diff --git a/src/libnest2d/include/libnest2d/backends/clipper/geometries.hpp b/src/libnest2d/include/libnest2d/backends/clipper/geometries.hpp
index 15eeb57b8..232668f61 100644
--- a/src/libnest2d/include/libnest2d/backends/clipper/geometries.hpp
+++ b/src/libnest2d/include/libnest2d/backends/clipper/geometries.hpp
@@ -331,8 +331,15 @@ inline std::vector<PolygonImpl> clipper_execute(
 
     auto processPoly = [&retv, &processHole](ClipperLib::PolyNode *pptr) {
         PolygonImpl poly;
-        poly.Contour.swap(pptr->Contour); auto front_p = poly.Contour.front();
-        poly.Contour.emplace_back(std::move(front_p));
+        poly.Contour.swap(pptr->Contour);
+
+        assert(!pptr->IsHole());
+
+        if(pptr->IsOpen()) {
+            auto front_p = poly.Contour.front();
+            poly.Contour.emplace_back(front_p);
+        }
+
         for(auto h : pptr->Childs) { processHole(h, poly); }
         retv.push_back(poly);
     };
@@ -340,8 +347,14 @@ inline std::vector<PolygonImpl> clipper_execute(
     processHole = [&processPoly](ClipperLib::PolyNode *pptr, PolygonImpl& poly)
     {
         poly.Holes.emplace_back(std::move(pptr->Contour));
-        auto front_p = poly.Holes.back().front();
-        poly.Holes.back().emplace_back(std::move(front_p));
+
+        assert(pptr->IsHole());
+
+        if(pptr->IsOpen()) {
+            auto front_p = poly.Holes.back().front();
+            poly.Holes.back().emplace_back(front_p);
+        }
+
         for(auto c : pptr->Childs) processPoly(c);
     };
 
diff --git a/src/libslic3r/Rasterizer/Rasterizer.cpp b/src/libslic3r/Rasterizer/Rasterizer.cpp
index ab584821f..496a584a2 100644
--- a/src/libslic3r/Rasterizer/Rasterizer.cpp
+++ b/src/libslic3r/Rasterizer/Rasterizer.cpp
@@ -71,16 +71,41 @@ public:
         clear();
     }
 
-    template<class Geometry> inline void draw(const Geometry &poly) {
+    void draw(const ExPolygon &poly) {
         agg::rasterizer_scanline_aa<> ras;
         agg::scanline_p8 scanlines;
 
-        auto&& path = to_path(poly);
+        auto&& path = to_path(poly.contour);
 
         if(m_o == Origin::TOP_LEFT) flipy(path);
 
         ras.add_path(path);
 
+        for(auto h : poly.holes) {
+            auto&& holepath = to_path(h);
+            if(m_o == Origin::TOP_LEFT) flipy(holepath);
+            ras.add_path(holepath);
+        }
+
+        agg::render_scanlines(ras, scanlines, m_renderer);
+    }
+
+    void draw(const ClipperLib::Polygon &poly) {
+        agg::rasterizer_scanline_aa<> ras;
+        agg::scanline_p8 scanlines;
+
+        auto&& path = to_path(poly.Contour);
+
+        if(m_o == Origin::TOP_LEFT) flipy(path);
+
+        ras.add_path(path);
+
+        for(auto h : poly.Holes) {
+            auto&& holepath = to_path(h);
+            if(m_o == Origin::TOP_LEFT) flipy(holepath);
+            ras.add_path(holepath);
+        }
+
         agg::render_scanlines(ras, scanlines, m_renderer);
     }
 
@@ -186,14 +211,12 @@ void Raster::clear()
 
 void Raster::draw(const ExPolygon &expoly)
 {
-    m_impl->draw(expoly.contour);
-    for(auto& h : expoly.holes) m_impl->draw(h);
+    m_impl->draw(expoly);
 }
 
 void Raster::draw(const ClipperLib::Polygon &poly)
 {
-    m_impl->draw(poly.Contour);
-    for(auto& h : poly.Holes) m_impl->draw(h);
+    m_impl->draw(poly);
 }
 
 void Raster::save(std::ostream& stream, Compression comp)
diff --git a/src/libslic3r/SLAPrint.cpp b/src/libslic3r/SLAPrint.cpp
index a5aa826c9..424881491 100644
--- a/src/libslic3r/SLAPrint.cpp
+++ b/src/libslic3r/SLAPrint.cpp
@@ -1084,9 +1084,15 @@ void SLAPrint::process()
         SpinMutex mutex;
         using Lock = std::lock_guard<SpinMutex>;
 
-//        for (PrintLayer& layer : m_printer_input)
-        auto printlayerfn = [this, get_all_polygons, polyunion, polydiff, areafn, area_fill, display_area, exp_time, init_exp_time, fast_tilt, slow_tilt, delta_fade_time,
-                            &mutex, &models_volume, &supports_volume, &estim_time, &slow_layers, &fast_layers, &fade_layer_time](size_t sliced_layer_cnt)
+        // Going to parallel:
+        auto printlayerfn = [this,
+                // functions and read only vars
+                get_all_polygons, polyunion, polydiff, areafn,
+                area_fill, display_area, exp_time, init_exp_time, fast_tilt, slow_tilt, delta_fade_time,
+
+                // write vars
+                &mutex, &models_volume, &supports_volume, &estim_time, &slow_layers,
+                &fast_layers, &fade_layer_time](size_t sliced_layer_cnt)
         {
             PrintLayer& layer = m_printer_input[sliced_layer_cnt];
 
@@ -1141,9 +1147,9 @@ void SLAPrint::process()
             }
 
             if(!supports_polygons.empty()) {
-                /*if(model_polygons.empty()) */supports_polygons = polyunion(supports_polygons);
-                /*else */ if(!model_polygons.empty())supports_polygons = polydiff(supports_polygons, model_polygons);
-                // allegedly, union of subject is done withing the diff
+                if(model_polygons.empty()) supports_polygons = polyunion(supports_polygons);
+                else supports_polygons = polydiff(supports_polygons, model_polygons);
+                // allegedly, union of subject is done withing the diff according to the pftPositive polyFillType
             }
 
             double layer_support_area = 0;
@@ -1190,8 +1196,9 @@ void SLAPrint::process()
             }
         };
 
-        for(size_t i = 0; i < m_printer_input.size(); ++i) printlayerfn(i);
-//        tbb::parallel_for<size_t, decltype(printlayerfn)>(0, m_printer_input.size(), printlayerfn);
+        // sequential version for debugging:
+        // for(size_t i = 0; i < m_printer_input.size(); ++i) printlayerfn(i);
+        tbb::parallel_for<size_t, decltype(printlayerfn)>(0, m_printer_input.size(), printlayerfn);
 
         m_print_statistics.support_used_material = supports_volume * SCALING_FACTOR * SCALING_FACTOR;
         m_print_statistics.objects_used_material = models_volume  * SCALING_FACTOR * SCALING_FACTOR;