separated}});
+    TriangleMesh mesh = internal_bridge_step();
+    if (separated) {
+        TriangleMesh second = internal_bridge_step();
+        second.translate(50, 0, 0);
+        mesh.merge(second);
+    }
+    Print print;
+    Model model;
+    init_print({mesh}, print, model, config, nullptr, false);
+    print.process();
+
+    // Orca: Check final extrusion endpoints after polygon cleanup and fill generation.
+    // A correct bridge angle and correct sparse anchors alone do not guarantee contact.
+    const PrintObject &object = *print.objects().front();
+    size_t checked = 0;
+    for (size_t i = 1; i < object.layer_count(); ++i) {
+        Polygons support;
+        Polylines walls;
+        for (const LayerRegion *region : object.get_layer(i - 1)->regions()) {
+            region->perimeters.polygons_covered_by_width(support, 0.f);
+            region->fills.polygons_covered_by_width(support, 0.f);
+            region->perimeters.collect_polylines(walls);
+        }
+        REQUIRE_FALSE(support.empty());
+        const AABBTreeLines::LinesDistancer<Line> support_tree(to_lines(union_(support)));
+        const AABBTreeLines::LinesDistancer<Line> wall_tree(to_lines(walls));
+        for (const LayerRegion *region : object.get_layer(i)->regions())
+            for (const ExtrusionEntity *entity : region->fills.flatten().entities) {
+                if (entity->role() != erInternalBridgeInfill)
+                    continue;
+                const auto *path = dynamic_cast<const ExtrusionPath *>(entity);
+                REQUIRE(path != nullptr);
+                for (const Line &line : path->polyline.to_polyline().lines()) {
+                    // Orca: Sample span ends, excluding short connectors and wall overlap.
+                    if (line.length() < scale_(std::max(0.7, 3. * path->width)))
+                        continue;
+                    for (const Point &point : {line.a, line.b}) {
+                        if (wall_tree.distance_from_lines<false>(point) <= scale_(0.5))
+                            continue;
+                        CAPTURE(i, point.x(), point.y());
+                        const double gap = unscale<double>(support_tree.distance_from_lines<true>(point)) - 0.5 * path->width;
+                        CHECK(gap <= 0.1);
+                        ++checked;
+                    }
+                }
+            }
+    }
+    REQUIRE(checked > 0);
+}
+
+TEST_CASE("Enabling separated infill recomputes body origins", "[PrintObject][InternalBridge][Regression]")
+{
+    const std::string pattern = GENERATE("hilbertcurve", "octagramspiral", "archimedeanchords");
+    CAPTURE(pattern);
+    auto footprint = [&](bool reslice) {
+        auto config = internal_bridge_config(pattern, 2);
+        config.set_deserialize_strict({{"separated_infills", !reslice}});
+        TriangleMesh mesh = internal_bridge_step();
+        TriangleMesh second = internal_bridge_step();
+        second.translate(50, 0, 0);
+        mesh.merge(second);
+        Print print;
+        Model model;
+        init_print({mesh}, print, model, config, nullptr, false);
+        print.process();
+        if (reslice) {
+            // Orca: Enabling centering after a completed slice must rebuild the body
+            // origins now shared by bridge preparation and printed infill.
+            config.set_deserialize_strict({{"separated_infills", true}});
+            print.apply(model, config);
+            print.process();
+        }
+        Polygons result;
+        for (const LayerRegion *region : print.objects().front()->get_layer(4)->regions())
+            region->fills.polygons_covered_by_width(result, 0.f);
+        return union_(result);
+    };
+    const Polygons fresh = footprint(false);
+    const Polygons resliced = footprint(true);
+    REQUIRE_FALSE(fresh.empty());
+    CHECK(area(diff(fresh, resliced)) < scaled<double>(1.) * scaled<double>(1.) * 1e-6);
+    CHECK(area(diff(resliced, fresh)) < scaled<double>(1.) * scaled<double>(1.) * 1e-6);
+}
+
+TEST_CASE("Surface centering survives changes to separated infill settings", "[PrintObject][SurfaceInfill][Regression]")
+{
+    const std::string pattern = GENERATE("archimedeanchords", "octagramspiral");
+    const std::string initial_center = GENERATE("each_surface", "each_model", "each_assembly");
+    const std::string final_center = GENERATE("each_surface", "each_model", "each_assembly");
+    const bool separated = GENERATE(false, true);
+    const std::string top_order = GENERATE("default", "outward", "inward");
+    const std::string bottom_order = top_order == "outward" ? "inward" : top_order == "inward" ? "outward" : "default";
+    const std::string density = GENERATE("80%", "100%");
+    const bool change_center = initial_center != final_center;
+    CAPTURE(pattern, initial_center, final_center, separated, top_order, bottom_order, density);
+
+    auto config = DynamicPrintConfig::full_print_config();
+    config.set_deserialize_strict({{"top_surface_pattern", pattern},
+                                   {"bottom_surface_pattern", pattern},
+                                   {"top_surface_fill_order", top_order},
+                                   {"bottom_surface_fill_order", bottom_order},
+                                   {"top_surface_density", density},
+                                   {"bottom_surface_density", density},
+                                   {"center_of_surface_pattern", initial_center},
+                                   {"separated_infills", change_center ? separated : !separated},
+                                   {"sparse_infill_pattern", "rectilinear"},
+                                   {"sparse_infill_density", "15%"},
+                                   {"top_shell_layers", 2},
+                                   {"bottom_shell_layers", 2},
+                                   {"top_shell_thickness", 0},
+                                   {"bottom_shell_thickness", 0},
+                                   {"layer_height", 0.2},
+                                   {"initial_layer_print_height", 0.2}});
+
+    // Orca: Two disconnected bodies exercise per-body centering. The offset tower also
+    // makes each-surface and each-model centering differ on the top surfaces.
+    TriangleMesh mesh = make_cube(30, 24, 2);
+    TriangleMesh tower = make_cube(12, 10, 1);
+    tower.translate(4, 3, 2);
+    mesh.merge(tower);
+    TriangleMesh second = mesh;
+    second.translate(50, 0, 0);
+    mesh.merge(second);
+
+    // Orca: Equal footprints can hide reordered or reversed paths. Retain their point
+    // sequences and ordering protection to cover the directional surface behavior too.
+    struct SurfaceFillSnapshot {
+        std::map<bool, std::vector<Points>> paths;
+        bool protected_order = true;
+    };
+    auto surface_fills = [](const Print &print) {
+        std::map<std::pair<size_t, ExtrusionRole>, SurfaceFillSnapshot> result;
+        const PrintObject &object = *print.objects().front();
+        for (size_t i = 0; i < object.layer_count(); ++i) {
+            auto collect = [&](const auto &self, const ExtrusionEntity &entity, bool no_sort) -> void {
+                if (const auto *collection = dynamic_cast<const ExtrusionEntityCollection *>(&entity)) {
+                    for (const ExtrusionEntity *child : collection->entities)
+                        self(self, *child, no_sort || collection->no_sort);
+                } else if (entity.role() == erTopSolidInfill || entity.role() == erBottomSurface) {
+                    const auto *path = dynamic_cast<const ExtrusionPath *>(&entity);
+                    REQUIRE(path != nullptr);
+                    auto &snapshot = result[{i, entity.role()}];
+                    // Orca: The centered test model has one body on either side of X=0.
+                    // Their traversal order may vary; preserve path order within each body.
+                    Points points = path->polyline.to_polyline().points;
+                    REQUIRE_FALSE(points.empty());
+                    snapshot.paths[points.front().x() > 0].push_back(std::move(points));
+                    snapshot.protected_order &= no_sort && !path->can_reverse();
+                }
+            };
+            for (const LayerRegion *region : object.get_layer(i)->regions())
+                collect(collect, region->fills, false);
+        }
+        return result;
+    };
+
+    Print print;
+    Model model;
+    init_print({mesh}, print, model, config, nullptr, false);
+    print.process();
+    const auto initial = surface_fills(print);
+    config.set_deserialize_strict({{"center_of_surface_pattern", final_center}, {"separated_infills", separated}});
+    print.apply(model, config);
+    // Orca: Preparation owns the body origins, and its invalidation must also force
+    // regeneration of top/bottom extrusion paths, even when sparse infill is unchanged.
+    CHECK_FALSE(print.objects().front()->is_step_done(posPrepareInfill));
+    CHECK_FALSE(print.objects().front()->is_step_done(posInfill));
+    print.process();
+    const auto resliced = surface_fills(print);
+
+    Print fresh_print;
+    Model fresh_model;
+    init_print({mesh}, fresh_print, fresh_model, config, nullptr, false);
+    fresh_print.process();
+    const auto fresh = surface_fills(fresh_print);
+    REQUIRE_FALSE(fresh.empty());
+    REQUIRE(resliced.size() == fresh.size());
+    std::set<ExtrusionRole> roles;
+    bool changed_paths = false;
+    for (const auto &entry : fresh) {
+        CAPTURE(entry.first.first, entry.first.second);
+        REQUIRE_FALSE(entry.second.paths.empty());
+        roles.insert(entry.first.second);
+        REQUIRE(resliced.count(entry.first) == 1);
+        REQUIRE(initial.count(entry.first) == 1);
+        const auto &actual = resliced.at(entry.first);
+        const auto &expected = entry.second;
+        const auto &before = initial.at(entry.first);
+        CHECK((actual.paths == expected.paths));
+        if (!change_center)
+            CHECK((actual.paths == before.paths));
+        if (top_order != "default") {
+            CHECK(expected.protected_order);
+            CHECK(actual.protected_order);
+            CHECK(before.protected_order);
+        }
+        changed_paths |= expected.paths != before.paths;
+    }
+    CHECK(roles.count(erTopSolidInfill) == 1);
+    CHECK(roles.count(erBottomSurface) == 1);
+    // Orca: Guard against a vacuous comparison: changing surface centering must change
+    // the printed pattern, while toggling separated sparse infill must leave it alone.
+    CHECK(changed_paths == change_center);
+}
+
+TEST_CASE("Separated infill keeps fragmented and nested bodies independent", "[PrintObject][SurfaceInfill][Regression]")
+{
+    constexpr size_t grid_size = 8;
+    TriangleMesh mesh;
+    auto add_box = [&](double x, double y, double width, double depth) {
+        TriangleMesh box = make_cube(width, depth, 0.6);
+        box.translate(x, y, 0);
+        mesh.merge(box);
+    };
+    // Orca: Many small islands exercise spatial pruning and the tree's original
+    // island indices. A pillar inside a frame also overlaps its bounding box,
+    // but must remain a separate body because it lies entirely inside the hole.
+    for (size_t x = 0; x < grid_size; ++ x)
+        for (size_t y = 0; y < grid_size; ++ y)
+            add_box(6 * x, 6 * y, 3, 3);
+    add_box(54, 0, 20, 4);
+    add_box(54, 16, 20, 4);
+    add_box(54, 0, 4, 20);
+    add_box(70, 0, 4, 20);
+    add_box(62, 8, 4, 4);
+
+    auto config = DynamicPrintConfig::full_print_config();
+    config.set_deserialize_strict({{"separated_infills", true},
+                                   {"center_of_surface_pattern", "each_surface"},
+                                   {"layer_height", 0.2},
+                                   {"initial_layer_print_height", 0.2},
+                                   {"elefant_foot_compensation", 0},
+                                   {"wall_loops", 1}});
+    Print print;
+    Model model;
+    init_print({mesh}, print, model, config, nullptr, false);
+    // Orca: Prepare body bounds through the public pipeline, then inspect the object read-only.
+    print.process();
+    const PrintObject &object = *print.objects().front();
+    REQUIRE(object.layer_count() > 1);
+    for (const Layer *layer : object.layers()) {
+        REQUIRE(layer->lslices.size() == grid_size * grid_size + 2);
+        REQUIRE(layer->lslices_separated_component_bboxes.size() == layer->lslices.size());
+        size_t holes = 0;
+        for (size_t i = 0; i < layer->lslices.size(); ++ i) {
+            const BoundingBox &body = layer->lslices_separated_component_bboxes[i];
+            const BoundingBox &island = layer->lslices_bboxes[i];
+            CHECK(body.min == island.min);
+            CHECK(body.max == island.max);
+            holes += layer->lslices[i].holes.size();
+        }
+        CHECK(holes == 1);
+    }
+}
+
+TEST_CASE("Body centering survives islands merging and splitting between layers", "[PrintObject][SurfaceInfill][Regression]")
+{
+    const bool separated = GENERATE(false, true);
+    CAPTURE(separated);
+    // Orca: Four posts join through horizontal then vertical rails, creating a
+    // cycle of overlaps before splitting into four islands again. This exercises
+    // redundant connections and indexing either adjacent layer. A fifth post
+    // stays separate at every height.
+    TriangleMesh mesh;
+    for (int x : {0, 8})
+        for (int y : {0, 8}) {
+            TriangleMesh post = make_cube(4, 4, 1);
+            post.translate(x, y, 0);
+            mesh.merge(post);
+        }
+    for (int y : {0, 8}) {
+        TriangleMesh rail = make_cube(12, 4, 0.2);
+        rail.translate(0, y, 0.2);
+        mesh.merge(rail);
+    }
+    for (int x : {0, 8}) {
+        TriangleMesh rail = make_cube(4, 12, 0.2);
+        rail.translate(x, 0, 0.4);
+        mesh.merge(rail);
+    }
+    TriangleMesh isolated = make_cube(4, 4, 1);
+    isolated.translate(20, 0, 0);
+    mesh.merge(isolated);
+
+    auto config = DynamicPrintConfig::full_print_config();
+    config.set_deserialize_strict({{"separated_infills", separated},
+                                   {"center_of_surface_pattern", separated ? "each_surface" : "each_model"},
+                                   {"layer_height", 0.2},
+                                   {"initial_layer_print_height", 0.2},
+                                   {"elefant_foot_compensation", 0},
+                                   {"wall_loops", 1}});
+    Print print;
+    Model model;
+    init_print({mesh}, print, model, config, nullptr, false);
+    // Orca: Prepare body bounds through the public pipeline, then inspect the object read-only.
+    print.process();
+    const PrintObject &object = *print.objects().front();
+    REQUIRE(object.layer_count() == 5);
+    REQUIRE(object.get_layer(0)->lslices.size() == 5);
+    REQUIRE(object.get_layer(1)->lslices.size() == 3);
+    REQUIRE(object.get_layer(2)->lslices.size() == 3);
+    REQUIRE(object.get_layer(4)->lslices.size() == 5);
+
+    BoundingBox isolated_bbox = object.get_layer(0)->lslices_bboxes.front();
+    for (const BoundingBox &bbox : object.get_layer(0)->lslices_bboxes)
+        if (bbox.min.x() > isolated_bbox.min.x())
+            isolated_bbox = bbox;
+    BoundingBox connected_bbox;
+    for (const Layer *layer : object.layers())
+        for (const BoundingBox &bbox : layer->lslices_bboxes)
+            if (bbox.min.x() < isolated_bbox.min.x())
+                connected_bbox.merge(bbox);
+    for (const Layer *layer : object.layers()) {
+        REQUIRE(layer->lslices_separated_component_bboxes.size() == layer->lslices.size());
+        for (size_t i = 0; i < layer->lslices.size(); ++ i) {
+            const BoundingBox &expected = layer->lslices_bboxes[i].min.x() < isolated_bbox.min.x() ? connected_bbox : isolated_bbox;
+            const BoundingBox &actual = layer->lslices_separated_component_bboxes[i];
+            CHECK(actual.min == expected.min);
+            CHECK(actual.max == expected.max);
+        }
+    }
+}