Fix of #8472: The middle perimeter was missing for some specific configurations due to numeric rounding errors.

src/libslic3r/Arachne/BeadingStrategy/DistributedBeadingStrategy.cpp

@@ -21,7 +21,7 @@ DistributedBeadingStrategy::DistributedBeadingStrategy(const coord_t optimal_wid
     name = "DistributedBeadingStrategy";
 }
 
-DistributedBeadingStrategy::Beading DistributedBeadingStrategy::compute(coord_t thickness, coord_t bead_count) const
+DistributedBeadingStrategy::Beading DistributedBeadingStrategy::compute(const coord_t thickness, const coord_t bead_count) const
 {
     Beading ret;
 
@@ -41,17 +41,23 @@ DistributedBeadingStrategy::Beading DistributedBeadingStrategy::compute(coord_t
             weights[bead_idx] = getWeight(bead_idx);
 
         const float total_weight      = std::accumulate(weights.cbegin(), weights.cend(), 0.f);
+        coord_t     accumulated_width = 0;
         for (coord_t bead_idx = 0; bead_idx < bead_count; bead_idx++) {
             const float   weight_fraction          = weights[bead_idx] / total_weight;
             const coord_t splitup_left_over_weight = to_be_divided * weight_fraction;
-            const coord_t width = optimal_width + splitup_left_over_weight;
+            const coord_t width                    = (bead_idx == bead_count - 1) ? thickness - accumulated_width : optimal_width + splitup_left_over_weight;
+
+            // Be aware that toolpath_locations is computed by dividing the width by 2, so toolpath_locations
+            // could be off by 1 because of rounding errors.
             if (bead_idx == 0)
                 ret.toolpath_locations.emplace_back(width / 2);
             else
                 ret.toolpath_locations.emplace_back(ret.toolpath_locations.back() + (ret.bead_widths.back() + width) / 2);
             ret.bead_widths.emplace_back(width);
+            accumulated_width += width;
         }
         ret.left_over = 0;
+        assert((accumulated_width + ret.left_over) == thickness);
     } else if (bead_count == 2) {
         const coord_t outer_width = thickness / 2;
         ret.bead_widths.emplace_back(outer_width);
@@ -68,6 +74,13 @@ DistributedBeadingStrategy::Beading DistributedBeadingStrategy::compute(coord_t
         ret.left_over = thickness;
     }
 
+    assert(([&ret = std::as_const(ret), thickness]() -> bool {
+        coord_t total_bead_width = 0;
+        for (const coord_t &bead_width : ret.bead_widths)
+            total_bead_width += bead_width;
+        return (total_bead_width + ret.left_over) == thickness;
+    }()));
+
     return ret;
 }
 

src/libslic3r/Arachne/SkeletalTrapezoidation.cpp

@@ -525,7 +525,7 @@ void SkeletalTrapezoidation::constructFromPolygons(const Polygons& polys)
 #endif
 
 #ifdef ARACHNE_DEBUG
-    assert(is_voronoi_diagram_planar_intersection(voronoi_diagram));
+    assert(Geometry::VoronoiUtilsCgal::is_voronoi_diagram_planar_intersection(voronoi_diagram));
 #endif
 
     // Try to detect cases when some Voronoi vertex is missing and when
@@ -1911,7 +1911,10 @@ void SkeletalTrapezoidation::generateJunctions(ptr_vector_t<BeadingPropagation>&
         for (junction_idx = (std::max(size_t(1), beading->toolpath_locations.size()) - 1) / 2; junction_idx < num_junctions; junction_idx--)
         {
             coord_t bead_R = beading->toolpath_locations[junction_idx];
-            if (bead_R <= start_R)
+            // toolpath_locations computed inside DistributedBeadingStrategy be off by 1 because of rounding errors.
+            // In GH issue #8472, these roundings errors caused missing the middle extrusion.
+            // Adding some epsilon should help resolve those cases.
+            if (bead_R <= start_R + scaled<coord_t>(0.005))
             { // Junction coinciding with start node is used in this function call
                 break;
             }

tests/libslic3r/test_arachne.cpp

@@ -73,3 +73,32 @@ TEST_CASE("Arachne - Closed ExtrusionLine", "[ArachneClosedExtrusionLine]") {
             }
 }
 
+// This test case was distilled from GitHub issue #8472.
+// Where for wall_distribution_count == 3 sometime middle perimeter was missing.
+TEST_CASE("Arachne - Missing perimeter - #8472", "[ArachneMissingPerimeter8472]") {
+    Polygon poly = {
+        Point(-9000000,  8054793),
+        Point( 7000000,  8054793),
+        Point( 7000000, 10211874),
+        Point(-8700000, 10211874),
+        Point(-9000000,  9824444)
+    };
+
+    Polygons polygons    = {poly};
+    coord_t  spacing     = 437079;
+    coord_t  inset_count = 3;
+
+    PrintObjectConfig print_object_config = PrintObjectConfig::defaults();
+    print_object_config.wall_distribution_count.setInt(3);
+
+    Arachne::WallToolPaths wallToolPaths(polygons, spacing, spacing, inset_count, 0, print_object_config, PrintConfig::defaults());
+    wallToolPaths.generate();
+    std::vector<Arachne::VariableWidthLines> perimeters = wallToolPaths.getToolPaths();
+
+#ifdef ARACHNE_DEBUG_OUT
+    draw_extrusion(debug_out_path("arachne-missing-perimeter-8472.svg"), polygons, perimeters, union_ex(wallToolPaths.getInnerContour()));
+#endif
+
+    REQUIRE(perimeters.size() == 3);
+}
+