No fuzzy skin on bridge like overhangs perimeters (#13891)

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Ian Bassi <ian.bassi@outlook.com>

src/libslic3r/Feature/FuzzySkin/FuzzySkin.cpp

@@ -464,6 +464,16 @@ void group_region_by_fuzzify(PerimeterGenerator& g)
         }
     }
 
+    g.fuzzy_supported_area.reset();
+    if ((g.has_fuzzy_skin || g.has_fuzzy_hole) && g.lower_slices != nullptr) {
+        coord_t max_thickness = 0;
+        for (const auto& region : regions)
+            if (should_fuzzify(region.config, g.layer_id, 0, true) || should_fuzzify(region.config, g.layer_id, 0, false))
+                max_thickness = std::max(max_thickness, region.config.thickness);
+        // Walls farther than a line width plus the noise amplitude from the layer below are bridging; keep them smooth.
+        g.fuzzy_supported_area = offset_ex(*g.lower_slices, float(g.ext_perimeter_flow.scaled_width() + max_thickness));
+    }
+
     if (regions.size() == 1) { // optimization
         g.regions_by_fuzzify.push_back({regions.front().config, {}});
         return;
@@ -560,13 +570,23 @@ static std::vector<MergedFuzzyRegion> collect_merged_fuzzy_regions(const std::ve
     return merged_regions;
 }
 
+// Afterwards an empty region means nothing to fuzzify, no longer full coverage.
+static void restrict_to_supported(std::vector<MergedFuzzyRegion>& merged_regions, const std::optional<ExPolygons>& supported)
+{
+    if (!supported)
+        return;
+    for (auto& merged_region : merged_regions)
+        merged_region.expolygons = merged_region.expolygons.empty() ? *supported : intersection_ex(merged_region.expolygons, *supported);
+}
+
 Polygon apply_fuzzy_skin(const Polygon& polygon, const PerimeterGenerator& perimeter_generator, const size_t loop_idx, const bool is_contour)
 {
     Polygon fuzzified;
 
     const auto  slice_z = perimeter_generator.slice_z;
     const auto& regions = perimeter_generator.regions_by_fuzzify;
-    if (regions.size() == 1) { // optimization
+    const auto& supported = perimeter_generator.fuzzy_supported_area;
+    if (regions.size() == 1 && !supported) { // optimization
         const auto& config  = regions.begin()->first;
         const bool  fuzzify = should_fuzzify(config, perimeter_generator.layer_id, loop_idx, is_contour);
         if (!fuzzify) {
@@ -590,7 +610,7 @@ Polygon apply_fuzzy_skin(const Polygon& polygon, const PerimeterGenerator& perim
     // Fast path: single merged region — apply directly without splitting
     if (merged_regions.size() == 1) {
         const auto& mr = merged_regions.front();
-        if (mr.expolygons.empty()) {
+        if (mr.expolygons.empty() && !supported) {
             fuzzified = polygon;
             fuzzy_polyline(fuzzified.points, true, slice_z, *mr.config);
             return fuzzified;
@@ -626,6 +646,8 @@ Polygon apply_fuzzy_skin(const Polygon& polygon, const PerimeterGenerator& perim
             if (!merged_regions[i].expolygons.empty() && !merged_regions[j].expolygons.empty())
                 merged_regions[i].expolygons = diff_ex(merged_regions[i].expolygons, merged_regions[j].expolygons);
 
+    restrict_to_supported(merged_regions, supported);
+
     // Split the loops into lines with different config, and fuzzy them separately
     fuzzified = polygon;
     for (const auto& r : merged_regions) {
@@ -689,7 +711,8 @@ void apply_fuzzy_skin(Arachne::ExtrusionLine* extrusion, const PerimeterGenerato
     const auto  slice_z = perimeter_generator.slice_z;
     const auto  layer_height = perimeter_generator.layer_height;
     const auto& regions = perimeter_generator.regions_by_fuzzify;
-    if (regions.size() == 1) { // optimization
+    const auto& supported = perimeter_generator.fuzzy_supported_area;
+    if (regions.size() == 1 && !supported) { // optimization
         const auto& config  = regions.begin()->first;
         const bool  fuzzify = should_fuzzify(config, perimeter_generator.layer_id, extrusion->inset_idx, is_contour);
         if (fuzzify)
@@ -703,7 +726,7 @@ void apply_fuzzy_skin(Arachne::ExtrusionLine* extrusion, const PerimeterGenerato
         if (!merged_regions.empty()) {
 
             // Fast path: single merged region — apply directly without splitting
-            if (merged_regions.size() == 1 && merged_regions.front().expolygons.empty()) {
+            if (merged_regions.size() == 1 && merged_regions.front().expolygons.empty() && !supported) {
                 fuzzy_extrusion_line(extrusion->junctions, slice_z, perimeter_generator.layer_height, *merged_regions.front().config, closed);
                 return;
             }
@@ -753,6 +776,8 @@ void apply_fuzzy_skin(Arachne::ExtrusionLine* extrusion, const PerimeterGenerato
                     if (!merged_regions[i].expolygons.empty() && !merged_regions[j].expolygons.empty())
                         merged_regions[i].expolygons = diff_ex(merged_regions[i].expolygons, merged_regions[j].expolygons);
 
+            restrict_to_supported(merged_regions, supported);
+
             // Split the loops into lines with different config, and fuzzy them separately
             for (const auto& r : merged_regions) {
                 const auto splitted = Algorithm::split_line(*extrusion, r.expolygons, false);

src/libslic3r/PerimeterGenerator.hpp

@@ -2,6 +2,7 @@
 #define slic3r_PerimeterGenerator_hpp_
 
 #include "libslic3r.h"
+#include <optional>
 #include <vector>
 #include "Layer.hpp"
 #include "Flow.hpp"
@@ -105,6 +106,8 @@ public:
     bool                                            has_fuzzy_hole = false;
     // Preserve construction order so overlap precedence remains deterministic.
     std::vector<std::pair<FuzzySkinConfig, ExPolygons>> regions_by_fuzzify;
+    // Area resting on the layer below, where fuzzy skin is allowed. Unset means no restriction.
+    std::optional<ExPolygons>                       fuzzy_supported_area;
     
     PerimeterGenerator(
         // Input:

tests/fff_print/test_perimeters.cpp

@@ -630,3 +630,97 @@ TEST_CASE("A lower layer sliver too thin to print does not support the wall abov
     // A rib that does get printed takes the 20mm outer wall running along it out of the overhangs.
     CHECK(printable < no_rib - scale_(15.));
 }
+
+namespace {
+
+// Every setting the fuzzy skin assertions below depend on.
+DynamicPrintConfig fuzzy_skin_config(const char *wall_generator)
+{
+    DynamicPrintConfig config = DynamicPrintConfig::full_print_config();
+    config.set_deserialize_strict({
+        { "wall_generator",             wall_generator },
+        { "layer_height",               0.2 },
+        { "initial_layer_print_height", 0.2 },
+        // One wall, so every wall point along the long sides belongs to the fuzzed outer wall.
+        { "wall_loops",                 1 },
+        { "fuzzy_skin",                 "external" },
+        { "fuzzy_skin_noise_type",      "classic" },
+        { "fuzzy_skin_thickness",       0.3 },
+        { "fuzzy_skin_point_distance",  0.8 },
+    });
+    return config;
+}
+
+// How far the wall points over the middle 60% of the layer's length stray across its width, worst side.
+// A negative result means there is no layer at `print_z`.
+double mid_span_wall_spread(const Print &print, double print_z)
+{
+    for (const Layer *layer : print.objects().front()->layers()) {
+        if (std::abs(layer->print_z - print_z) > 1e-4)
+            continue;
+        const BoundingBox bbox  = get_extents(layer->lslices);
+        const coord_t     x_min = bbox.min.x() + bbox.size().x() / 5;
+        const coord_t     x_max = bbox.max.x() - bbox.size().x() / 5;
+        Points            points;
+        for (const LayerRegion *region : layer->regions())
+            region->perimeters.collect_points(points);
+        coord_t spread = 0;
+        for (const bool south : { true, false }) {
+            coord_t lo = bbox.max.y(), hi = bbox.min.y();
+            for (const Point &p : points)
+                if (p.x() > x_min && p.x() < x_max && (p.y() < bbox.center().y()) == south) {
+                    lo = std::min(lo, p.y());
+                    hi = std::max(hi, p.y());
+                }
+            spread = std::max(spread, hi - lo);
+        }
+        return unscale<double>(spread);
+    }
+    return -1.;
+}
+
+} // namespace
+
+// TestMesh::bridge is a 50x10mm deck from z=5 to z=8 on two 5mm-wide pillars, leaving a 40mm span. The deck's
+// first layer (print_z 5.2) crosses the span unsupported; the layers above it rest on the deck.
+TEST_CASE("Fuzzy skin leaves the walls of a bridge smooth", "[Perimeters]")
+{
+    const char *wall_generator = GENERATE("classic", "arachne");
+    CAPTURE(wall_generator);
+
+    Print print;
+    init_and_process_print({ TestMesh::bridge }, print, fuzzy_skin_config(wall_generator));
+    REQUIRE_FALSE(print.objects().empty());
+
+    // Control: one deck layer up the same walls rest on the deck, so they are fuzzed.
+    CHECK(mid_span_wall_spread(print, 5.6) > 0.1);
+    // Over the unsupported span the walls stay straight.
+    const double bridged = mid_span_wall_spread(print, 5.2);
+    CHECK(bridged >= 0.);
+    CHECK(bridged < 0.001);
+}
+
+// One object: a 20x20x3mm block on the bed and a second one floating above it from z=5 to z=8. The layers in
+// the gap are empty, so the floating block's first layer (print_z 5.2) has a layer below it with nothing
+// printed on it; the layers above rest on the floating block.
+TEST_CASE("Fuzzy skin leaves the walls over an empty layer smooth", "[Perimeters]")
+{
+    const char *wall_generator = GENERATE("classic", "arachne");
+    CAPTURE(wall_generator);
+
+    TriangleMesh mesh     = make_cube(20., 20., 3.);
+    TriangleMesh floating = make_cube(20., 20., 3.);
+    floating.translate(0.f, 0.f, 5.f);
+    mesh.merge(floating);
+
+    Print print;
+    init_and_process_print({ mesh }, print, fuzzy_skin_config(wall_generator));
+    REQUIRE_FALSE(print.objects().empty());
+
+    // Control: one layer up the walls rest on the floating block, so they are fuzzed.
+    CHECK(mid_span_wall_spread(print, 5.6) > 0.1);
+    // Nothing is printed under the first floating layer, so its walls stay straight.
+    const double floating_first_layer = mid_span_wall_spread(print, 5.2);
+    CHECK(floating_first_layer >= 0.);
+    CHECK(floating_first_layer < 0.001);
+}