Fix some overhang detection (#15694)

src/libslic3r/LayerRegion.cpp

@@ -120,9 +120,6 @@ void LayerRegion::make_perimeters(const SurfaceCollection &slices, const LayerRe
         fill_no_overlap
     );
     
-    if (this->layer()->lower_layer != nullptr)
-        // Cummulative sum of polygons over all the regions.
-        g.lower_slices = &this->layer()->lower_layer->lslices;
     if (this->layer()->upper_layer != NULL)
         g.upper_slices = &this->layer()->upper_layer->lslices;
 
@@ -135,6 +132,13 @@ void LayerRegion::make_perimeters(const SurfaceCollection &slices, const LayerRe
     g.overhang_flow         = this->bridging_flow(frPerimeter, object_config.thick_bridges);
     g.solid_infill_flow     = this->flow(frSolidInfill);
 
+    // Cumulative sum of polygons over all the regions, less what the lower layer could not print.
+    ExPolygons lower_slices;
+    if (this->layer()->lower_layer != nullptr) {
+        lower_slices   = g.printable_slices(this->layer()->lower_layer->lslices);
+        g.lower_slices = &lower_slices;
+    }
+
     if (this->layer()->object()->config().wall_generator.value == PerimeterGeneratorType::Arachne && !spiral_mode)
         g.process_arachne();
     else

src/libslic3r/PerimeterGenerator.cpp

@@ -2885,6 +2885,21 @@ bool PerimeterGeneratorLoop::is_internal_contour() const
     return true;
 }
 
+// ORCA: Arachne drops features below min_feature_size, classic builds nothing thinner than a third of the
+// nozzle. Both describe the layer below, a union of regions sharing neither nozzle nor generator, so every
+// ambiguity resolves low: it may keep a sliver that was never printed, but it never drops one that was.
+ExPolygons PerimeterGenerator::printable_slices(const ExPolygons &slices) const
+{
+    double min_width = *std::min_element(print_config->nozzle_diameter.values.begin(),
+                                         print_config->nozzle_diameter.values.end()) / 3.;
+    if (object_config->wall_generator.value == PerimeterGeneratorType::Arachne) {
+        const double min_feature_size = Arachne::make_paths_params(layer_id, *object_config, *print_config).min_feature_size;
+        // Spiral vase can put a classic layer under an Arachne one, so there both limits apply.
+        min_width = print_config->spiral_mode ? std::min(min_width, min_feature_size) : min_feature_size;
+    }
+    return min_width > EPSILON ? opening_ex(slices, float(scale_(min_width / 2.))) : slices;
+}
+
 std::vector<Polygons> PerimeterGenerator::generate_lower_polygons_series(float width)
 {
     float nozzle_diameter = print_config->nozzle_diameter.get_at(config->outer_wall_filament_id - 1);

src/libslic3r/PerimeterGenerator.hpp

@@ -149,6 +149,8 @@ public:
     //BBS
     double      smaller_width_ext_mm3_per_mm()   const { return m_ext_mm3_per_mm_smaller_width; }
     Polygons    lower_slices_polygons() const { return m_lower_slices_polygons; }
+    // ORCA: the slices less the slivers the wall generator prints nothing for, so they never count as support.
+    ExPolygons  printable_slices(const ExPolygons &slices) const;
 
 private:
     std::vector<Polygons>     generate_lower_polygons_series(float width);