Fix for to clipper::union without enough offset

src/clipper/clipper.cpp

@@ -166,6 +166,25 @@ double Area(const Path &poly)
   return -a * 0.5;
 }
 //------------------------------------------------------------------------------
+IntPoint Centroid(const Path& poly, double area)
+{
+    double x_temp = 0;
+    double y_temp = 0;
+
+    const int max = poly.size() - 1;
+    size_t i = 0;
+    for (; i < max; ++i)
+    {
+        size_t j = i + 1;
+        x_temp += (double)(poly[i].X + poly[j].X) * ((double)poly[i].X * poly[j].Y - (double)poly[j].X * poly[i].Y);
+        y_temp += (double)(poly[i].Y + poly[j].Y) * ((double)poly[i].X * poly[j].Y - (double)poly[j].X * poly[i].Y);
+    }
+    x_temp += (double)(poly[i].X + poly[0].X) * ((double)poly[i].X * poly[0].Y - (double)poly[0].X * poly[i].Y);
+    y_temp += (double)(poly[i].Y + poly[0].Y) * ((double)poly[i].X * poly[0].Y - (double)poly[0].X * poly[i].Y);
+
+    return IntPoint(x_temp / (6 * area), y_temp / (6 * area));
+}
+//------------------------------------------------------------------------------
 
 double Area(const OutRec &outRec)
 {

src/clipper/clipper.hpp

@@ -188,6 +188,7 @@ class PolyTree: public PolyNode
 };
 
 double Area(const Path &poly);
+IntPoint Centroid(const Path& poly, double area);
 inline bool Orientation(const Path &poly) { return Area(poly) >= 0; }
 int PointInPolygon(const IntPoint &pt, const Path &path);
 

src/libslic3r/ClipperUtils.cpp

@@ -579,6 +579,22 @@ T _clipper_do(const ClipperLib::ClipType     clipType,
     return retval;
 }
 
+bool test_path(const ClipperLib::Path &path) {
+
+    double area = std::abs(ClipperLib::Area(path));
+    // get highest dist, but as it's n² in complexity, i use 2*dist to center wich is 2n in complexity
+    ClipperLib::cInt max_dist_sqrd = 0;
+    ClipperLib::IntPoint centroid = ClipperLib::Centroid(path, area);
+    for (const ClipperLib::IntPoint& pt : path) {
+        // &0x3FFFFFFF to let  (dx * dx + dy * dy) be storable into a int64
+        ClipperLib::cInt dx = (pt.X - centroid.X) & 0x3FFFFFFF;
+        ClipperLib::cInt dy = (pt.Y - centroid.Y) & 0x3FFFFFFF;
+        ClipperLib::cInt dist_sqrd = (dx * dx + dy * dy);
+        max_dist_sqrd = std::max(max_dist_sqrd, dist_sqrd);
+    }
+    return (area < (SCALED_EPSILON + SCALED_EPSILON) * std::sqrt(max_dist_sqrd));
+}
+
 // Fix of #117: A large fractal pyramid takes ages to slice
 // The Clipper library has difficulties processing overlapping polygons.
 // Namely, the function ClipperLib::JoinCommonEdges() has potentially a terrible time complexity if the output
@@ -609,6 +625,24 @@ inline ClipperLib::PolyTree _clipper_do_polytree2(const ClipperLib::ClipType cli
     clipper.AddPaths(input_subject, ClipperLib::ptSubject, true);
     ClipperLib::PolyTree retval;
     clipper.Execute(ClipperLib::ctUnion, retval, fillType, fillType);
+
+    // if safety_offset_, remove too small polygons & holes
+    if (safety_offset_)
+        for (int idx_poly = 0; idx_poly < retval.ChildCount(); ++idx_poly) {
+            ClipperLib::PolyNode* ex_polygon = retval.Childs[idx_poly];
+            if (test_path(ex_polygon->Contour)) {
+                retval.Childs.erase(retval.Childs.begin() + idx_poly);
+                --idx_poly;
+            } else {
+                for (int i = 0; i < ex_polygon->ChildCount(); ++i)
+                {
+                    if (test_path(ex_polygon->Childs[i]->Contour)) {
+                        ex_polygon->Childs.erase(ex_polygon->Childs.begin() + i);
+                        --i;
+                    }
+                }
+            }
+        }
     return retval;
 }