Fix thin wall fuzzy (#14309)

Co-authored-by: Rodrigo Faselli <162915171+RF47@users.noreply.github.com>

src/libslic3r/Feature/FuzzySkin/FuzzySkin.cpp

@@ -682,7 +682,7 @@ Polygon apply_fuzzy_skin(const Polygon& polygon, const PerimeterGenerator& perim
     return fuzzified;
 }
 
-void apply_fuzzy_skin(Arachne::ExtrusionLine* extrusion, const PerimeterGenerator& perimeter_generator, const bool is_contour)
+void apply_fuzzy_skin(Arachne::ExtrusionLine* extrusion, const PerimeterGenerator& perimeter_generator, const bool is_contour, const bool closed)
 {
     const auto  slice_z = perimeter_generator.slice_z;
     const auto& regions = perimeter_generator.regions_by_fuzzify;
@@ -690,7 +690,7 @@ void apply_fuzzy_skin(Arachne::ExtrusionLine* extrusion, const PerimeterGenerato
         const auto& config  = regions.begin()->first;
         const bool  fuzzify = should_fuzzify(config, perimeter_generator.layer_id, extrusion->inset_idx, is_contour);
         if (fuzzify)
-            fuzzy_extrusion_line(extrusion->junctions, slice_z, config);
+            fuzzy_extrusion_line(extrusion->junctions, slice_z, config, closed);
     } else {
         // Merge regions that produce identical fuzzy effects (differ only in type).
         // When the style (e.g. External) and a painted region (All) both fuzzify this loop
@@ -701,10 +701,19 @@ void apply_fuzzy_skin(Arachne::ExtrusionLine* extrusion, const PerimeterGenerato
 
             // Fast path: single merged region — apply directly without splitting
             if (merged_regions.size() == 1 && merged_regions.front().expolygons.empty()) {
-                fuzzy_extrusion_line(extrusion->junctions, slice_z, *merged_regions.front().config);
+                fuzzy_extrusion_line(extrusion->junctions, slice_z, *merged_regions.front().config, closed);
                 return;
             }
 
+            // Open path means this is a thin wall that collapsed into a single thick line, in this case the path will go exactly
+            // between the middle two sides of the object. And since the paint segmentation never goes beyond the middle line because
+            // it uses voronoi diagram, we need to expand the segmentation a little bit to make sure it covers the path.
+            if (!closed) {
+                for (auto& r : merged_regions) {
+                    r.expolygons = offset_ex(r.expolygons, perimeter_generator.ext_perimeter_flow.scaled_width() / 10);
+                }
+            }
+
 #ifdef DEBUG_FUZZY
             {
                 int i = 0;
@@ -752,7 +761,7 @@ void apply_fuzzy_skin(Arachne::ExtrusionLine* extrusion, const PerimeterGenerato
                 // Fuzzy splitted extrusion
                 if (std::all_of(splitted.begin(), splitted.end(), [](const Algorithm::SplitLineJunction& j) { return j.clipped; })) {
                     // The entire polygon is fuzzified
-                    fuzzy_extrusion_line(extrusion->junctions, slice_z, *r.config);
+                    fuzzy_extrusion_line(extrusion->junctions, slice_z, *r.config, closed);
                     continue;
                 } else {
                     const auto                              current_ext = extrusion->junctions;
@@ -803,7 +812,7 @@ void apply_fuzzy_skin(Arachne::ExtrusionLine* extrusion, const PerimeterGenerato
                     }
 
                     //Orca: ensure the loop is closed after fuzzy
-                    if (!extrusion->junctions.empty() && extrusion->junctions.front().p != extrusion->junctions.back().p) {
+                    if (closed && !extrusion->junctions.empty() && extrusion->junctions.front().p != extrusion->junctions.back().p) {
                         extrusion->junctions.back().p = extrusion->junctions.front().p;
                         extrusion->junctions.back().w = extrusion->junctions.front().w;
                     }

src/libslic3r/Feature/FuzzySkin/FuzzySkin.hpp

@@ -16,7 +16,7 @@ void group_region_by_fuzzify(PerimeterGenerator& g);
 bool should_fuzzify(const FuzzySkinConfig& config, int layer_id, size_t loop_idx, bool is_contour);
 
 Polygon apply_fuzzy_skin(const Polygon& polygon, const PerimeterGenerator& perimeter_generator, size_t loop_idx, bool is_contour);
-void    apply_fuzzy_skin(Arachne::ExtrusionLine* extrusion, const PerimeterGenerator& perimeter_generator, bool is_contour);
+void    apply_fuzzy_skin(Arachne::ExtrusionLine* extrusion, const PerimeterGenerator& perimeter_generator, bool is_contour, bool closed = true);
 
 } // namespace Slic3r::Feature::FuzzySkin
 

src/libslic3r/PerimeterGenerator.cpp

@@ -229,6 +229,22 @@ static ExtrusionEntityCollection traverse_loops(const PerimeterGenerator &perime
 
     // Append thin walls to the nearest-neighbor search (only for first iteration)
     if (! thin_walls.empty()) {
+        // Orca: apply fuzzy skin to thin walls as well
+        for (auto& thin_wall : thin_walls) {
+            // First, we convert the ThickPolyline into Arachne::ExtrusionLine so we could reuse our existing fuzzy code
+            Arachne::ExtrusionLine el(0, true);
+            el.junctions.reserve(thin_wall.points.size());
+            for (int i = 0; i < thin_wall.points.size(); i++) {
+                el.junctions.emplace_back(thin_wall.points[i], thin_wall.width[i], 0);
+            }
+
+            // Then we fuzzy it
+            apply_fuzzy_skin(&el, perimeter_generator, true, thin_wall.is_closed());
+
+            // Then convert the result back to ThickPolyline
+            thin_wall = Arachne::to_thick_polyline(el);
+        }
+
         variable_width(thin_walls, erExternalPerimeter, perimeter_generator.ext_perimeter_flow, coll.entities);
         thin_walls.clear();
     }