Fixed polygons_simplify() to correctly handle holes.

This fixes some missing tree / organic supports in the middle
of an object surrounded by walls all around.

src/libslic3r/Polygon.cpp

@@ -99,9 +99,11 @@ void Polygon::douglas_peucker(double tolerance)
     this->points = std::move(p);
 }
 
-// this only works on CCW polygons as CW will be ripped out by Clipper's simplify_polygons()
 Polygons Polygon::simplify(double tolerance) const
 {
+    // Works on CCW polygons only, CW contour will be reoriented to CCW by Clipper's simplify_polygons()!
+    assert(this->is_counter_clockwise());
+
     // repeat first point at the end in order to apply Douglas-Peucker
     // on the whole polygon
     Points points = this->points;
@@ -114,13 +116,6 @@ Polygons Polygon::simplify(double tolerance) const
     return simplify_polygons(pp);
 }
 
-void Polygon::simplify(double tolerance, Polygons &polygons) const
-{
-    Polygons pp = this->simplify(tolerance);
-    polygons.reserve(polygons.size() + pp.size());
-    polygons.insert(polygons.end(), pp.begin(), pp.end());
-}
-
 // Only call this on convex polygons or it will return invalid results
 void Polygon::triangulate_convex(Polygons* polygons) const
 {
@@ -562,6 +557,27 @@ void remove_collinear(Polygons &polys)
 		remove_collinear(poly);
 }
 
+Polygons polygons_simplify(const Polygons &source_polygons, double tolerance)
+{
+    Polygons out;
+    out.reserve(source_polygons.size());
+    for (const Polygon &source_polygon : source_polygons) {
+        // Run Douglas / Peucker simplification algorithm on an open polyline (by repeating the first point at the end of the polyline),
+        Points simplified = MultiPoint::_douglas_peucker(to_polyline(source_polygon).points, tolerance);
+        // then remove the last (repeated) point.
+        simplified.pop_back();
+        // Simplify the decimated contour by ClipperLib.
+        bool ccw = ClipperLib::Area(simplified) > 0.;
+        for (Points &path : ClipperLib::SimplifyPolygons(ClipperUtils::SinglePathProvider(simplified), ClipperLib::pftNonZero)) {
+            if (! ccw)
+                // ClipperLib likely reoriented negative area contours to become positive. Reverse holes back to CW.
+                std::reverse(path.begin(), path.end());
+            out.emplace_back(std::move(path));
+        }
+    }
+    return out;
+}
+
 // Do polygons match? If they match, they must have the same topology,
 // however their contours may be rotated.
 bool polygons_match(const Polygon &l, const Polygon &r)

src/libslic3r/Polygon.hpp

@@ -67,8 +67,8 @@ public:
     bool on_boundary(const Point &point, double eps) const
         { return (this->point_projection(point) - point).cast<double>().squaredNorm() < eps * eps; }
 
+    // Works on CCW polygons only, CW contour will be reoriented to CCW by Clipper's simplify_polygons()!
     Polygons simplify(double tolerance) const;
-    void simplify(double tolerance, Polygons &polygons) const;
     void densify(float min_length, std::vector<float>* lengths = nullptr);
     void triangulate_convex(Polygons* polygons) const;
     Point centroid() const;
@@ -148,14 +148,7 @@ inline void polygons_append(Polygons &dst, Polygons &&src)
     }
 }
 
-inline Polygons polygons_simplify(const Polygons &polys, double tolerance)
-{
-	Polygons out;
-	out.reserve(polys.size());
-	for (const Polygon &p : polys)
-		polygons_append(out, p.simplify(tolerance));
-	return out;
-}
+Polygons polygons_simplify(const Polygons &polys, double tolerance);
 
 inline void polygons_rotate(Polygons &polys, double angle)
 {
@@ -216,18 +209,22 @@ inline Lines to_lines(const Polygons &polys)
     return lines;
 }
 
-inline Polylines to_polylines(const Polygons &polys)
+inline Polyline to_polyline(const Polygon &polygon)
 {
-    Polylines polylines;
-    polylines.assign(polys.size(), Polyline());
-    size_t idx = 0;
-    for (Polygons::const_iterator it = polys.begin(); it != polys.end(); ++ it) {
-        Polyline &pl = polylines[idx ++];
-        pl.points = it->points;
-        pl.points.push_back(it->points.front());
-    }
-    assert(idx == polylines.size());
-    return polylines;
+    Polyline out;
+    out.points.reserve(polygon.size() + 1);
+    out.points.assign(polygon.points.begin(), polygon.points.end());
+    out.points.push_back(polygon.points.front());
+    return out;
+}
+
+inline Polylines to_polylines(const Polygons &polygons)
+{
+    Polylines out;
+    out.reserve(polygons.size());
+    for (const Polygon &polygon : polygons)
+        out.emplace_back(to_polyline(polygon));
+    return out;
 }
 
 inline Polylines to_polylines(Polygons &&polys)