diff --git a/src/libslic3r/Brim.cpp b/src/libslic3r/Brim.cpp
index 62d886785..061cf1423 100644
--- a/src/libslic3r/Brim.cpp
+++ b/src/libslic3r/Brim.cpp
@@ -53,7 +53,7 @@ static ExPolygons get_print_object_bottom_layer_expolygons(const PrintObject &pr
 {
     ExPolygons ex_polygons;
     for (LayerRegion *region : print_object.layers().front()->regions())
-        Slic3r::append(ex_polygons, closing_ex(region->slices.surfaces, float(SCALED_EPSILON)));
+        Slic3r::append(ex_polygons, closing_ex(region->slices().surfaces, float(SCALED_EPSILON)));
     return ex_polygons;
 }
 
diff --git a/src/libslic3r/Color.cpp b/src/libslic3r/Color.cpp
index 4d3bc6793..6d8daa00a 100644
--- a/src/libslic3r/Color.cpp
+++ b/src/libslic3r/Color.cpp
@@ -218,7 +218,7 @@ ColorRGBA ColorRGBA::operator * (float value) const
 	for (size_t i = 0; i < 3; ++i) {
 		ret.m_data[i] = std::clamp(value * m_data[i], 0.0f, 1.0f);
 	}
-	ret.m_data[3] = this->m_data[3];
+	ret.m_data[3] = m_data[3];
 	return ret;
 }
 
diff --git a/src/libslic3r/ExtrusionEntityCollection.hpp b/src/libslic3r/ExtrusionEntityCollection.hpp
index 1ecda7dd5..a344bb4a9 100644
--- a/src/libslic3r/ExtrusionEntityCollection.hpp
+++ b/src/libslic3r/ExtrusionEntityCollection.hpp
@@ -44,7 +44,14 @@ public:
     }
     ~ExtrusionEntityCollection() override { clear(); }
     explicit operator ExtrusionPaths() const;
-    
+
+    ExtrusionEntitiesPtr::const_iterator    cbegin() const { return this->entities.cbegin(); }
+    ExtrusionEntitiesPtr::const_iterator    cend()   const { return this->entities.cend(); }
+    ExtrusionEntitiesPtr::const_iterator    begin()  const { return this->entities.cbegin(); }
+    ExtrusionEntitiesPtr::const_iterator    end()    const { return this->entities.cend(); }
+    ExtrusionEntitiesPtr::iterator          begin()        { return this->entities.begin(); }
+    ExtrusionEntitiesPtr::iterator          end()          { return this->entities.end(); }
+
     bool is_collection() const override { return true; }
     ExtrusionRole role() const override {
         ExtrusionRole out = erNone;
@@ -102,6 +109,9 @@ public:
         { Polygons out; this->polygons_covered_by_width(out, scaled_epsilon); return out; }
     Polygons polygons_covered_by_spacing(const float scaled_epsilon = 0.f) const
         { Polygons out; this->polygons_covered_by_spacing(out, scaled_epsilon); return out; }
+    size_t size() const { return entities.size(); }
+    // Recursively count paths and loops contained in this collection. 
+    // this->items_count() >= this->size()
     size_t items_count() const;
     /// Returns a flattened copy of this ExtrusionEntityCollection. That is, all of the items in its entities vector are not collections.
     /// You should be iterating over flatten().entities if you are interested in the underlying ExtrusionEntities (and don't care about hierarchy).
diff --git a/src/libslic3r/Fill/Fill.cpp b/src/libslic3r/Fill/Fill.cpp
index 8278db960..5ae71616c 100644
--- a/src/libslic3r/Fill/Fill.cpp
+++ b/src/libslic3r/Fill/Fill.cpp
@@ -121,8 +121,8 @@ std::vector<SurfaceFill> group_fills(const Layer &layer)
     bool 												has_internal_voids = false;
 	for (size_t region_id = 0; region_id < layer.regions().size(); ++ region_id) {
 		const LayerRegion  &layerm = *layer.regions()[region_id];
-		region_to_surface_params[region_id].assign(layerm.fill_surfaces.size(), nullptr);
-	    for (const Surface &surface : layerm.fill_surfaces.surfaces)
+		region_to_surface_params[region_id].assign(layerm.fill_surfaces().size(), nullptr);
+	    for (const Surface &surface : layerm.fill_surfaces())
 	        if (surface.surface_type == stInternalVoid)
 	        	has_internal_voids = true;
 	        else {
@@ -180,7 +180,7 @@ std::vector<SurfaceFill> group_fills(const Layer &layer)
 		        auto it_params = set_surface_params.find(params);
 		        if (it_params == set_surface_params.end())
 		        	it_params = set_surface_params.insert(it_params, params);
-		        region_to_surface_params[region_id][&surface - &layerm.fill_surfaces.surfaces.front()] = &(*it_params);
+		        region_to_surface_params[region_id][&surface - &layerm.fill_surfaces().surfaces.front()] = &(*it_params);
 		    }
 	}
 
@@ -192,9 +192,9 @@ std::vector<SurfaceFill> group_fills(const Layer &layer)
 
 	for (size_t region_id = 0; region_id < layer.regions().size(); ++ region_id) {
 		const LayerRegion &layerm = *layer.regions()[region_id];
-	    for (const Surface &surface : layerm.fill_surfaces.surfaces)
+	    for (const Surface &surface : layerm.fill_surfaces())
 	        if (surface.surface_type != stInternalVoid) {
-	        	const SurfaceFillParams *params = region_to_surface_params[region_id][&surface - &layerm.fill_surfaces.surfaces.front()];
+	        	const SurfaceFillParams *params = region_to_surface_params[region_id][&surface - &layerm.fill_surfaces().surfaces.front()];
 				if (params != nullptr) {
 	        		SurfaceFill &fill = surface_fills[params->idx];
                     if (fill.region_id == size_t(-1)) {
@@ -325,7 +325,7 @@ void export_group_fills_to_svg(const char *path, const std::vector<SurfaceFill>
 void Layer::make_fills(FillAdaptive::Octree* adaptive_fill_octree, FillAdaptive::Octree* support_fill_octree, FillLightning::Generator* lightning_generator)
 {
 	for (LayerRegion *layerm : m_regions)
-		layerm->fills.clear();
+		layerm->m_fills.clear();
 
 
 #ifdef SLIC3R_DEBUG_SLICE_PROCESSING
@@ -420,7 +420,7 @@ void Layer::make_fills(FillAdaptive::Octree* adaptive_fill_octree, FillAdaptive:
 		        }
 		        // Save into layer.
 				ExtrusionEntityCollection* eec = nullptr;
-		        m_regions[surface_fill.region_id]->fills.entities.push_back(eec = new ExtrusionEntityCollection());
+		        m_regions[surface_fill.region_id]->m_fills.entities.push_back(eec = new ExtrusionEntityCollection());
 		        // Only concentric fills are not sorted.
 		        eec->no_sort = f->no_sort();
                 if (params.use_arachne) {
@@ -453,16 +453,16 @@ void Layer::make_fills(FillAdaptive::Octree* adaptive_fill_octree, FillAdaptive:
     // The path type could be ExtrusionPath, ExtrusionLoop or ExtrusionEntityCollection.
     // Why the paths are unpacked?
 	for (LayerRegion *layerm : m_regions)
-	    for (const ExtrusionEntity *thin_fill : layerm->thin_fills.entities) {
+	    for (const ExtrusionEntity *thin_fill : layerm->thin_fills().entities) {
 	        ExtrusionEntityCollection &collection = *(new ExtrusionEntityCollection());
-	        layerm->fills.entities.push_back(&collection);
+	        layerm->m_fills.entities.push_back(&collection);
 	        collection.entities.push_back(thin_fill->clone());
 	    }
 
 #ifndef NDEBUG
 	for (LayerRegion *layerm : m_regions)
-	    for (size_t i = 0; i < layerm->fills.entities.size(); ++ i)
-    	    assert(dynamic_cast<ExtrusionEntityCollection*>(layerm->fills.entities[i]) != nullptr);
+	    for (const ExtrusionEntity *e : layerm->fills())
+    	    assert(dynamic_cast<const ExtrusionEntityCollection*>(e) != nullptr);
 #endif
 }
 
@@ -539,7 +539,7 @@ void Layer::make_ironing()
     double default_layer_height = this->object()->config().layer_height;
 
 	for (LayerRegion *layerm : m_regions)
-		if (! layerm->slices.empty()) {
+		if (! layerm->slices().empty()) {
 			IroningParams ironing_params;
 			const PrintRegionConfig &config = layerm->region().config();
 			if (config.ironing && 
@@ -602,7 +602,7 @@ void Layer::make_ironing()
 				if (iron_everything) {
 					// Check whether there is any non-solid hole in the regions.
 					bool internal_infill_solid = region_config.fill_density.value > 95.;
-					for (const Surface &surface : ironing_params.layerm->fill_surfaces.surfaces)
+					for (const Surface &surface : ironing_params.layerm->fill_surfaces())
 						if ((! internal_infill_solid && surface.surface_type == stInternal) || surface.surface_type == stInternalBridge || surface.surface_type == stInternalVoid) {
 							// Some fill region is not quite solid. Don't iron over the whole surface.
 							iron_completely = false;
@@ -611,10 +611,10 @@ void Layer::make_ironing()
 				}
 				if (iron_completely) {
 					// Iron everything. This is likely only good for solid transparent objects.
-					for (const Surface &surface : ironing_params.layerm->slices.surfaces)
+					for (const Surface &surface : ironing_params.layerm->slices())
 						polygons_append(polys, surface.expolygon);
 				} else {
-					for (const Surface &surface : ironing_params.layerm->slices.surfaces)
+					for (const Surface &surface : ironing_params.layerm->slices())
 						if (surface.surface_type == stTop || (iron_everything && surface.surface_type == stBottom))
 							// stBottomBridge is not being ironed on purpose, as it would likely destroy the bridges.
 							polygons_append(polys, surface.expolygon);
@@ -622,7 +622,7 @@ void Layer::make_ironing()
 				if (iron_everything && ! iron_completely) {
 					// Add solid fill surfaces. This may not be ideal, as one will not iron perimeters touching these
 					// solid fill surfaces, but it is likely better than nothing.
-					for (const Surface &surface : ironing_params.layerm->fill_surfaces.surfaces)
+					for (const Surface &surface : ironing_params.layerm->fill_surfaces())
 						if (surface.surface_type == stInternalSolid)
 							polygons_append(infills, surface.expolygon);
 				}
@@ -659,7 +659,7 @@ void Layer::make_ironing()
 	        if (! polylines.empty()) {
 		        // Save into layer.
 				ExtrusionEntityCollection *eec = nullptr;
-		        ironing_params.layerm->fills.entities.push_back(eec = new ExtrusionEntityCollection());
+		        ironing_params.layerm->m_fills.entities.push_back(eec = new ExtrusionEntityCollection());
 		        // Don't sort the ironing infill lines as they are monotonicly ordered.
 				eec->no_sort = true;
 		        extrusion_entities_append_paths(
diff --git a/src/libslic3r/Fill/FillAdaptive.cpp b/src/libslic3r/Fill/FillAdaptive.cpp
index 29b343db0..f935d0edf 100644
--- a/src/libslic3r/Fill/FillAdaptive.cpp
+++ b/src/libslic3r/Fill/FillAdaptive.cpp
@@ -316,9 +316,9 @@ std::pair<double, double> adaptive_fill_line_spacing(const PrintObject &print_ob
         for (const Layer *layer : print_object.layers())
             for (size_t region_id = 0; region_id < layer->regions().size(); ++ region_id) {
                 RegionFillData &rd = region_fill_data[region_id];
-                if (rd.has_adaptive_infill == Tristate::Maybe && ! layer->regions()[region_id]->fill_surfaces.empty())
+                if (rd.has_adaptive_infill == Tristate::Maybe && ! layer->regions()[region_id]->fill_surfaces().empty())
                     rd.has_adaptive_infill = Tristate::Yes;
-                if (rd.has_support_infill == Tristate::Maybe && ! layer->regions()[region_id]->fill_surfaces.empty())
+                if (rd.has_support_infill == Tristate::Maybe && ! layer->regions()[region_id]->fill_surfaces().empty())
                     rd.has_support_infill = Tristate::Yes;
             }
 
diff --git a/src/libslic3r/Fill/Lightning/Generator.cpp b/src/libslic3r/Fill/Lightning/Generator.cpp
index bd83bcfff..185cb60af 100644
--- a/src/libslic3r/Fill/Lightning/Generator.cpp
+++ b/src/libslic3r/Fill/Lightning/Generator.cpp
@@ -62,7 +62,7 @@ void Generator::generateInitialInternalOverhangs(const PrintObject &print_object
         throw_on_cancel_callback();
         Polygons infill_area_here;
         for (const LayerRegion* layerm : print_object.get_layer(layer_nr)->regions())
-            for (const Surface& surface : layerm->fill_surfaces.surfaces)
+            for (const Surface& surface : layerm->fill_surfaces())
                 if (surface.surface_type == stInternal || surface.surface_type == stInternalVoid)
                     append(infill_area_here, to_polygons(surface.expolygon));
 
@@ -93,7 +93,7 @@ void Generator::generateTrees(const PrintObject &print_object, const std::functi
     for (int layer_id = int(print_object.layers().size()) - 1; layer_id >= 0; layer_id--) {
         throw_on_cancel_callback();
         for (const LayerRegion *layerm : print_object.get_layer(layer_id)->regions())
-            for (const Surface &surface : layerm->fill_surfaces.surfaces)
+            for (const Surface &surface : layerm->fill_surfaces())
                 if (surface.surface_type == stInternal || surface.surface_type == stInternalVoid)
                     append(infill_outlines[layer_id], to_polygons(surface.expolygon));
 
diff --git a/src/libslic3r/GCode.cpp b/src/libslic3r/GCode.cpp
index e2bd6ef65..c45b6daa9 100644
--- a/src/libslic3r/GCode.cpp
+++ b/src/libslic3r/GCode.cpp
@@ -849,7 +849,7 @@ namespace DoExport {
 	                    region.config().get_abs_value("small_perimeter_speed") == 0 ||
 	                    region.config().get_abs_value("external_perimeter_speed") == 0 ||
 	                    region.config().get_abs_value("bridge_speed") == 0)
-	                    mm3_per_mm.push_back(layerm->perimeters.min_mm3_per_mm());
+	                    mm3_per_mm.push_back(layerm->perimeters().min_mm3_per_mm());
 	                if (region.config().get_abs_value("infill_speed") == 0 ||
 	                    region.config().get_abs_value("solid_infill_speed") == 0 ||
 	                    region.config().get_abs_value("top_solid_infill_speed") == 0 ||
@@ -866,7 +866,7 @@ namespace DoExport {
                             return min;
                         };
 
-                        mm3_per_mm.push_back(min_mm3_per_mm_no_ironing(layerm->fills));
+                        mm3_per_mm.push_back(min_mm3_per_mm_no_ironing(layerm->fills()));
                     }
 	            }
 	        }
@@ -1502,7 +1502,7 @@ void GCode::process_layers(
             }
         });
     const auto spiral_vase = tbb::make_filter<LayerResult, LayerResult>(slic3r_tbb_filtermode::serial_in_order,
-        [&spiral_vase = *this->m_spiral_vase](LayerResult in) -> LayerResult {
+        [&spiral_vase = *m_spiral_vase](LayerResult in) -> LayerResult {
             if (in.nop_layer_result)
                 return in;
 
@@ -1510,18 +1510,18 @@ void GCode::process_layers(
             return { spiral_vase.process_layer(std::move(in.gcode)), in.layer_id, in.spiral_vase_enable, in.cooling_buffer_flush};
         });
     const auto pressure_equalizer = tbb::make_filter<LayerResult, LayerResult>(slic3r_tbb_filtermode::serial_in_order,
-        [&pressure_equalizer = *this->m_pressure_equalizer](LayerResult in) -> LayerResult {
+        [&pressure_equalizer = *m_pressure_equalizer](LayerResult in) -> LayerResult {
             return pressure_equalizer.process_layer(std::move(in));
         });
     const auto cooling = tbb::make_filter<LayerResult, std::string>(slic3r_tbb_filtermode::serial_in_order,
-        [&cooling_buffer = *this->m_cooling_buffer](LayerResult in) -> std::string {
+        [&cooling_buffer = *m_cooling_buffer](LayerResult in) -> std::string {
              if (in.nop_layer_result)
                 return in.gcode;
 
              return cooling_buffer.process_layer(std::move(in.gcode), in.layer_id, in.cooling_buffer_flush);
         });
     const auto find_replace = tbb::make_filter<std::string, std::string>(slic3r_tbb_filtermode::serial_in_order,
-        [&self = *this->m_find_replace](std::string s) -> std::string {
+        [&self = *m_find_replace](std::string s) -> std::string {
             return self.process_layer(std::move(s));
         });
     const auto output = tbb::make_filter<std::string, void>(slic3r_tbb_filtermode::serial_in_order,
@@ -1584,24 +1584,24 @@ void GCode::process_layers(
             }
         });
     const auto spiral_vase = tbb::make_filter<LayerResult, LayerResult>(slic3r_tbb_filtermode::serial_in_order,
-        [&spiral_vase = *this->m_spiral_vase](LayerResult in)->LayerResult {
+        [&spiral_vase = *m_spiral_vase](LayerResult in)->LayerResult {
             if (in.nop_layer_result)
                 return in;
             spiral_vase.enable(in.spiral_vase_enable);
             return { spiral_vase.process_layer(std::move(in.gcode)), in.layer_id, in.spiral_vase_enable, in.cooling_buffer_flush };
         });
     const auto pressure_equalizer = tbb::make_filter<LayerResult, LayerResult>(slic3r_tbb_filtermode::serial_in_order,
-        [&pressure_equalizer = *this->m_pressure_equalizer](LayerResult in) -> LayerResult {
+        [&pressure_equalizer = *m_pressure_equalizer](LayerResult in) -> LayerResult {
              return pressure_equalizer.process_layer(std::move(in));
         });
     const auto cooling = tbb::make_filter<LayerResult, std::string>(slic3r_tbb_filtermode::serial_in_order,
-        [&cooling_buffer = *this->m_cooling_buffer](LayerResult in)->std::string {
+        [&cooling_buffer = *m_cooling_buffer](LayerResult in)->std::string {
             if (in.nop_layer_result)
                 return in.gcode;
             return cooling_buffer.process_layer(std::move(in.gcode), in.layer_id, in.cooling_buffer_flush);
         });
     const auto find_replace = tbb::make_filter<std::string, std::string>(slic3r_tbb_filtermode::serial_in_order,
-        [&self = *this->m_find_replace](std::string s) -> std::string {
+        [&self = *m_find_replace](std::string s) -> std::string {
             return self.process_layer(std::move(s));
         });
     const auto output = tbb::make_filter<std::string, void>(slic3r_tbb_filtermode::serial_in_order,
@@ -2108,8 +2108,8 @@ LayerResult GCode::process_layer(
         if (enable) {
             for (const LayerRegion *layer_region : layer.regions())
                 if (size_t(layer_region->region().config().bottom_solid_layers.value) > layer.id() ||
-                    layer_region->perimeters.items_count() > 1u ||
-                    layer_region->fills.items_count() > 0) {
+                    layer_region->perimeters().items_count() > 1u ||
+                    layer_region->fills().items_count() > 0) {
                     enable = false;
                     break;
                 }
@@ -2252,20 +2252,11 @@ LayerResult GCode::process_layer(
             //   option
             // (Still, we have to keep track of regions because we need to apply their config)
             size_t n_slices = layer.lslices.size();
-            const std::vector<BoundingBox> &layer_surface_bboxes = layer.lslices_bboxes;
+            const LayerSlices &layer_surfaces = layer.lslices_ex;
             // Traverse the slices in an increasing order of bounding box size, so that the islands inside another islands are tested first,
             // so we can just test a point inside ExPolygon::contour and we may skip testing the holes.
-            std::vector<size_t> slices_test_order;
-            slices_test_order.reserve(n_slices);
-            for (size_t i = 0; i < n_slices; ++ i)
-                slices_test_order.emplace_back(i);
-            std::sort(slices_test_order.begin(), slices_test_order.end(), [&layer_surface_bboxes](size_t i, size_t j) {
-                const Vec2d s1 = layer_surface_bboxes[i].size().cast<double>();
-                const Vec2d s2 = layer_surface_bboxes[j].size().cast<double>();
-                return s1.x() * s1.y() < s2.x() * s2.y();
-            });
-            auto point_inside_surface = [&layer, &layer_surface_bboxes](const size_t i, const Point &point) {
-                const BoundingBox &bbox = layer_surface_bboxes[i];
+            auto point_inside_surface = [&layer, &layer_surfaces](const size_t i, const Point &point) {
+                const BoundingBox &bbox = layer_surfaces[i].bbox;
                 return point(0) >= bbox.min(0) && point(0) < bbox.max(0) &&
                        point(1) >= bbox.min(1) && point(1) < bbox.max(1) &&
                        layer.lslices[i].contour.contains(point);
@@ -2284,7 +2275,7 @@ LayerResult GCode::process_layer(
                 // The process is almost the same for perimeters and infills - we will do it in a cycle that repeats twice:
                 std::vector<unsigned int> printing_extruders;
                 for (const ObjectByExtruder::Island::Region::Type entity_type : { ObjectByExtruder::Island::Region::INFILL, ObjectByExtruder::Island::Region::PERIMETERS }) {
-                    for (const ExtrusionEntity *ee : (entity_type == ObjectByExtruder::Island::Region::INFILL) ? layerm->fills.entities : layerm->perimeters.entities) {
+                    for (const ExtrusionEntity *ee : (entity_type == ObjectByExtruder::Island::Region::INFILL) ? layerm->fills() : layerm->perimeters()) {
                         // extrusions represents infill or perimeter extrusions of a single island.
                         assert(dynamic_cast<const ExtrusionEntityCollection*>(ee) != nullptr);
                         const auto *extrusions = static_cast<const ExtrusionEntityCollection*>(ee);
@@ -2329,7 +2320,9 @@ LayerResult GCode::process_layer(
                                 layers.size(), n_slices+1);
                             for (size_t i = 0; i <= n_slices; ++ i) {
                                 bool   last = i == n_slices;
-                                size_t island_idx = last ? n_slices : slices_test_order[i];
+                                // Traverse lslices back to front: lslices are produced by traversing ClipperLib::PolyTree, emitting parent contour before its children.
+                                // Therefore traversing layer_surfaces back to front will traverse children contours before their parents.
+                                size_t island_idx = last ? n_slices : layer_surfaces.size() - i - 1;
                                 if (// extrusions->first_point does not fit inside any slice
                                     last ||
                                     // extrusions->first_point fits inside ith slice
diff --git a/src/libslic3r/GCode/AvoidCrossingPerimeters.cpp b/src/libslic3r/GCode/AvoidCrossingPerimeters.cpp
index 9edb35ee8..c866e13e4 100644
--- a/src/libslic3r/GCode/AvoidCrossingPerimeters.cpp
+++ b/src/libslic3r/GCode/AvoidCrossingPerimeters.cpp
@@ -487,7 +487,7 @@ static float get_perimeter_spacing(const Layer &layer)
     size_t regions_count     = 0;
     float  perimeter_spacing = 0.f;
     for (const LayerRegion *layer_region : layer.regions())
-        if (layer_region != nullptr && !layer_region->slices.empty()) {
+        if (layer_region != nullptr && ! layer_region->slices().empty()) {
             perimeter_spacing += layer_region->flow(frPerimeter).scaled_spacing();
             ++regions_count;
         }
@@ -508,7 +508,7 @@ static float get_perimeter_spacing_external(const Layer &layer)
     for (const PrintObject *object : layer.object()->print()->objects())
         if (const Layer *l = object->get_layer_at_printz(layer.print_z, EPSILON); l)
             for (const LayerRegion *layer_region : l->regions())
-                if (layer_region != nullptr && !layer_region->slices.empty()) {
+                if (layer_region != nullptr && ! layer_region->slices().empty()) {
                     perimeter_spacing += layer_region->flow(frPerimeter).scaled_spacing();
                     ++ regions_count;
                 }
@@ -527,7 +527,7 @@ static float get_external_perimeter_width(const Layer &layer)
     size_t regions_count     = 0;
     float  perimeter_width   = 0.f;
     for (const LayerRegion *layer_region : layer.regions())
-        if (layer_region != nullptr && !layer_region->slices.empty()) {
+        if (layer_region != nullptr && ! layer_region->slices().empty()) {
             perimeter_width += float(layer_region->flow(frExternalPerimeter).scaled_width());
             ++regions_count;
         }
@@ -1070,14 +1070,14 @@ static ExPolygons get_boundary(const Layer &layer)
     // Collect all top layers that will not be crossed.
     size_t      polygons_count    = 0;
     for (const LayerRegion *layer_region : layer.regions())
-        for (const Surface &surface : layer_region->fill_surfaces.surfaces)
+        for (const Surface &surface : layer_region->fill_surfaces())
             if (surface.is_top()) ++polygons_count;
 
     if (polygons_count > 0) {
         ExPolygons top_layer_polygons;
         top_layer_polygons.reserve(polygons_count);
         for (const LayerRegion *layer_region : layer.regions())
-            for (const Surface &surface : layer_region->fill_surfaces.surfaces)
+            for (const Surface &surface : layer_region->fill_surfaces())
                 if (surface.is_top()) top_layer_polygons.emplace_back(surface.expolygon);
 
         top_layer_polygons = union_ex(top_layer_polygons);
diff --git a/src/libslic3r/GCode/GCodeProcessor.cpp b/src/libslic3r/GCode/GCodeProcessor.cpp
index c82fd38e5..d9492cb2a 100644
--- a/src/libslic3r/GCode/GCodeProcessor.cpp
+++ b/src/libslic3r/GCode/GCodeProcessor.cpp
@@ -2735,8 +2735,8 @@ void GCodeProcessor::process_G1(const GCodeReader::GCodeLine& line)
 
     if (volume_extruded_filament != 0.)
         m_used_filaments.increase_caches(volume_extruded_filament,
-            this->m_extruder_id, area_filament_cross_section * this->m_parking_position,
-            area_filament_cross_section * this->m_extra_loading_move);
+            m_extruder_id, area_filament_cross_section * m_parking_position,
+            area_filament_cross_section * m_extra_loading_move);
 
     const EMoveType type = move_type(delta_pos);
     if (type == EMoveType::Extrude) {
@@ -4303,7 +4303,7 @@ void GCodeProcessor::process_filaments(CustomGCode::Type code)
         m_used_filaments.process_color_change_cache();
 
     if (code == CustomGCode::ToolChange)
-        m_used_filaments.process_extruder_cache(this->m_extruder_id);
+        m_used_filaments.process_extruder_cache(m_extruder_id);
 }
 
 void GCodeProcessor::simulate_st_synchronize(float additional_time)
diff --git a/src/libslic3r/GCode/PressureEqualizer.cpp b/src/libslic3r/GCode/PressureEqualizer.cpp
index 7135c0a5c..c8a9618ef 100644
--- a/src/libslic3r/GCode/PressureEqualizer.cpp
+++ b/src/libslic3r/GCode/PressureEqualizer.cpp
@@ -690,7 +690,7 @@ inline bool is_just_line_with_extrude_set_speed_tag(const std::string &line)
 
 void PressureEqualizer::push_line_to_output(const size_t line_idx, const float new_feedrate, const char *comment)
 {
-    const GCodeLine &line = this->m_gcode_lines[line_idx];
+    const GCodeLine &line = m_gcode_lines[line_idx];
     if (line_idx > 0 && output_buffer_length > 0) {
         const std::string prev_line_str = std::string(output_buffer.begin() + int(this->output_buffer_prev_length),
                                                       output_buffer.begin() + int(this->output_buffer_length) + 1);
diff --git a/src/libslic3r/GCode/PrintExtents.cpp b/src/libslic3r/GCode/PrintExtents.cpp
index a86411519..9a01dc8a6 100644
--- a/src/libslic3r/GCode/PrintExtents.cpp
+++ b/src/libslic3r/GCode/PrintExtents.cpp
@@ -113,8 +113,8 @@ BoundingBoxf get_print_object_extrusions_extents(const PrintObject &print_object
             break;
         BoundingBoxf bbox_this;
         for (const LayerRegion *layerm : layer->regions()) {
-            bbox_this.merge(extrusionentity_extents(layerm->perimeters));
-            for (const ExtrusionEntity *ee : layerm->fills.entities)
+            bbox_this.merge(extrusionentity_extents(layerm->perimeters()));
+            for (const ExtrusionEntity *ee : layerm->fills())
                 // fill represents infill extrusions of a single island.
                 bbox_this.merge(extrusionentity_extents(*dynamic_cast<const ExtrusionEntityCollection*>(ee)));
         }
diff --git a/src/libslic3r/GCode/SeamPlacer.cpp b/src/libslic3r/GCode/SeamPlacer.cpp
index 1d7fd5b3c..c92ff8212 100644
--- a/src/libslic3r/GCode/SeamPlacer.cpp
+++ b/src/libslic3r/GCode/SeamPlacer.cpp
@@ -401,7 +401,7 @@ struct GlobalModelInfo {
 Polygons extract_perimeter_polygons(const Layer *layer, std::vector<const LayerRegion*> &corresponding_regions_out) {
     Polygons polygons;
     for (const LayerRegion *layer_region : layer->regions()) {
-        for (const ExtrusionEntity *ex_entity : layer_region->perimeters.entities) {
+        for (const ExtrusionEntity *ex_entity : layer_region->perimeters()) {
             if (ex_entity->is_collection()) { //collection of inner, outer, and overhang perimeters
                 for (const ExtrusionEntity *perimeter : static_cast<const ExtrusionEntityCollection*>(ex_entity)->entities) {
                     ExtrusionRole role = perimeter->role();
@@ -1060,7 +1060,7 @@ void SeamPlacer::calculate_overhangs_and_layer_embedding(const PrintObject *po)
                 for (size_t layer_idx = r.begin(); layer_idx < r.end(); ++layer_idx) {
                     size_t regions_with_perimeter = 0;
                     for (const LayerRegion *region : po->layers()[layer_idx]->regions()) {
-                        if (region->perimeters.entities.size() > 0) {
+                        if (region->perimeters().size() > 0) {
                             regions_with_perimeter++;
                         }
                     };
diff --git a/src/libslic3r/GCode/ToolOrdering.cpp b/src/libslic3r/GCode/ToolOrdering.cpp
index c5554c2fa..6c5d0667a 100644
--- a/src/libslic3r/GCode/ToolOrdering.cpp
+++ b/src/libslic3r/GCode/ToolOrdering.cpp
@@ -226,12 +226,12 @@ void ToolOrdering::collect_extruders(const PrintObject &object, const std::vecto
         for (const LayerRegion *layerm : layer->regions()) {
             const PrintRegion &region = layerm->region();
 
-            if (! layerm->perimeters.entities.empty()) {
+            if (! layerm->perimeters().empty()) {
                 bool something_nonoverriddable = true;
 
                 if (m_print_config_ptr) { // in this case complete_objects is false (see ToolOrdering constructors)
                     something_nonoverriddable = false;
-                    for (const auto& eec : layerm->perimeters.entities) // let's check if there are nonoverriddable entities
+                    for (const ExtrusionEntity *eec : layerm->perimeters()) // let's check if there are nonoverriddable entities
                         if (!layer_tools.wiping_extrusions().is_overriddable_and_mark(dynamic_cast<const ExtrusionEntityCollection&>(*eec), *m_print_config_ptr, object, region))
                             something_nonoverriddable = true;
                 }
@@ -245,7 +245,7 @@ void ToolOrdering::collect_extruders(const PrintObject &object, const std::vecto
             bool has_infill       = false;
             bool has_solid_infill = false;
             bool something_nonoverriddable = false;
-            for (const ExtrusionEntity *ee : layerm->fills.entities) {
+            for (const ExtrusionEntity *ee : layerm->fills()) {
                 // fill represents infill extrusions of a single island.
                 const auto *fill = dynamic_cast<const ExtrusionEntityCollection*>(ee);
                 ExtrusionRole role = fill->entities.empty() ? erNone : fill->entities.front()->role();
@@ -692,7 +692,7 @@ float WipingExtrusions::mark_wiping_extrusions(const Print& print, unsigned int
 
                 bool wipe_into_infill_only = ! object->config().wipe_into_objects && region.config().wipe_into_infill;
                 if (print.config().infill_first != perimeters_done || wipe_into_infill_only) {
-                    for (const ExtrusionEntity* ee : layerm->fills.entities) {                      // iterate through all infill Collections
+                    for (const ExtrusionEntity* ee : layerm->fills()) { // iterate through all infill Collections
                         auto* fill = dynamic_cast<const ExtrusionEntityCollection*>(ee);
 
                         if (!is_overriddable(*fill, print.config(), *object, region))
@@ -716,7 +716,7 @@ float WipingExtrusions::mark_wiping_extrusions(const Print& print, unsigned int
                 // Now the same for perimeters - see comments above for explanation:
                 if (object->config().wipe_into_objects && print.config().infill_first == perimeters_done)
                 {
-                    for (const ExtrusionEntity* ee : layerm->perimeters.entities) {
+                    for (const ExtrusionEntity* ee : layerm->perimeters()) {
                         auto* fill = dynamic_cast<const ExtrusionEntityCollection*>(ee);
                         if (is_overriddable(*fill, print.config(), *object, region) && !is_entity_overridden(fill, copy) && fill->total_volume() > min_infill_volume) {
                             set_extruder_override(fill, copy, new_extruder, num_of_copies);
@@ -762,7 +762,7 @@ void WipingExtrusions::ensure_perimeters_infills_order(const Print& print)
                 if (!region.config().wipe_into_infill && !object->config().wipe_into_objects)
                     continue;
 
-                for (const ExtrusionEntity* ee : layerm->fills.entities) {                      // iterate through all infill Collections
+                for (const ExtrusionEntity* ee : layerm->fills()) { // iterate through all infill Collections
                     auto* fill = dynamic_cast<const ExtrusionEntityCollection*>(ee);
 
                     if (!is_overriddable(*fill, print.config(), *object, region)
@@ -785,7 +785,7 @@ void WipingExtrusions::ensure_perimeters_infills_order(const Print& print)
                 }
 
                 // Now the same for perimeters - see comments above for explanation:
-                for (const ExtrusionEntity* ee : layerm->perimeters.entities) {                      // iterate through all perimeter Collections
+                for (const ExtrusionEntity* ee : layerm->perimeters()) { // iterate through all perimeter Collections
                     auto* fill = dynamic_cast<const ExtrusionEntityCollection*>(ee);
                     if (is_overriddable(*fill, print.config(), *object, region) && ! is_entity_overridden(fill, copy))
                         set_extruder_override(fill, copy, (print.config().infill_first ? last_nonsoluble_extruder : first_nonsoluble_extruder), num_of_copies);
diff --git a/src/libslic3r/Layer.cpp b/src/libslic3r/Layer.cpp
index d273fde96..3bd2566a5 100644
--- a/src/libslic3r/Layer.cpp
+++ b/src/libslic3r/Layer.cpp
@@ -22,7 +22,7 @@ Layer::~Layer()
 bool Layer::empty() const
 {
 	for (const LayerRegion *layerm : m_regions)
-        if (layerm != nullptr && ! layerm->slices.empty())
+        if (layerm != nullptr && ! layerm->slices().empty())
             // Non empty layer.
             return false;
     return true;
@@ -40,11 +40,11 @@ void Layer::make_slices()
     ExPolygons slices;
     if (m_regions.size() == 1) {
         // optimization: if we only have one region, take its slices
-        slices = to_expolygons(m_regions.front()->slices.surfaces);
+        slices = to_expolygons(m_regions.front()->slices().surfaces);
     } else {
         Polygons slices_p;
         for (LayerRegion *layerm : m_regions)
-            polygons_append(slices_p, to_polygons(layerm->slices.surfaces));
+            polygons_append(slices_p, to_polygons(layerm->slices().surfaces));
         slices = union_safety_offset_ex(slices_p);
     }
     
@@ -65,6 +65,245 @@ void Layer::make_slices()
         this->lslices.emplace_back(std::move(slices[i]));
 }
 
+// used by Layer::build_up_down_graph()
+[[nodiscard]] static ClipperLib_Z::Paths expolygons_to_zpaths(const ExPolygons &expolygons, coord_t isrc)
+{
+    size_t num_paths = 0;
+    for (const ExPolygon &expolygon : expolygons)
+        num_paths += expolygon.num_contours();
+
+    ClipperLib_Z::Paths out;
+    out.reserve(num_paths);
+
+    for (const ExPolygon &expolygon : expolygons) {
+        for (size_t icontour = 0; icontour < expolygon.num_contours(); ++ icontour) {
+            const Polygon &contour = expolygon.contour_or_hole(icontour);
+            out.emplace_back();
+            ClipperLib_Z::Path &path = out.back();
+            path.reserve(contour.size());
+            for (const Point &p : contour.points)
+                path.push_back({ p.x(), p.y(), isrc });
+        }
+        ++ isrc;
+    }
+
+    return out;
+}
+
+// used by Layer::build_up_down_graph()
+static void connect_layer_slices(
+    Layer                                           &below,
+    Layer                                           &above,
+    const ClipperLib_Z::PolyTree                    &polytree,
+    const std::vector<std::pair<coord_t, coord_t>>  &intersections,
+    const coord_t                                    offset_below,
+    const coord_t                                    offset_above,
+    const coord_t                                    offset_end)
+{
+    class Visitor {
+    public:
+        Visitor(const std::vector<std::pair<coord_t, coord_t>> &intersections, 
+            Layer &below, Layer &above, const coord_t offset_below, const coord_t offset_above, const coord_t offset_end) :
+            m_intersections(intersections), m_below(below), m_above(above), m_offset_below(offset_below), m_offset_above(offset_above), m_offset_end(offset_end) {}
+
+        void visit(const ClipperLib_Z::PolyNode &polynode)
+        {
+            if (polynode.Contour.size() >= 3) {
+                int32_t i = 0, j = 0;
+                double  area = 0;
+                for (int icontour = 0; icontour <= polynode.ChildCount(); ++ icontour) {
+                    const ClipperLib_Z::Path &contour = icontour == 0 ? polynode.Contour : polynode.Childs[icontour - 1]->Contour;
+                    if (contour.size() >= 3) {
+                        area = ClipperLib_Z::Area(contour);
+                        int32_t i = contour.front().z();
+                        int32_t j = i;
+                        if (i < 0) {
+                            std::tie(i, j) = m_intersections[-i - 1];
+                        } else {
+                            for (const ClipperLib_Z::IntPoint& pt : contour) {
+                                j = pt.z();
+                                if (j < 0) {
+                                    std::tie(i, j) = m_intersections[-j - 1];
+                                    goto end;
+                                }
+                                else if (i != j)
+                                    goto end;
+                            }
+                        }
+                    }
+                }
+            end:
+                bool found = false;
+                if (i == j) {
+                    // The contour is completely inside another contour.
+                    Point pt(polynode.Contour.front().x(), polynode.Contour.front().y());
+                    if (i < m_offset_above) {
+                        // Index of an island below. Look-it up in the island above.
+                        assert(i >= m_offset_below);
+                        i -= m_offset_below;
+                        for (int l = int(m_above.lslices_ex.size()) - 1; l >= 0; -- l) {
+                            LayerSlice &lslice = m_above.lslices_ex[l];
+                            if (lslice.bbox.contains(pt) && m_above.lslices[l].contains(pt)) {
+                                found = true;
+                                j = l;
+                                break;
+                            }
+                        }
+                    } else {
+                        // Index of an island above. Look-it up in the island below.
+                        assert(j < m_offset_end);
+                        j -= m_offset_below;
+                        for (int l = int(m_below.lslices_ex.size()) - 1; l >= 0; -- l) {
+                            LayerSlice &lslice = m_below.lslices_ex[l];
+                            if (lslice.bbox.contains(pt) && m_below.lslices[l].contains(pt)) {
+                                found = true;
+                                i = l;
+                                break;
+                            }
+                        }
+                    }
+                } else {
+                    if (i > j)
+                        std::swap(i, j);
+                    assert(i >= m_offset_below);
+                    assert(i < m_offset_above);
+                    i -= m_offset_below;
+                    assert(j >= m_offset_above);
+                    assert(j < m_offset_end);
+                    j -= m_offset_above;
+                    found = true;
+                }
+                if (found) {
+                    // Subtract area of holes from the area of outer contour.
+                    for (int icontour = 0; icontour < polynode.ChildCount(); ++ icontour)
+                        area -= ClipperLib_Z::Area(polynode.Childs[icontour]->Contour);
+                    // Store the links and area into the contours.
+                    LayerSlice::Links &links_below = m_below.lslices_ex[i].overlaps_above;
+                    LayerSlice::Links &links_above = m_above.lslices_ex[i].overlaps_below;
+                    LayerSlice::Link key{ j };
+                    auto it_below = std::lower_bound(links_below.begin(), links_below.end(), key, [](auto &l, auto &r){ return l.slice_idx < r.slice_idx; });
+                    if (it_below != links_below.end() && it_below->slice_idx == j) {
+                        it_below->area += area;
+                    } else {
+                        auto it_above = std::lower_bound(links_above.begin(), links_above.end(), key, [](auto &l, auto &r){ return l.slice_idx < r.slice_idx; });
+                        if (it_above != links_above.end() && it_above->slice_idx == i) {
+                            it_above->area += area;
+                        } else {
+                            // Insert into one of the two vectors.
+                            bool take_below = false;
+                            if (links_below.size() < LayerSlice::LinksStaticSize)
+                                take_below = false;
+                            else if (links_above.size() >= LayerSlice::LinksStaticSize) {
+                                size_t shift_below = links_below.end() - it_below;
+                                size_t shift_above = links_above.end() - it_above;
+                                take_below = shift_below < shift_above;
+                            }
+                            if (take_below)
+                                links_below.insert(it_below, { j, float(area) });
+                            else
+                                links_above.insert(it_above, { i, float(area) });
+                        }
+                    }
+                }
+            }
+            for (int i = 0; i < polynode.ChildCount(); ++ i)
+                for (int j = 0; j < polynode.Childs[i]->ChildCount(); ++ j)
+                    this->visit(*polynode.Childs[i]->Childs[j]);
+        }
+
+    private:
+        const std::vector<std::pair<coord_t, coord_t>> &m_intersections;
+        Layer                                          &m_below;
+        Layer                                          &m_above;
+        const coord_t                                   m_offset_below;
+        const coord_t                                   m_offset_above;
+        const coord_t                                   m_offset_end;
+    } visitor(intersections, below, above, offset_below, offset_above, offset_end);
+
+    for (int i = 0; i < polytree.ChildCount(); ++ i)
+        visitor.visit(*polytree.Childs[i]);
+
+#ifndef NDEBUG
+    // Verify that only one directional link is stored: either from bottom slice up or from upper slice down.
+    for (int32_t islice = 0; islice < below.lslices_ex.size(); ++ islice) {
+        LayerSlice::Links &links1 = below.lslices_ex[islice].overlaps_above;
+        for (LayerSlice::Link &link1 : links1) {
+            LayerSlice::Links &links2 = above.lslices_ex[link1.slice_idx].overlaps_below;
+            assert(! std::binary_search(links2.begin(), links2.end(), link1, [](auto &l, auto &r){ return l.slice_idx < r.slice_idx; }));
+        }
+    }
+    for (int32_t islice = 0; islice < above.lslices_ex.size(); ++ islice) {
+        LayerSlice::Links &links1 = above.lslices_ex[islice].overlaps_above;
+        for (LayerSlice::Link &link1 : links1) {
+            LayerSlice::Links &links2 = below.lslices_ex[link1.slice_idx].overlaps_below;
+            assert(! std::binary_search(links2.begin(), links2.end(), link1, [](auto &l, auto &r){ return l.slice_idx < r.slice_idx; }));
+        }
+    }
+#endif // NDEBUG
+
+    // Scatter the links, but don't sort them yet.
+    for (int32_t islice = 0; islice < below.lslices_ex.size(); ++ islice)
+        for (LayerSlice::Link &link : below.lslices_ex[islice].overlaps_above)
+            above.lslices_ex[link.slice_idx].overlaps_below.push_back({ islice, link.area });
+    for (int32_t islice = 0; islice < above.lslices_ex.size(); ++ islice)
+        for (LayerSlice::Link &link : above.lslices_ex[islice].overlaps_below)
+            below.lslices_ex[link.slice_idx].overlaps_above.push_back({ islice, link.area });
+    // Sort the links.
+    for (LayerSlice &lslice : below.lslices_ex)
+        std::sort(lslice.overlaps_above.begin(), lslice.overlaps_above.end(), [](const LayerSlice::Link &l, const LayerSlice::Link &r){ return l.slice_idx < r.slice_idx; });
+    for (LayerSlice &lslice : above.lslices_ex)
+        std::sort(lslice.overlaps_below.begin(), lslice.overlaps_below.end(), [](const LayerSlice::Link &l, const LayerSlice::Link &r){ return l.slice_idx < r.slice_idx; });
+}
+
+void Layer::build_up_down_graph(Layer& below, Layer& above)
+{
+    coord_t             paths_below_offset = 0;
+    ClipperLib_Z::Paths paths_below = expolygons_to_zpaths(below.lslices, paths_below_offset);
+    coord_t             paths_above_offset = paths_below_offset + coord_t(below.lslices.size());
+    ClipperLib_Z::Paths paths_above = expolygons_to_zpaths(above.lslices, paths_above_offset);
+    coord_t             paths_end = paths_above_offset + coord_t(above.lslices.size());
+
+    class ZFill {
+    public:
+        ZFill() = default;
+        void reset() { m_intersections.clear(); }
+        void operator()(
+            const ClipperLib_Z::IntPoint& e1bot, const ClipperLib_Z::IntPoint& e1top,
+            const ClipperLib_Z::IntPoint& e2bot, const ClipperLib_Z::IntPoint& e2top,
+            ClipperLib_Z::IntPoint& pt) {
+            coord_t srcs[4]{ e1bot.z(), e1top.z(), e2bot.z(), e2top.z() };
+            coord_t* begin = srcs;
+            coord_t* end = srcs + 4;
+            std::sort(begin, end);
+            end = std::unique(begin, end);
+            assert(begin + 2 == end);
+            if (begin + 1 == end)
+                pt.z() = *begin;
+            else if (begin + 2 <= end) {
+                // store a -1 based negative index into the "intersections" vector here.
+                m_intersections.emplace_back(srcs[0], srcs[1]);
+                pt.z() = -coord_t(m_intersections.size());
+            }
+        }
+        const std::vector<std::pair<coord_t, coord_t>>& intersections() const { return m_intersections; }
+
+    private:
+        std::vector<std::pair<coord_t, coord_t>> m_intersections;
+    } zfill;
+
+    ClipperLib_Z::Clipper  clipper;
+    ClipperLib_Z::PolyTree result;
+    clipper.ZFillFunction(
+        [&zfill](const ClipperLib_Z::IntPoint &e1bot, const ClipperLib_Z::IntPoint &e1top, 
+                 const ClipperLib_Z::IntPoint &e2bot, const ClipperLib_Z::IntPoint &e2top, ClipperLib_Z::IntPoint &pt)
+        { return zfill(e1bot, e1top, e2bot, e2top, pt); });
+    clipper.AddPaths(paths_below, ClipperLib_Z::ptSubject, true);
+    clipper.AddPaths(paths_above, ClipperLib_Z::ptClip, true);
+    clipper.Execute(ClipperLib_Z::ctIntersection, result, ClipperLib_Z::pftNonZero, ClipperLib_Z::pftNonZero);
+
+    connect_layer_slices(below, above, result, zfill.intersections(), paths_below_offset, paths_above_offset, paths_end);
+}
+
 static inline bool layer_needs_raw_backup(const Layer *layer)
 {
     return ! (layer->regions().size() == 1 && (layer->id() > 0 || layer->object()->config().elefant_foot_compensation.value == 0));
@@ -74,10 +313,10 @@ void Layer::backup_untyped_slices()
 {
     if (layer_needs_raw_backup(this)) {
         for (LayerRegion *layerm : m_regions)
-            layerm->raw_slices = to_expolygons(layerm->slices.surfaces);
+            layerm->m_raw_slices = to_expolygons(layerm->slices().surfaces);
     } else {
         assert(m_regions.size() == 1);
-        m_regions.front()->raw_slices.clear();
+        m_regions.front()->m_raw_slices.clear();
     }
 }
 
@@ -85,10 +324,10 @@ void Layer::restore_untyped_slices()
 {
     if (layer_needs_raw_backup(this)) {
         for (LayerRegion *layerm : m_regions)
-            layerm->slices.set(layerm->raw_slices, stInternal);
+            layerm->m_slices.set(layerm->m_raw_slices, stInternal);
     } else {
         assert(m_regions.size() == 1);
-        m_regions.front()->slices.set(this->lslices, stInternal);
+        m_regions.front()->m_slices.set(this->lslices, stInternal);
     }
 }
 
@@ -101,13 +340,13 @@ void Layer::restore_untyped_slices_no_extra_perimeters()
     if (layer_needs_raw_backup(this)) {
         for (LayerRegion *layerm : m_regions)
         	if (! layerm->region().config().extra_perimeters.value)
-            	layerm->slices.set(layerm->raw_slices, stInternal);
+            	layerm->m_slices.set(layerm->m_raw_slices, stInternal);
     } else {
     	assert(m_regions.size() == 1);
     	LayerRegion *layerm = m_regions.front();
     	// This optimization is correct, as extra_perimeters are only reused by prepare_infill() with multi-regions.
         //if (! layerm->region().config().extra_perimeters.value)
-        	layerm->slices.set(this->lslices, stInternal);
+        	layerm->m_slices.set(this->lslices, stInternal);
     }
 }
 
@@ -125,7 +364,7 @@ ExPolygons Layer::merged(float offset_scaled) const
 		const PrintRegionConfig &config = layerm->region().config();
 		// Our users learned to bend Slic3r to produce empty volumes to act as subtracters. Only add the region if it is non-empty.
 		if (config.bottom_solid_layers > 0 || config.top_solid_layers > 0 || config.fill_density > 0. || config.perimeters > 0)
-			append(polygons, offset(layerm->slices.surfaces, offset_scaled));
+			append(polygons, offset(layerm->slices().surfaces, offset_scaled));
 	}
     ExPolygons out = union_ex(polygons);
 	if (offset_scaled2 != 0.f)
@@ -143,11 +382,12 @@ void Layer::make_perimeters()
     // keep track of regions whose perimeters we have already generated
     std::vector<unsigned char> done(m_regions.size(), false);
     
+    LayerRegionPtrs layerms;
     for (LayerRegionPtrs::iterator layerm = m_regions.begin(); layerm != m_regions.end(); ++ layerm) 
-    	if ((*layerm)->slices.empty()) {
- 			(*layerm)->perimeters.clear();
- 			(*layerm)->fills.clear();
- 			(*layerm)->thin_fills.clear();
+    	if ((*layerm)->slices().empty()) {
+ 			(*layerm)->m_perimeters.clear();
+ 			(*layerm)->m_fills.clear();
+ 			(*layerm)->m_thin_fills.clear();
     	} else {
 	        size_t region_id = layerm - m_regions.begin();
 	        if (done[region_id])
@@ -157,10 +397,10 @@ void Layer::make_perimeters()
 	        const PrintRegionConfig &config = (*layerm)->region().config();
 	        
 	        // find compatible regions
-	        LayerRegionPtrs layerms;
+            layerms.clear();
 	        layerms.push_back(*layerm);
 	        for (LayerRegionPtrs::const_iterator it = layerm + 1; it != m_regions.end(); ++it)
-	            if (! (*it)->slices.empty()) {
+	            if (! (*it)->slices().empty()) {
 		            LayerRegion* other_layerm = *it;
 		            const PrintRegionConfig &other_config = other_layerm->region().config();
 		            if (config.perimeter_extruder             == other_config.perimeter_extruder
@@ -178,18 +418,20 @@ void Layer::make_perimeters()
                         && config.fuzzy_skin_thickness        == other_config.fuzzy_skin_thickness
                         && config.fuzzy_skin_point_dist       == other_config.fuzzy_skin_point_dist)
 		            {
-			 			other_layerm->perimeters.clear();
-			 			other_layerm->fills.clear();
-			 			other_layerm->thin_fills.clear();
+			 			other_layerm->m_perimeters.clear();
+			 			other_layerm->m_fills.clear();
+			 			other_layerm->m_thin_fills.clear();
 		                layerms.push_back(other_layerm);
 		                done[it - m_regions.begin()] = true;
 		            }
 		        }
 	        
+            SurfaceCollection fill_surfaces;
 	        if (layerms.size() == 1) {  // optimization
-	            (*layerm)->fill_surfaces.surfaces.clear();
-	            (*layerm)->make_perimeters((*layerm)->slices, &(*layerm)->fill_surfaces);
-	            (*layerm)->fill_expolygons = to_expolygons((*layerm)->fill_surfaces.surfaces);
+                (*layerm)->m_fill_expolygons.clear();
+	            (*layerm)->m_fill_surfaces.clear();
+	            (*layerm)->make_perimeters((*layerm)->slices(), &fill_surfaces);
+	            (*layerm)->m_fill_expolygons = to_expolygons(fill_surfaces.surfaces);
 	        } else {
 	            SurfaceCollection new_slices;
 	            // Use the region with highest infill rate, as the make_perimeters() function below decides on the gap fill based on the infill existence.
@@ -198,29 +440,25 @@ void Layer::make_perimeters()
 	                // group slices (surfaces) according to number of extra perimeters
 	                std::map<unsigned short, Surfaces> slices;  // extra_perimeters => [ surface, surface... ]
 	                for (LayerRegion *layerm : layerms) {
-	                    for (const Surface &surface : layerm->slices.surfaces)
+	                    for (const Surface &surface : layerm->slices())
 	                        slices[surface.extra_perimeters].emplace_back(surface);
 	                    if (layerm->region().config().fill_density > layerm_config->region().config().fill_density)
 	                    	layerm_config = layerm;
+                        layerm->m_fill_surfaces.clear();
+                        layerm->m_fill_expolygons.clear();
 	                }
 	                // merge the surfaces assigned to each group
 	                for (std::pair<const unsigned short,Surfaces> &surfaces_with_extra_perimeters : slices)
 	                    new_slices.append(offset_ex(surfaces_with_extra_perimeters.second, ClipperSafetyOffset), surfaces_with_extra_perimeters.second.front());
 	            }
-	            
 	            // make perimeters
-	            SurfaceCollection fill_surfaces;
 	            layerm_config->make_perimeters(new_slices, &fill_surfaces);
-
 	            // assign fill_surfaces to each layer
-	            if (!fill_surfaces.surfaces.empty()) { 
-	                for (LayerRegionPtrs::iterator l = layerms.begin(); l != layerms.end(); ++l) {
-	                    // Separate the fill surfaces.
-	                    ExPolygons expp = intersection_ex(fill_surfaces.surfaces, (*l)->slices.surfaces);
-	                    (*l)->fill_expolygons = expp;
-	                    (*l)->fill_surfaces.set(std::move(expp), fill_surfaces.surfaces.front());
-	                }
-	            }
+                if (! fill_surfaces.empty()) {
+                    // Separate the fill surfaces.
+                    for (LayerRegion *l : layerms)
+                       l->m_fill_expolygons = intersection_ex(fill_surfaces.surfaces, l->slices().surfaces);
+                }
 	        }
 	    }
     BOOST_LOG_TRIVIAL(trace) << "Generating perimeters for layer " << this->id() << " - Done";
@@ -230,7 +468,7 @@ void Layer::export_region_slices_to_svg(const char *path) const
 {
     BoundingBox bbox;
     for (const auto *region : m_regions)
-        for (const auto &surface : region->slices.surfaces)
+        for (const auto &surface : region->slices())
             bbox.merge(get_extents(surface.expolygon));
     Point legend_size = export_surface_type_legend_to_svg_box_size();
     Point legend_pos(bbox.min(0), bbox.max(1));
@@ -239,7 +477,7 @@ void Layer::export_region_slices_to_svg(const char *path) const
     SVG svg(path, bbox);
     const float transparency = 0.5f;
     for (const auto *region : m_regions)
-        for (const auto &surface : region->slices.surfaces)
+        for (const auto &surface : region->slices())
             svg.draw(surface.expolygon, surface_type_to_color_name(surface.surface_type), transparency);
     export_surface_type_legend_to_svg(svg, legend_pos);
     svg.Close(); 
@@ -256,7 +494,7 @@ void Layer::export_region_fill_surfaces_to_svg(const char *path) const
 {
     BoundingBox bbox;
     for (const auto *region : m_regions)
-        for (const auto &surface : region->slices.surfaces)
+        for (const auto &surface : region->slices())
             bbox.merge(get_extents(surface.expolygon));
     Point legend_size = export_surface_type_legend_to_svg_box_size();
     Point legend_pos(bbox.min(0), bbox.max(1));
@@ -265,7 +503,7 @@ void Layer::export_region_fill_surfaces_to_svg(const char *path) const
     SVG svg(path, bbox);
     const float transparency = 0.5f;
     for (const auto *region : m_regions)
-        for (const auto &surface : region->slices.surfaces)
+        for (const auto &surface : region->slices())
             svg.draw(surface.expolygon, surface_type_to_color_name(surface.surface_type), transparency);
     export_surface_type_legend_to_svg(svg, legend_pos);
     svg.Close();
@@ -281,9 +519,9 @@ void Layer::export_region_fill_surfaces_to_svg_debug(const char *name) const
 BoundingBox get_extents(const LayerRegion &layer_region)
 {
     BoundingBox bbox;
-    if (!layer_region.slices.surfaces.empty()) {
-        bbox = get_extents(layer_region.slices.surfaces.front());
-        for (auto it = layer_region.slices.surfaces.cbegin() + 1; it != layer_region.slices.surfaces.cend(); ++it)
+    if (! layer_region.slices().empty()) {
+        bbox = get_extents(layer_region.slices().surfaces.front());
+        for (auto it = layer_region.slices().surfaces.cbegin() + 1; it != layer_region.slices().surfaces.cend(); ++ it)
             bbox.merge(get_extents(*it));
     }
     return bbox;
diff --git a/src/libslic3r/Layer.hpp b/src/libslic3r/Layer.hpp
index e11c740e6..9a1a77aea 100644
--- a/src/libslic3r/Layer.hpp
+++ b/src/libslic3r/Layer.hpp
@@ -2,10 +2,13 @@
 #define slic3r_Layer_hpp_
 
 #include "libslic3r.h"
+#include "BoundingBox.hpp"
 #include "Flow.hpp"
 #include "SurfaceCollection.hpp"
 #include "ExtrusionEntityCollection.hpp"
 
+#include <boost/container/small_vector.hpp>
+
 namespace Slic3r {
 
 class ExPolygon;
@@ -25,50 +28,38 @@ namespace FillLightning {
     class Generator;
 };
 
-namespace FillLightning {
-    class Generator;
-};
-
 class LayerRegion
 {
 public:
-    Layer*                      layer()         { return m_layer; }
-    const Layer*                layer() const   { return m_layer; }
-    const PrintRegion&          region() const  { return *m_region; }
+    [[nodiscard]] Layer*                            layer()         { return m_layer; }
+    [[nodiscard]] const Layer*                      layer() const   { return m_layer; }
+    [[nodiscard]] const PrintRegion&                region() const  { return *m_region; }
 
     // collection of surfaces generated by slicing the original geometry
     // divided by type top/bottom/internal
-    SurfaceCollection           slices;
-    // Backed up slices before they are split into top/bottom/internal.
-    // Only backed up for multi-region layers or layers with elephant foot compensation.
-    //FIXME Review whether not to simplify the code by keeping the raw_slices all the time.
-    ExPolygons                  raw_slices;
+    [[nodiscard]] const SurfaceCollection&          slices() const { return m_slices; }
+
+    [[nodiscard]] const ExPolygons&                 fill_expolygons() const { return m_fill_expolygons; }
+
+    // collection of surfaces generated by slicing the original geometry
+    // divided by type top/bottom/internal
+    [[nodiscard]] const SurfaceCollection&          fill_surfaces() const { return m_fill_surfaces; }
 
     // collection of extrusion paths/loops filling gaps
     // These fills are generated by the perimeter generator.
     // They are not printed on their own, but they are copied to this->fills during infill generation.
-    ExtrusionEntityCollection   thin_fills;
-
-    // Unspecified fill polygons, used for overhang detection ("ensure vertical wall thickness feature")
-    // and for re-starting of infills.
-    ExPolygons                  fill_expolygons;
-    // collection of surfaces for infill generation
-    SurfaceCollection           fill_surfaces;
-
-    // collection of expolygons representing the bridged areas (thus not
-    // needing support material)
-//    Polygons                    bridged;
+    [[nodiscard]] const ExtrusionEntityCollection&  thin_fills() const { return m_thin_fills; }
 
     // collection of polylines representing the unsupported bridge edges
-    Polylines          			unsupported_bridge_edges;
+    [[nodiscard]] const Polylines&                  unsupported_bridge_edges() const { return m_unsupported_bridge_edges; }
 
     // ordered collection of extrusion paths/loops to build all perimeters
     // (this collection contains only ExtrusionEntityCollection objects)
-    ExtrusionEntityCollection   perimeters;
+    [[nodiscard]] const ExtrusionEntityCollection&  perimeters() const { return m_perimeters; }
 
     // ordered collection of extrusion paths to fill surfaces
     // (this collection contains only ExtrusionEntityCollection objects)
-    ExtrusionEntityCollection   fills;
+    [[nodiscard]] const ExtrusionEntityCollection&  fills() const { return m_fills; }
     
     Flow    flow(FlowRole role) const;
     Flow    flow(FlowRole role, double layer_height) const;
@@ -92,20 +83,165 @@ public:
     void    export_region_fill_surfaces_to_svg_debug(const char *name) const;
 
     // Is there any valid extrusion assigned to this LayerRegion?
-    bool    has_extrusions() const { return ! this->perimeters.entities.empty() || ! this->fills.entities.empty(); }
+    bool    has_extrusions() const { return ! this->perimeters().empty() || ! this->fills().empty(); }
 
 protected:
     friend class Layer;
     friend class PrintObject;
 
     LayerRegion(Layer *layer, const PrintRegion *region) : m_layer(layer), m_region(region) {}
-    ~LayerRegion() {}
+    ~LayerRegion() = default;
 
 private:
-    Layer             *m_layer;
-    const PrintRegion *m_region;
+    // Modifying m_slices
+    friend std::string fix_slicing_errors(LayerPtrs&, const std::function<void()>&);
+    template<typename ThrowOnCancel>
+    friend void apply_mm_segmentation(PrintObject& print_object, ThrowOnCancel throw_on_cancel);
+
+    Layer                      *m_layer;
+    const PrintRegion          *m_region;
+
+    // Backed up slices before they are split into top/bottom/internal.
+    // Only backed up for multi-region layers or layers with elephant foot compensation.
+    //FIXME Review whether not to simplify the code by keeping the raw_slices all the time.
+    ExPolygons                  m_raw_slices;
+
+//FIXME make m_slices public for unit tests
+public:
+    // collection of surfaces generated by slicing the original geometry
+    // divided by type top/bottom/internal
+    SurfaceCollection           m_slices;
+
+private:
+    // Unspecified fill polygons, used for overhang detection ("ensure vertical wall thickness feature")
+    // and for re-starting of infills.
+    ExPolygons                  m_fill_expolygons;
+
+    // collection of surfaces for infill generation
+    SurfaceCollection           m_fill_surfaces;
+
+    // collection of extrusion paths/loops filling gaps
+    // These fills are generated by the perimeter generator.
+    // They are not printed on their own, but they are copied to this->fills during infill generation.
+    ExtrusionEntityCollection   m_thin_fills;
+
+    // collection of polylines representing the unsupported bridge edges
+    Polylines                   m_unsupported_bridge_edges;
+
+    // ordered collection of extrusion paths/loops to build all perimeters
+    // (this collection contains only ExtrusionEntityCollection objects)
+    ExtrusionEntityCollection   m_perimeters;
+
+    // ordered collection of extrusion paths to fill surfaces
+    // (this collection contains only ExtrusionEntityCollection objects)
+    ExtrusionEntityCollection   m_fills;
+
+    // collection of expolygons representing the bridged areas (thus not
+    // needing support material)
+//  Polygons                    bridged;
 };
 
+// Range of two indices, providing support for range based loops.
+template<typename T>
+class IndexRange
+{
+public:
+    IndexRange(T ibegin, T iend) : m_begin(ibegin), m_end(iend) {}
+    IndexRange() = default;
+
+    T    begin()  const { assert(m_begin <= m_end); return m_begin; };
+    T    end()    const { assert(m_begin <= m_end); return m_end; };
+
+    bool empty()  const { assert(m_begin <= m_end); return m_begin >= m_end; }
+    T    size()   const { assert(m_begin <= m_end); return m_end - m_begin; }
+
+private:
+    // Index of the first extrusion in LayerRegion.
+    T    m_begin   { 0 };
+    // Index of the last extrusion in LayerRegion.
+    T    m_end     { 0 };
+};
+
+class LayerExtrusionRange : public IndexRange<uint32_t>
+{
+public:
+    LayerExtrusionRange(uint32_t iregion, uint32_t ibegin, uint32_t iend) : m_region(iregion), IndexRange<uint32_t>(ibegin, iend) {}
+    LayerExtrusionRange() = default;
+
+    // Index of LayerRegion in Layer.
+    uint32_t    region() const { return m_region; };
+
+private:
+    // Index of LayerRegion in Layer.
+    uint32_t    m_region  { 0 };
+};
+
+static constexpr const size_t LayerExtrusionRangesStaticSize = 1;
+using LayerExtrusionRanges =
+#ifdef NDEBUG
+    // To reduce memory allocation in release mode.
+    boost::container::small_vector<LayerExtrusionRange, LayerExtrusionRangesStaticSize>;
+#else // NDEBUG
+    // To ease debugging.
+    std::vector<LayerExtrusionRange>;
+#endif // NDEBUG
+
+using ExPolygonRange = IndexRange<uint32_t>();
+
+// LayerSlice contains one or more LayerIsland objects,
+// each LayerIsland containing a set of perimeter extrusions extruded with one particular PrintRegionConfig parameters
+// and one or multiple 
+struct LayerIsland
+{
+    // Perimeter extrusions in LayerRegion belonging to this island.
+    LayerExtrusionRange   perimeters;
+    // Infill + gapfill extrusions in LayerRegion belonging to this island.
+    LayerExtrusionRanges  fills;
+    // Region that is to be filled with the fills above.
+    ExPolygonRange        fill_expolygons;
+    // Centroid of this island used for path planning.
+    Point                 centroid;
+
+    bool                  has_extrusions() const { return ! this->perimeters.empty() || ! this->fills.empty(); }
+};
+
+static constexpr const size_t LayerIslandsStaticSize = 1;
+using LayerIslands =
+#ifdef NDEBUG
+    // To reduce memory allocation in release mode.
+    boost::container::small_vector<LayerIsland, LayerIslandsStaticSize>;
+#else // NDEBUG
+    // To ease debugging.
+    std::vector<LayerIsland>;
+#endif // NDEBUG
+
+//
+struct LayerSlice
+{
+    struct Link {
+        int32_t     slice_idx;
+        float       area;
+    };
+    static constexpr const size_t LinksStaticSize = 4;
+    using Links =
+#ifdef NDEBUG
+        // To reduce memory allocation in release mode.
+        boost::container::small_vector<Link, LinksStaticSize>;
+#else // NDEBUG
+        // To ease debugging.
+        std::vector<Link>;
+#endif // NDEBUG
+
+    BoundingBox         bbox;
+    Links               overlaps_above;
+    Links               overlaps_below;
+    LayerIslands        islands;
+
+    bool    has_extrusions() const { for (const LayerIsland &island : islands) if (island.has_extrusions()) return true; return false; }
+};
+
+using LayerSlices = std::vector<LayerSlice>;
+
 class Layer 
 {
 public:
@@ -131,8 +267,8 @@ public:
     // order will be applied by the G-code generator to the extrusions fitting into these lslices.
     // These lslices are also used to detect overhangs and overlaps between successive layers, therefore it is important
     // that the 1st lslice is not compensated by the Elephant foot compensation algorithm.
-    ExPolygons 				 lslices;
-    std::vector<BoundingBox> lslices_bboxes;
+    ExPolygons 				lslices;
+    LayerSlices             lslices_ex;
 
     size_t                  region_count() const { return m_regions.size(); }
     const LayerRegion*      get_region(int idx) const { return m_regions[idx]; }
@@ -142,6 +278,8 @@ public:
     // Test whether whether there are any slices assigned to this layer.
     bool                    empty() const;    
     void                    make_slices();
+    // After creating the slices on all layers, chain the islands overlapping in Z.
+    static void             build_up_down_graph(Layer &below, Layer &above);
     // Backup and restore raw sliced regions if needed.
     //FIXME Review whether not to simplify the code by keeping the raw_slices all the time.
     void                    backup_untyped_slices();
@@ -151,11 +289,11 @@ public:
     // Slices merged into islands, to be used by the elephant foot compensation to trim the individual surfaces with the shrunk merged slices.
     ExPolygons              merged(float offset) const;
     template <class T> bool any_internal_region_slice_contains(const T &item) const {
-        for (const LayerRegion *layerm : m_regions) if (layerm->slices.any_internal_contains(item)) return true;
+        for (const LayerRegion *layerm : m_regions) if (layerm->slices().any_internal_contains(item)) return true;
         return false;
     }
     template <class T> bool any_bottom_region_slice_contains(const T &item) const {
-        for (const LayerRegion *layerm : m_regions) if (layerm->slices.any_bottom_contains(item)) return true;
+        for (const LayerRegion *layerm : m_regions) if (layerm->slices().any_bottom_contains(item)) return true;
         return false;
     }
     void                    make_perimeters();
@@ -172,6 +310,7 @@ public:
 
     // Is there any valid extrusion assigned to this LayerRegion?
     virtual bool            has_extrusions() const { for (auto layerm : m_regions) if (layerm->has_extrusions()) return true; return false; }
+//    virtual bool            has_extrusions() const { for (const LayerSlice &lslice : lslices_ex) if (lslice.has_extrusions()) return true; return false; }
 
 protected:
     friend class PrintObject;
diff --git a/src/libslic3r/LayerRegion.cpp b/src/libslic3r/LayerRegion.cpp
index 1996a58b5..b0bfc961e 100644
--- a/src/libslic3r/LayerRegion.cpp
+++ b/src/libslic3r/LayerRegion.cpp
@@ -43,30 +43,26 @@ Flow LayerRegion::bridging_flow(FlowRole role) const
     }
 }
 
-// Fill in layerm->fill_surfaces by trimming the layerm->slices by the cummulative layerm->fill_surfaces.
+// Fill in layerm->fill_surfaces by trimming the layerm->slices by layerm->fill_expolygons.
 void LayerRegion::slices_to_fill_surfaces_clipped()
 {
-    // Note: this method should be idempotent, but fill_surfaces gets modified 
-    // in place. However we're now only using its boundaries (which are invariant)
-    // so we're safe. This guarantees idempotence of prepare_infill() also in case
-    // that combine_infill() turns some fill_surface into VOID surfaces.
     // Collect polygons per surface type.
-    std::array<SurfacesPtr, size_t(stCount)> by_surface;
-    for (Surface &surface : this->slices.surfaces)
+    std::array<std::vector<const Surface*>, size_t(stCount)> by_surface;
+    for (const Surface &surface : this->slices())
         by_surface[size_t(surface.surface_type)].emplace_back(&surface);
     // Trim surfaces by the fill_boundaries.
-    this->fill_surfaces.surfaces.clear();
+    m_fill_surfaces.surfaces.clear();
     for (size_t surface_type = 0; surface_type < size_t(stCount); ++ surface_type) {
-        const SurfacesPtr &this_surfaces = by_surface[surface_type];
+        const std::vector<const Surface*> &this_surfaces = by_surface[surface_type];
         if (! this_surfaces.empty())
-            this->fill_surfaces.append(intersection_ex(this_surfaces, this->fill_expolygons), SurfaceType(surface_type));
+            m_fill_surfaces.append(intersection_ex(this_surfaces, this->fill_expolygons()), SurfaceType(surface_type));
     }
 }
 
 void LayerRegion::make_perimeters(const SurfaceCollection &slices, SurfaceCollection* fill_surfaces)
 {
-    this->perimeters.clear();
-    this->thin_fills.clear();
+    m_perimeters.clear();
+    m_thin_fills.clear();
 
     const PrintConfig       &print_config  = this->layer()->object()->print()->config();
     const PrintRegionConfig &region_config = this->region().config();
@@ -87,8 +83,8 @@ void LayerRegion::make_perimeters(const SurfaceCollection &slices, SurfaceCollec
         spiral_vase,
         
         // output:
-        &this->perimeters,
-        &this->thin_fills,
+        &m_perimeters,
+        &m_thin_fills,
         fill_surfaces
     );
     
@@ -131,7 +127,7 @@ void LayerRegion::process_external_surfaces(const Layer *lower_layer, const Poly
     // Internal surfaces, not grown.
     Surfaces                    internal;
     // Areas, where an infill of various types (top, bottom, bottom bride, sparse, void) could be placed.
-    Polygons                    fill_boundaries = to_polygons(this->fill_expolygons);
+    Polygons                    fill_boundaries = to_polygons(this->fill_expolygons());
     Polygons  					lower_layer_covered_tmp;
 
     // Collect top surfaces and internal surfaces.
@@ -141,7 +137,7 @@ void LayerRegion::process_external_surfaces(const Layer *lower_layer, const Poly
     {
         // Voids are sparse infills if infill rate is zero.
         Polygons voids;
-        for (const Surface &surface : this->fill_surfaces.surfaces) {
+        for (const Surface &surface : this->fill_surfaces()) {
             if (surface.is_top()) {
                 // Collect the top surfaces, inflate them and trim them by the bottom surfaces.
                 // This gives the priority to bottom surfaces.
@@ -292,7 +288,7 @@ void LayerRegion::process_external_surfaces(const Layer *lower_layer, const Poly
                     bridges[idx_last].bridge_angle = bd.angle;
                     if (this->layer()->object()->has_support()) {
 //                        polygons_append(this->bridged, bd.coverage());
-                        append(this->unsupported_bridge_edges, bd.unsupported_edges());
+                        append(m_unsupported_bridge_edges, bd.unsupported_edges());
                     }
 				} else if (custom_angle > 0) {
 					// Bridge was not detected (likely it is only supported at one side). Still it is a surface filled in
@@ -365,7 +361,7 @@ void LayerRegion::process_external_surfaces(const Layer *lower_layer, const Poly
         surfaces_append(new_surfaces, std::move(new_expolys), s1);
     }
     
-    this->fill_surfaces.surfaces = std::move(new_surfaces);
+    m_fill_surfaces.surfaces = std::move(new_surfaces);
 
 #ifdef SLIC3R_DEBUG_SLICE_PROCESSING
     export_region_fill_surfaces_to_svg_debug("3_process_external_surfaces-final");
@@ -389,12 +385,12 @@ void LayerRegion::prepare_fill_surfaces()
     // For Lightning infill, infill_only_where_needed is ignored because both
     // do a similar thing, and their combination doesn't make much sense.
     if (! spiral_vase && this->region().config().top_solid_layers == 0) {
-        for (Surface &surface : this->fill_surfaces.surfaces)
+        for (Surface &surface : m_fill_surfaces)
             if (surface.is_top())
                 surface.surface_type = this->layer()->object()->config().infill_only_where_needed && this->region().config().fill_pattern != ipLightning ? stInternalVoid : stInternal;
     }
     if (this->region().config().bottom_solid_layers == 0) {
-        for (Surface &surface : this->fill_surfaces.surfaces)
+        for (Surface &surface : m_fill_surfaces)
             if (surface.is_bottom()) // (surface.surface_type == stBottom)
                 surface.surface_type = stInternal;
     }
@@ -403,7 +399,7 @@ void LayerRegion::prepare_fill_surfaces()
     if (! spiral_vase && this->region().config().fill_density.value > 0) {
         // scaling an area requires two calls!
         double min_area = scale_(scale_(this->region().config().solid_infill_below_area.value));
-        for (Surface &surface : this->fill_surfaces.surfaces)
+        for (Surface &surface : m_fill_surfaces)
             if (surface.surface_type == stInternal && surface.area() <= min_area)
                 surface.surface_type = stInternalSolid;
     }
@@ -423,38 +419,38 @@ double LayerRegion::infill_area_threshold() const
 void LayerRegion::trim_surfaces(const Polygons &trimming_polygons)
 {
 #ifndef NDEBUG
-    for (const Surface &surface : this->slices.surfaces)
+    for (const Surface &surface : this->slices())
         assert(surface.surface_type == stInternal);
 #endif /* NDEBUG */
-	this->slices.set(intersection_ex(this->slices.surfaces, trimming_polygons), stInternal);
+	m_slices.set(intersection_ex(this->slices().surfaces, trimming_polygons), stInternal);
 }
 
 void LayerRegion::elephant_foot_compensation_step(const float elephant_foot_compensation_perimeter_step, const Polygons &trimming_polygons)
 {
 #ifndef NDEBUG
-    for (const Surface &surface : this->slices.surfaces)
+    for (const Surface &surface : this->slices())
         assert(surface.surface_type == stInternal);
 #endif /* NDEBUG */
-    Polygons tmp = intersection(this->slices.surfaces, trimming_polygons);
-    append(tmp, diff(this->slices.surfaces, opening(this->slices.surfaces, elephant_foot_compensation_perimeter_step)));
-    this->slices.set(union_ex(tmp), stInternal);
+    Polygons tmp = intersection(this->slices().surfaces, trimming_polygons);
+    append(tmp, diff(this->slices().surfaces, opening(this->slices().surfaces, elephant_foot_compensation_perimeter_step)));
+    m_slices.set(union_ex(tmp), stInternal);
 }
 
 void LayerRegion::export_region_slices_to_svg(const char *path) const
 {
     BoundingBox bbox;
-    for (Surfaces::const_iterator surface = this->slices.surfaces.begin(); surface != this->slices.surfaces.end(); ++surface)
-        bbox.merge(get_extents(surface->expolygon));
+    for (const Surface &surface : this->slices())
+        bbox.merge(get_extents(surface.expolygon));
     Point legend_size = export_surface_type_legend_to_svg_box_size();
     Point legend_pos(bbox.min(0), bbox.max(1));
     bbox.merge(Point(std::max(bbox.min(0) + legend_size(0), bbox.max(0)), bbox.max(1) + legend_size(1)));
 
     SVG svg(path, bbox);
     const float transparency = 0.5f;
-    for (Surfaces::const_iterator surface = this->slices.surfaces.begin(); surface != this->slices.surfaces.end(); ++surface)
-        svg.draw(surface->expolygon, surface_type_to_color_name(surface->surface_type), transparency);
-    for (Surfaces::const_iterator surface = this->fill_surfaces.surfaces.begin(); surface != this->fill_surfaces.surfaces.end(); ++surface)
-        svg.draw(surface->expolygon.lines(), surface_type_to_color_name(surface->surface_type));
+    for (const Surface &surface : this->slices())
+        svg.draw(surface.expolygon, surface_type_to_color_name(surface.surface_type), transparency);
+    for (const Surface &surface : this->fill_surfaces())
+        svg.draw(surface.expolygon.lines(), surface_type_to_color_name(surface.surface_type));
     export_surface_type_legend_to_svg(svg, legend_pos);
     svg.Close();
 }
@@ -470,15 +466,15 @@ void LayerRegion::export_region_slices_to_svg_debug(const char *name) const
 void LayerRegion::export_region_fill_surfaces_to_svg(const char *path) const
 {
     BoundingBox bbox;
-    for (Surfaces::const_iterator surface = this->fill_surfaces.surfaces.begin(); surface != this->fill_surfaces.surfaces.end(); ++surface)
-        bbox.merge(get_extents(surface->expolygon));
+    for (const Surface &surface : this->fill_surfaces())
+        bbox.merge(get_extents(surface.expolygon));
     Point legend_size = export_surface_type_legend_to_svg_box_size();
     Point legend_pos(bbox.min(0), bbox.max(1));
     bbox.merge(Point(std::max(bbox.min(0) + legend_size(0), bbox.max(0)), bbox.max(1) + legend_size(1)));
 
     SVG svg(path, bbox);
     const float transparency = 0.5f;
-    for (const Surface &surface : this->fill_surfaces.surfaces) {
+    for (const Surface &surface : this->fill_surfaces()) {
         svg.draw(surface.expolygon, surface_type_to_color_name(surface.surface_type), transparency);
         svg.draw_outline(surface.expolygon, "black", "blue", scale_(0.05)); 
     }
diff --git a/src/libslic3r/MultiMaterialSegmentation.cpp b/src/libslic3r/MultiMaterialSegmentation.cpp
index 21b53c40d..661c2d0fd 100644
--- a/src/libslic3r/MultiMaterialSegmentation.cpp
+++ b/src/libslic3r/MultiMaterialSegmentation.cpp
@@ -713,7 +713,7 @@ struct MMU_Graph
 
             [[nodiscard]] const Vec2d &point_double() const { return m_point_double; }
             [[nodiscard]] const Point &point() const { return m_point; }
-            bool operator==(const CPoint &rhs) const { return this->m_point_double == rhs.m_point_double && this->m_contour_idx == rhs.m_contour_idx && this->m_point_idx == rhs.m_point_idx; }
+            bool operator==(const CPoint &rhs) const { return m_point_double == rhs.m_point_double && m_contour_idx == rhs.m_contour_idx && m_point_idx == rhs.m_point_idx; }
         };
         struct CPointAccessor { const Point* operator()(const CPoint &pt) const { return &pt.point(); }};
         typedef ClosestPointInRadiusLookup<CPoint, CPointAccessor> CPointLookupType;
@@ -1697,7 +1697,7 @@ std::vector<std::vector<ExPolygons>> multi_material_segmentation_by_painting(con
             throw_on_cancel_callback();
             ExPolygons ex_polygons;
             for (LayerRegion *region : layers[layer_idx]->regions())
-                for (const Surface &surface : region->slices.surfaces)
+                for (const Surface &surface : region->slices())
                     Slic3r::append(ex_polygons, offset_ex(surface.expolygon, float(10 * SCALED_EPSILON)));
             // All expolygons are expanded by SCALED_EPSILON, merged, and then shrunk again by SCALED_EPSILON
             // to ensure that very close polygons will be merged.
diff --git a/src/libslic3r/Preset.hpp b/src/libslic3r/Preset.hpp
index f0a81a71a..74ba2e0f5 100644
--- a/src/libslic3r/Preset.hpp
+++ b/src/libslic3r/Preset.hpp
@@ -418,7 +418,7 @@ public:
     size_t          first_compatible_idx(PreferedCondition prefered_condition) const
     {
         size_t i = m_default_suppressed ? m_num_default_presets : 0;
-        size_t n = this->m_presets.size();
+        size_t n = m_presets.size();
         size_t i_compatible = n;
         int    match_quality = -1;
         for (; i < n; ++ i)
diff --git a/src/libslic3r/PrintBase.cpp b/src/libslic3r/PrintBase.cpp
index b60401e96..b0aa7bd1e 100644
--- a/src/libslic3r/PrintBase.cpp
+++ b/src/libslic3r/PrintBase.cpp
@@ -103,7 +103,7 @@ std::string PrintBase::output_filepath(const std::string &path, const std::strin
 
 void PrintBase::status_update_warnings(int step, PrintStateBase::WarningLevel /* warning_level */, const std::string &message, const PrintObjectBase* print_object)
 {
-    if (this->m_status_callback) {
+    if (m_status_callback) {
         auto status = print_object ? SlicingStatus(*print_object, step) : SlicingStatus(*this, step);
         m_status_callback(status);
     }
diff --git a/src/libslic3r/PrintObject.cpp b/src/libslic3r/PrintObject.cpp
index bdcb034ab..d3ac219b0 100644
--- a/src/libslic3r/PrintObject.cpp
+++ b/src/libslic3r/PrintObject.cpp
@@ -157,7 +157,7 @@ void PrintObject::make_perimeters()
                     m_print->throw_if_canceled();
                     LayerRegion &layerm                     = *m_layers[layer_idx]->get_region(region_id);
                     const LayerRegion &upper_layerm         = *m_layers[layer_idx+1]->get_region(region_id);
-                    const Polygons upper_layerm_polygons    = to_polygons(upper_layerm.slices.surfaces);
+                    const Polygons upper_layerm_polygons    = to_polygons(upper_layerm.slices().surfaces);
                     // Filter upper layer polygons in intersection_ppl by their bounding boxes?
                     // my $upper_layerm_poly_bboxes= [ map $_->bounding_box, @{$upper_layerm_polygons} ];
                     const double total_loop_length      = total_length(upper_layerm_polygons);
@@ -166,7 +166,8 @@ void PrintObject::make_perimeters()
                     const coord_t ext_perimeter_width   = ext_perimeter_flow.scaled_width();
                     const coord_t ext_perimeter_spacing = ext_perimeter_flow.scaled_spacing();
 
-                    for (Surface &slice : layerm.slices.surfaces) {
+                    // slice is not const because slice.extra_perimeters is being incremented.
+                    for (Surface &slice : layerm.m_slices.surfaces) {
                         for (;;) {
                             // compute the total thickness of perimeters
                             const coord_t perimeters_thickness = ext_perimeter_width/2 + ext_perimeter_spacing/2
@@ -418,7 +419,7 @@ void PrintObject::generate_support_spots()
         BOOST_LOG_TRIVIAL(debug)
         << "Searching support spots - start";
         m_print->set_status(75, L("Searching support spots"));
-        if (this->m_config.support_material && !this->m_config.support_material_auto &&
+        if (m_config.support_material && !m_config.support_material_auto &&
         std::all_of(this->model_object()->volumes.begin(), this->model_object()->volumes.end(),
                 [](const ModelVolume* mv){return mv->supported_facets.empty();})
         ) {
@@ -500,7 +501,7 @@ std::pair<FillAdaptive::OctreePtr, FillAdaptive::OctreePtr> PrintObject::prepare
                 m_print->throw_if_canceled();
                 const Layer *layer = this->layers()[idx_layer];
                 for (const LayerRegion *layerm : layer->regions())
-                    for (const Surface &surface : layerm->fill_surfaces.surfaces)
+                    for (const Surface &surface : layerm->fill_surfaces())
                         if (surface.surface_type == stInternalBridge)
                             append(out, triangulate_expolygon_3d(surface.expolygon, layer->bottom_z()));
             }
@@ -883,13 +884,13 @@ void PrintObject::detect_surfaces_type()
                     Surfaces top;
                     if (upper_layer) {
                         ExPolygons upper_slices = interface_shells ? 
-                            diff_ex(layerm->slices.surfaces, upper_layer->m_regions[region_id]->slices.surfaces, ApplySafetyOffset::Yes) :
-                            diff_ex(layerm->slices.surfaces, upper_layer->lslices, ApplySafetyOffset::Yes);
+                            diff_ex(layerm->slices().surfaces, upper_layer->m_regions[region_id]->slices().surfaces, ApplySafetyOffset::Yes) :
+                            diff_ex(layerm->slices().surfaces, upper_layer->lslices, ApplySafetyOffset::Yes);
                         surfaces_append(top, opening_ex(upper_slices, offset), stTop);
                     } else {
                         // if no upper layer, all surfaces of this one are solid
                         // we clone surfaces because we're going to clear the slices collection
-                        top = layerm->slices.surfaces;
+                        top = layerm->slices().surfaces;
                         for (Surface &surface : top)
                             surface.surface_type = stTop;
                     }
@@ -910,7 +911,7 @@ void PrintObject::detect_surfaces_type()
                         surfaces_append(
                             bottom,
                             opening_ex(
-                                diff_ex(layerm->slices.surfaces, lower_layer->lslices, ApplySafetyOffset::Yes),
+                                diff_ex(layerm->slices().surfaces, lower_layer->lslices, ApplySafetyOffset::Yes),
                                 offset),
                             surface_type_bottom_other);
                         // if user requested internal shells, we need to identify surfaces
@@ -922,8 +923,8 @@ void PrintObject::detect_surfaces_type()
                                 bottom,
                                 opening_ex(
                                     diff_ex(
-                                        intersection(layerm->slices.surfaces, lower_layer->lslices), // supported
-                                        lower_layer->m_regions[region_id]->slices.surfaces,
+                                        intersection(layerm->slices().surfaces, lower_layer->lslices), // supported
+                                        lower_layer->m_regions[region_id]->slices().surfaces,
                                         ApplySafetyOffset::Yes),
                                     offset),
                                 stBottom);
@@ -932,7 +933,7 @@ void PrintObject::detect_surfaces_type()
                     } else {
                         // if no lower layer, all surfaces of this one are solid
                         // we clone surfaces because we're going to clear the slices collection
-                        bottom = layerm->slices.surfaces;
+                        bottom = layerm->slices().surfaces;
                         for (Surface &surface : bottom)
                             surface.surface_type = stBottom;
                     }
@@ -961,13 +962,13 @@ void PrintObject::detect_surfaces_type()
         #endif /* SLIC3R_DEBUG_SLICE_PROCESSING */
                     
                     // save surfaces to layer
-                    Surfaces &surfaces_out = interface_shells ? surfaces_new[idx_layer] : layerm->slices.surfaces;
+                    Surfaces &surfaces_out = interface_shells ? surfaces_new[idx_layer] : layerm->slices().surfaces;
                     Surfaces  surfaces_backup;
                     if (! interface_shells) {
                         surfaces_backup = std::move(surfaces_out);
                         surfaces_out.clear();
                     }
-                    const Surfaces &surfaces_prev = interface_shells ? layerm->slices.surfaces : surfaces_backup;
+                    const Surfaces &surfaces_prev = interface_shells ? layerm->slices().surfaces : surfaces_backup;
 
                     // find internal surfaces (difference between top/bottom surfaces and others)
                     {
@@ -993,15 +994,15 @@ void PrintObject::detect_surfaces_type()
         if (interface_shells) {
             // Move surfaces_new to layerm->slices.surfaces
             for (size_t idx_layer = 0; idx_layer < num_layers; ++ idx_layer)
-                m_layers[idx_layer]->m_regions[region_id]->slices.surfaces = std::move(surfaces_new[idx_layer]);
+                m_layers[idx_layer]->m_regions[region_id]->m_slices.set(std::move(surfaces_new[idx_layer]));
         }
 
         if (spiral_vase) {
         	if (num_layers > 1)
 	        	// Turn the last bottom layer infill to a top infill, so it will be extruded with a proper pattern.
-	        	m_layers[num_layers - 1]->m_regions[region_id]->slices.set_type(stTop);
+	        	m_layers[num_layers - 1]->m_regions[region_id]->m_slices.set_type(stTop);
 	        for (size_t i = num_layers; i < m_layers.size(); ++ i)
-	        	m_layers[i]->m_regions[region_id]->slices.set_type(stInternal);
+	        	m_layers[i]->m_regions[region_id]->m_slices.set_type(stInternal);
         }
 
         BOOST_LOG_TRIVIAL(debug) << "Detecting solid surfaces for region " << region_id << " - clipping in parallel - start";
@@ -1048,7 +1049,7 @@ void PrintObject::process_external_surfaces()
 	    	bool expansions = false;
 	    	bool voids      = false;
 	    	for (const LayerRegion *layerm : layer->regions()) {
-	    		for (const Surface &surface : layerm->fill_surfaces.surfaces) {
+	    		for (const Surface &surface : layerm->fill_surfaces()) {
 	    			if (surface.surface_type == stInternal)
 	    				voids = true;
 	    			else
@@ -1073,7 +1074,7 @@ void PrintObject::process_external_surfaces()
 		                Polygons voids;
 		                for (const LayerRegion *layerm : m_layers[layer_idx]->regions()) {
 		                	if (layerm->region().config().fill_density.value == 0.)
-		                		for (const Surface &surface : layerm->fill_surfaces.surfaces)
+		                		for (const Surface &surface : layerm->fill_surfaces())
 		                			// Shrink the holes, let the layer above expand slightly inside the unsupported areas.
 		                			polygons_append(voids, offset(surface.expolygon, unsupported_width));
 		                }
@@ -1102,7 +1103,7 @@ void PrintObject::process_external_surfaces()
         m_print->throw_if_canceled();
         BOOST_LOG_TRIVIAL(debug) << "Processing external surfaces for region " << region_id << " in parallel - end";
     }
-}
+} // void PrintObject::process_external_surfaces()
 
 void PrintObject::discover_vertical_shells()
 {
@@ -1170,15 +1171,15 @@ void PrintObject::discover_vertical_shells()
                         LayerRegion &layerm                       = *layer.m_regions[region_id];
                         float        min_perimeter_infill_spacing = float(layerm.flow(frSolidInfill).scaled_spacing()) * 1.05f;
                         // Top surfaces.
-                        append(cache.top_surfaces, offset(layerm.slices.filter_by_type(stTop), min_perimeter_infill_spacing));
-                        append(cache.top_surfaces, offset(layerm.fill_surfaces.filter_by_type(stTop), min_perimeter_infill_spacing));
+                        append(cache.top_surfaces, offset(layerm.slices().filter_by_type(stTop), min_perimeter_infill_spacing));
+                        append(cache.top_surfaces, offset(layerm.fill_surfaces().filter_by_type(stTop), min_perimeter_infill_spacing));
                         // Bottom surfaces.
-                        append(cache.bottom_surfaces, offset(layerm.slices.filter_by_types(surfaces_bottom, 2), min_perimeter_infill_spacing));
-                        append(cache.bottom_surfaces, offset(layerm.fill_surfaces.filter_by_types(surfaces_bottom, 2), min_perimeter_infill_spacing));
+                        append(cache.bottom_surfaces, offset(layerm.slices().filter_by_types(surfaces_bottom, 2), min_perimeter_infill_spacing));
+                        append(cache.bottom_surfaces, offset(layerm.fill_surfaces().filter_by_types(surfaces_bottom, 2), min_perimeter_infill_spacing));
                         // Calculate the maximum perimeter offset as if the slice was extruded with a single extruder only.
                         // First find the maxium number of perimeters per region slice.
                         unsigned int perimeters = 0;
-                        for (Surface &s : layerm.slices.surfaces)
+                        for (const Surface &s : layerm.slices())
                             perimeters = std::max<unsigned int>(perimeters, s.extra_perimeters);
                         perimeters += layerm.region().config().perimeters.value;
                         // Then calculate the infill offset.
@@ -1189,7 +1190,7 @@ void PrintObject::discover_vertical_shells()
                                 0.5f * float(extflow.scaled_width() + extflow.scaled_spacing()) + (float(perimeters) - 1.f) * flow.scaled_spacing());
                             perimeter_min_spacing = std::min(perimeter_min_spacing, float(std::min(extflow.scaled_spacing(), flow.scaled_spacing())));
                         }
-                        polygons_append(cache.holes, to_polygons(layerm.fill_expolygons));
+                        polygons_append(cache.holes, to_polygons(layerm.fill_expolygons()));
                     }
                     // Save some computing time by reducing the number of polygons.
                     cache.top_surfaces    = union_(cache.top_surfaces);
@@ -1242,15 +1243,15 @@ void PrintObject::discover_vertical_shells()
                         float        min_perimeter_infill_spacing = float(layerm.flow(frSolidInfill).scaled_spacing()) * 1.05f;
                         // Top surfaces.
                         auto &cache = cache_top_botom_regions[idx_layer];
-                        cache.top_surfaces = offset(layerm.slices.filter_by_type(stTop), min_perimeter_infill_spacing);
-                        append(cache.top_surfaces, offset(layerm.fill_surfaces.filter_by_type(stTop), min_perimeter_infill_spacing));
+                        cache.top_surfaces = offset(layerm.slices().filter_by_type(stTop), min_perimeter_infill_spacing);
+                        append(cache.top_surfaces, offset(layerm.fill_surfaces().filter_by_type(stTop), min_perimeter_infill_spacing));
                         // Bottom surfaces.
-                        cache.bottom_surfaces = offset(layerm.slices.filter_by_types(surfaces_bottom, 2), min_perimeter_infill_spacing);
-                        append(cache.bottom_surfaces, offset(layerm.fill_surfaces.filter_by_types(surfaces_bottom, 2), min_perimeter_infill_spacing));
+                        cache.bottom_surfaces = offset(layerm.slices().filter_by_types(surfaces_bottom, 2), min_perimeter_infill_spacing);
+                        append(cache.bottom_surfaces, offset(layerm.fill_surfaces().filter_by_types(surfaces_bottom, 2), min_perimeter_infill_spacing));
                         // Holes over all regions. Only collect them once, they are valid for all region_id iterations.
                         if (cache.holes.empty()) {
                             for (size_t region_id = 0; region_id < layer.regions().size(); ++ region_id)
-                                polygons_append(cache.holes, to_polygons(layer.regions()[region_id]->fill_expolygons));
+                                polygons_append(cache.holes, to_polygons(layer.regions()[region_id]->fill_expolygons()));
                         }
                     }
                 });
@@ -1407,14 +1408,14 @@ void PrintObject::discover_vertical_shells()
 
                     // Trim the shells region by the internal & internal void surfaces.
                     const SurfaceType surfaceTypesInternal[] = { stInternal, stInternalVoid, stInternalSolid };
-                    const Polygons    polygonsInternal = to_polygons(layerm->fill_surfaces.filter_by_types(surfaceTypesInternal, 3));
+                    const Polygons    polygonsInternal = to_polygons(layerm->fill_surfaces().filter_by_types(surfaceTypesInternal, 3));
                     shell = intersection(shell, polygonsInternal, ApplySafetyOffset::Yes);
                     polygons_append(shell, diff(polygonsInternal, holes));
                     if (shell.empty())
                         continue;
 
                     // Append the internal solids, so they will be merged with the new ones.
-                    polygons_append(shell, to_polygons(layerm->fill_surfaces.filter_by_type(stInternalSolid)));
+                    polygons_append(shell, to_polygons(layerm->fill_surfaces().filter_by_type(stInternalSolid)));
 
                     // These regions will be filled by a rectilinear full infill. Currently this type of infill
                     // only fills regions, which fit at least a single line. To avoid gaps in the sparse infill,
@@ -1461,8 +1462,8 @@ void PrintObject::discover_vertical_shells()
 #endif /* SLIC3R_DEBUG_SLICE_PROCESSING */
 
                     // Trim the internal & internalvoid by the shell.
-                    Slic3r::ExPolygons new_internal = diff_ex(layerm->fill_surfaces.filter_by_type(stInternal), shell);
-                    Slic3r::ExPolygons new_internal_void = diff_ex(layerm->fill_surfaces.filter_by_type(stInternalVoid), shell);
+                    Slic3r::ExPolygons new_internal = diff_ex(layerm->fill_surfaces().filter_by_type(stInternal), shell);
+                    Slic3r::ExPolygons new_internal_void = diff_ex(layerm->fill_surfaces().filter_by_type(stInternalVoid), shell);
 
 #ifdef SLIC3R_DEBUG_SLICE_PROCESSING
                     {
@@ -1474,10 +1475,10 @@ void PrintObject::discover_vertical_shells()
 
                     // Assign resulting internal surfaces to layer.
                     const SurfaceType surfaceTypesKeep[] = { stTop, stBottom, stBottomBridge };
-                    layerm->fill_surfaces.keep_types(surfaceTypesKeep, sizeof(surfaceTypesKeep)/sizeof(SurfaceType));
-                    layerm->fill_surfaces.append(new_internal,       stInternal);
-                    layerm->fill_surfaces.append(new_internal_void,  stInternalVoid);
-                    layerm->fill_surfaces.append(new_internal_solid, stInternalSolid);
+                    layerm->m_fill_surfaces.keep_types(surfaceTypesKeep, sizeof(surfaceTypesKeep)/sizeof(SurfaceType));
+                    layerm->m_fill_surfaces.append(new_internal,       stInternal);
+                    layerm->m_fill_surfaces.append(new_internal_void,  stInternalVoid);
+                    layerm->m_fill_surfaces.append(new_internal_solid, stInternalSolid);
                 } // for each layer
             });
         m_print->throw_if_canceled();
@@ -1491,7 +1492,7 @@ void PrintObject::discover_vertical_shells()
 		}
 #endif /* SLIC3R_DEBUG_SLICE_PROCESSING */
     } // for each region
-}
+} // void PrintObject::discover_vertical_shells()
 
 // This method applies bridge flow to the first internal solid layer above sparse infill.
 void PrintObject::bridge_over_infill()
@@ -1514,7 +1515,7 @@ void PrintObject::bridge_over_infill()
     for (Layer *layer : m_layers) {
         Polygons sum;
         for (const LayerRegion *layerm : layer->m_regions)
-            layerm->fill_surfaces.filter_by_type(stInternal, &sum);
+            layerm->fill_surfaces().filter_by_type(stInternal, &sum);
         internals.emplace_back(std::move(sum));
     }
 
@@ -1531,7 +1532,7 @@ void PrintObject::bridge_over_infill()
 
             // Extract the stInternalSolid surfaces that might be transformed into bridges.
             ExPolygons internal_solid;
-            layerm->fill_surfaces.remove_type(stInternalSolid, &internal_solid);
+            layerm->m_fill_surfaces.remove_type(stInternalSolid, &internal_solid);
             if (internal_solid.empty())
                 // No internal solid -> no new bridges for this layer region.
                 continue;
@@ -1563,7 +1564,7 @@ void PrintObject::bridge_over_infill()
                 if (to_bridge_pp.empty()) {
                     // Restore internal_solid surfaces. 
                     for (ExPolygon &ex : internal_solid)
-                        layerm->fill_surfaces.surfaces.push_back(Surface(stInternalSolid, std::move(ex)));
+                        layerm->m_fill_surfaces.surfaces.push_back(Surface(stInternalSolid, std::move(ex)));
                     continue;
                 }
                 // convert into ExPolygons
@@ -1579,9 +1580,9 @@ void PrintObject::bridge_over_infill()
             to_bridge = intersection_ex(to_bridge, internal_solid, ApplySafetyOffset::Yes);
             // build the new collection of fill_surfaces
             for (ExPolygon &ex : to_bridge)
-                layerm->fill_surfaces.surfaces.push_back(Surface(stInternalBridge, std::move(ex)));
+                layerm->m_fill_surfaces.surfaces.push_back(Surface(stInternalBridge, std::move(ex)));
             for (ExPolygon &ex : not_to_bridge)
-                layerm->fill_surfaces.surfaces.push_back(Surface(stInternalSolid, std::move(ex)));
+                layerm->m_fill_surfaces.surfaces.push_back(Surface(stInternalSolid, std::move(ex)));
             /*
             # exclude infill from the layers below if needed
             # see discussion at https://github.com/alexrj/Slic3r/issues/240
@@ -1622,7 +1623,7 @@ void PrintObject::bridge_over_infill()
             m_print->throw_if_canceled();
         }
     });
-}
+} // void PrintObject::bridge_over_infill()
 
 static void clamp_exturder_to_default(ConfigOptionInt &opt, size_t num_extruders)
 {
@@ -1829,7 +1830,7 @@ void PrintObject::clip_fill_surfaces()
         // Solid surfaces to be supported.
         Polygons overhangs;
         for (const LayerRegion *layerm : layer->m_regions)
-            for (const Surface &surface : layerm->fill_surfaces.surfaces) {
+            for (const Surface &surface : layerm->fill_surfaces()) {
                 Polygons polygons = to_polygons(surface.expolygon);
                 if (surface.is_solid())
                     polygons_append(overhangs, polygons);
@@ -1838,7 +1839,7 @@ void PrintObject::clip_fill_surfaces()
         Polygons lower_layer_fill_surfaces;
         Polygons lower_layer_internal_surfaces;
         for (const LayerRegion *layerm : lower_layer->m_regions)
-            for (const Surface &surface : layerm->fill_surfaces.surfaces) {
+            for (const Surface &surface : layerm->fill_surfaces()) {
                 Polygons polygons = to_polygons(surface.expolygon);
                 if (surface.surface_type == stInternal || surface.surface_type == stInternalVoid)
                     polygons_append(lower_layer_internal_surfaces, polygons);
@@ -1873,12 +1874,12 @@ void PrintObject::clip_fill_surfaces()
                 continue;
             SurfaceType internal_surface_types[] = { stInternal, stInternalVoid };
             Polygons internal;
-            for (Surface &surface : layerm->fill_surfaces.surfaces)
+            for (Surface &surface : layerm->m_fill_surfaces.surfaces)
                 if (surface.surface_type == stInternal || surface.surface_type == stInternalVoid)
                     polygons_append(internal, std::move(surface.expolygon));
-            layerm->fill_surfaces.remove_types(internal_surface_types, 2);
-            layerm->fill_surfaces.append(intersection_ex(internal, upper_internal, ApplySafetyOffset::Yes), stInternal);
-            layerm->fill_surfaces.append(diff_ex        (internal, upper_internal, ApplySafetyOffset::Yes), stInternalVoid);
+            layerm->m_fill_surfaces.remove_types(internal_surface_types, 2);
+            layerm->m_fill_surfaces.append(intersection_ex(internal, upper_internal, ApplySafetyOffset::Yes), stInternal);
+            layerm->m_fill_surfaces.append(diff_ex        (internal, upper_internal, ApplySafetyOffset::Yes), stInternalVoid);
             // If there are voids it means that our internal infill is not adjacent to
             // perimeters. In this case it would be nice to add a loop around infill to
             // make it more robust and nicer. TODO.
@@ -1888,7 +1889,7 @@ void PrintObject::clip_fill_surfaces()
         }
         m_print->throw_if_canceled();
     }
-}
+} // void PrintObject::clip_fill_surfaces()
 
 void PrintObject::discover_horizontal_shells()
 {
@@ -1904,7 +1905,7 @@ void PrintObject::discover_horizontal_shells()
                 (i % region_config.solid_infill_every_layers) == 0) {
                 // Insert a solid internal layer. Mark stInternal surfaces as stInternalSolid or stInternalBridge.
                 SurfaceType type = (region_config.fill_density == 100 || region_config.solid_infill_every_layers == 1) ? stInternalSolid : stInternalBridge;
-                for (Surface &surface : layerm->fill_surfaces.surfaces)
+                for (Surface &surface : layerm->m_fill_surfaces.surfaces)
                     if (surface.surface_type == stInternal)
                         surface.surface_type = type;
             }
@@ -1935,11 +1936,11 @@ void PrintObject::discover_horizontal_shells()
                 // (not covered by a layer above / below).
                 // This does not contain the areas covered by perimeters!
                 Polygons solid;
-                for (const Surface &surface : layerm->slices.surfaces)
+                for (const Surface &surface : layerm->slices())
                     if (surface.surface_type == type)
                         polygons_append(solid, to_polygons(surface.expolygon));
                 // Infill areas (slices without the perimeters).
-                for (const Surface &surface : layerm->fill_surfaces.surfaces)
+                for (const Surface &surface : layerm->fill_surfaces())
                     if (surface.surface_type == type)
                         polygons_append(solid, to_polygons(surface.expolygon));
                 if (solid.empty())
@@ -1972,7 +1973,7 @@ void PrintObject::discover_horizontal_shells()
                     Polygons new_internal_solid;
                     {
                         Polygons internal;
-                        for (const Surface &surface : neighbor_layerm->fill_surfaces.surfaces)
+                        for (const Surface &surface : neighbor_layerm->fill_surfaces())
                             if (surface.surface_type == stInternal || surface.surface_type == stInternalSolid)
                                 polygons_append(internal, to_polygons(surface.expolygon));
                         new_internal_solid = intersection(solid, internal, ApplySafetyOffset::Yes);
@@ -2028,7 +2029,7 @@ void PrintObject::discover_horizontal_shells()
                             // additional area in the next shell too
                             // make sure our grown surfaces don't exceed the fill area
                             Polygons internal;
-                            for (const Surface &surface : neighbor_layerm->fill_surfaces.surfaces)
+                            for (const Surface &surface : neighbor_layerm->fill_surfaces())
                                 if (surface.is_internal() && !surface.is_bridge())
                                     polygons_append(internal, to_polygons(surface.expolygon));
                             polygons_append(new_internal_solid, 
@@ -2047,16 +2048,16 @@ void PrintObject::discover_horizontal_shells()
                     
                     // internal-solid are the union of the existing internal-solid surfaces
                     // and new ones
-                    SurfaceCollection backup = std::move(neighbor_layerm->fill_surfaces);
+                    SurfaceCollection backup = std::move(neighbor_layerm->m_fill_surfaces);
                     polygons_append(new_internal_solid, to_polygons(backup.filter_by_type(stInternalSolid)));
                     ExPolygons internal_solid = union_ex(new_internal_solid);
                     // assign new internal-solid surfaces to layer
-                    neighbor_layerm->fill_surfaces.set(internal_solid, stInternalSolid);
+                    neighbor_layerm->m_fill_surfaces.set(internal_solid, stInternalSolid);
                     // subtract intersections from layer surfaces to get resulting internal surfaces
                     Polygons polygons_internal = to_polygons(std::move(internal_solid));
                     ExPolygons internal = diff_ex(backup.filter_by_type(stInternal), polygons_internal, ApplySafetyOffset::Yes);
                     // assign resulting internal surfaces to layer
-                    neighbor_layerm->fill_surfaces.append(internal, stInternal);
+                    neighbor_layerm->m_fill_surfaces.append(internal, stInternal);
                     polygons_append(polygons_internal, to_polygons(std::move(internal)));
                     // assign top and bottom surfaces to layer
                     SurfaceType surface_types_solid[] = { stTop, stBottom, stBottomBridge };
@@ -2064,7 +2065,7 @@ void PrintObject::discover_horizontal_shells()
                     std::vector<SurfacesPtr> top_bottom_groups;
                     backup.group(&top_bottom_groups);
                     for (SurfacesPtr &group : top_bottom_groups)
-                        neighbor_layerm->fill_surfaces.append(
+                        neighbor_layerm->m_fill_surfaces.append(
                             diff_ex(group, polygons_internal),
                             // Use an existing surface as a template, it carries the bridge angle etc.
                             *group.front());
@@ -2083,7 +2084,7 @@ void PrintObject::discover_horizontal_shells()
         } // for each layer
     } // for each region
 #endif /* SLIC3R_DEBUG_SLICE_PROCESSING */
-}
+} // void PrintObject::discover_horizontal_shells()
 
 // combine fill surfaces across layers to honor the "infill every N layers" option
 // Idempotence of this method is guaranteed by the fact that we don't remove things from
@@ -2141,10 +2142,10 @@ void PrintObject::combine_infill()
                 layerms.emplace_back(m_layers[i]->regions()[region_id]);
             // We need to perform a multi-layer intersection, so let's split it in pairs.
             // Initialize the intersection with the candidates of the lowest layer.
-            ExPolygons intersection = to_expolygons(layerms.front()->fill_surfaces.filter_by_type(stInternal));
+            ExPolygons intersection = to_expolygons(layerms.front()->fill_surfaces().filter_by_type(stInternal));
             // Start looping from the second layer and intersect the current intersection with it.
             for (size_t i = 1; i < layerms.size(); ++ i)
-                intersection = intersection_ex(layerms[i]->fill_surfaces.filter_by_type(stInternal), intersection);
+                intersection = intersection_ex(layerms[i]->fill_surfaces().filter_by_type(stInternal), intersection);
             double area_threshold = layerms.front()->infill_area_threshold();
             if (! intersection.empty() && area_threshold > 0.)
                 intersection.erase(std::remove_if(intersection.begin(), intersection.end(), 
@@ -2173,9 +2174,9 @@ void PrintObject::combine_infill()
             for (ExPolygon &expoly : intersection)
                 polygons_append(intersection_with_clearance, offset(expoly, clearance_offset));
             for (LayerRegion *layerm : layerms) {
-                Polygons internal = to_polygons(std::move(layerm->fill_surfaces.filter_by_type(stInternal)));
-                layerm->fill_surfaces.remove_type(stInternal);
-                layerm->fill_surfaces.append(diff_ex(internal, intersection_with_clearance), stInternal);
+                Polygons internal = to_polygons(std::move(layerm->fill_surfaces().filter_by_type(stInternal)));
+                layerm->m_fill_surfaces.remove_type(stInternal);
+                layerm->m_fill_surfaces.append(diff_ex(internal, intersection_with_clearance), stInternal);
                 if (layerm == layerms.back()) {
                     // Apply surfaces back with adjusted depth to the uppermost layer.
                     Surface templ(stInternal, ExPolygon());
@@ -2183,17 +2184,17 @@ void PrintObject::combine_infill()
                     for (LayerRegion *layerm2 : layerms)
                         templ.thickness += layerm2->layer()->height;
                     templ.thickness_layers = (unsigned short)layerms.size();
-                    layerm->fill_surfaces.append(intersection, templ);
+                    layerm->m_fill_surfaces.append(intersection, templ);
                 } else {
                     // Save void surfaces.
-                    layerm->fill_surfaces.append(
+                    layerm->m_fill_surfaces.append(
                         intersection_ex(internal, intersection_with_clearance),
                         stInternalVoid);
                 }
             }
         }
     }
-}
+} // void PrintObject::combine_infill()
 
 void PrintObject::_generate_support_material()
 {
diff --git a/src/libslic3r/PrintObjectSlice.cpp b/src/libslic3r/PrintObjectSlice.cpp
index 4b91714e5..f5ba3b9d7 100644
--- a/src/libslic3r/PrintObjectSlice.cpp
+++ b/src/libslic3r/PrintObjectSlice.cpp
@@ -432,12 +432,12 @@ std::string fix_slicing_errors(LayerPtrs &layers, const std::function<void()> &t
                     const Surfaces *lower_surfaces = nullptr;
                     for (size_t j = idx_layer + 1; j < layers.size(); ++ j)
                         if (! layers[j]->slicing_errors) {
-                            upper_surfaces = &layers[j]->regions()[region_id]->slices.surfaces;
+                            upper_surfaces = &layers[j]->regions()[region_id]->slices().surfaces;
                             break;
                         }
                     for (int j = int(idx_layer) - 1; j >= 0; -- j)
                         if (! layers[j]->slicing_errors) {
-                            lower_surfaces = &layers[j]->regions()[region_id]->slices.surfaces;
+                            lower_surfaces = &layers[j]->regions()[region_id]->slices().surfaces;
                             break;
                         }
                     // Collect outer contours and holes from the valid layers above & below.
@@ -464,7 +464,7 @@ std::string fix_slicing_errors(LayerPtrs &layers, const std::function<void()> &t
                     if (lower_surfaces)
                         for (const auto &surface : *lower_surfaces)
                             polygons_append(holes, surface.expolygon.holes);
-                    layerm->slices.set(diff_ex(union_(outer), holes), stInternal);
+                    layerm->m_slices.set(diff_ex(union_(outer), holes), stInternal);
                 }
                 // Update layer slices after repairing the single regions.
                 layer->make_slices();
@@ -517,17 +517,26 @@ void PrintObject::slice()
     // Update bounding boxes, back up raw slices of complex models.
     tbb::parallel_for(
         tbb::blocked_range<size_t>(0, m_layers.size()),
-        [this](const tbb::blocked_range<size_t>& range) {
+        [this](const tbb::blocked_range<size_t> &range) {
             for (size_t layer_idx = range.begin(); layer_idx < range.end(); ++ layer_idx) {
                 m_print->throw_if_canceled();
                 Layer &layer = *m_layers[layer_idx];
-                layer.lslices_bboxes.clear();
-                layer.lslices_bboxes.reserve(layer.lslices.size());
+                layer.lslices_ex.clear();
+                layer.lslices_ex.reserve(layer.lslices.size());
                 for (const ExPolygon &expoly : layer.lslices)
-                	layer.lslices_bboxes.emplace_back(get_extents(expoly));
+                	layer.lslices_ex.push_back({ get_extents(expoly) });
                 layer.backup_untyped_slices();
             }
         });
+    // Interlink the lslices into a Z graph.
+    tbb::parallel_for(
+        tbb::blocked_range<size_t>(1, m_layers.size()),
+        [this](const tbb::blocked_range<size_t> &range) {
+            for (size_t layer_idx = range.begin(); layer_idx < range.end(); ++ layer_idx) {
+                m_print->throw_if_canceled();
+                Layer::build_up_down_graph(*m_layers[layer_idx - 1], *m_layers[layer_idx]);
+            }
+        });
     if (m_layers.empty())
         throw Slic3r::SlicingError("No layers were detected. You might want to repair your STL file(s) or check their size or thickness and retry.\n");    
     this->set_done(posSlice);
@@ -579,9 +588,9 @@ static inline void apply_mm_segmentation(PrintObject &print_object, ThrowOnCance
                 // layer_range.painted_regions are sorted by extruder ID and parent PrintObject region ID.
                 auto it_painted_region = layer_range.painted_regions.begin();
                 for (int region_id = 0; region_id < int(layer->region_count()); ++ region_id)
-                    if (LayerRegion &layerm = *layer->get_region(region_id); ! layerm.slices.surfaces.empty()) {
+                    if (LayerRegion &layerm = *layer->get_region(region_id); ! layerm.slices().empty()) {
                         assert(layerm.region().print_object_region_id() == region_id);
-                        const BoundingBox bbox = get_extents(layerm.slices.surfaces);
+                        const BoundingBox bbox = get_extents(layerm.slices().surfaces);
                         assert(it_painted_region < layer_range.painted_regions.end());
                         // Find the first it_painted_region which overrides this region.
                         for (; layer_range.volume_regions[it_painted_region->parent].region->print_object_region_id() < region_id; ++ it_painted_region)
@@ -604,7 +613,7 @@ static inline void apply_mm_segmentation(PrintObject &print_object, ThrowOnCance
                                 }
                                 // Steal from this region.
                                 int         target_region_id = it_painted_region->region->print_object_region_id();
-                                ExPolygons  stolen           = intersection_ex(layerm.slices.surfaces, segmented.expolygons);
+                                ExPolygons  stolen           = intersection_ex(layerm.slices().surfaces, segmented.expolygons);
                                 if (! stolen.empty()) {
                                     ByRegion &dst = by_region[target_region_id];
                                     if (dst.expolygons.empty()) {
@@ -622,7 +631,7 @@ static inline void apply_mm_segmentation(PrintObject &print_object, ThrowOnCance
                             }
                         if (! self_trimmed) {
                             // Trim slices of this LayerRegion with all the MMU regions.
-                            Polygons mine = to_polygons(std::move(layerm.slices.surfaces));
+                            Polygons mine = to_polygons(std::move(layerm.slices().surfaces));
                             for (auto &segmented : by_extruder)
                                 if (&segmented - by_extruder.data() + 1 != self_extruder_id && segmented.bbox.defined && bbox.overlap(segmented.bbox)) {
                                     mine = diff(mine, segmented.expolygons);
@@ -653,7 +662,7 @@ static inline void apply_mm_segmentation(PrintObject &print_object, ThrowOnCance
                     if (src.needs_merge)
                         // Multiple regions were merged into one.
                         src.expolygons = closing_ex(src.expolygons, float(scale_(10 * EPSILON)));
-                    layer->get_region(region_id)->slices.set(std::move(src.expolygons), stInternal);
+                    layer->get_region(region_id)->m_slices.set(std::move(src.expolygons), stInternal);
                 }
             }
         });
@@ -693,7 +702,7 @@ void PrintObject::slice_volumes()
     for (size_t region_id = 0; region_id < region_slices.size(); ++ region_id) {
         std::vector<ExPolygons> &by_layer = region_slices[region_id];
         for (size_t layer_id = 0; layer_id < by_layer.size(); ++ layer_id)
-            m_layers[layer_id]->regions()[region_id]->slices.append(std::move(by_layer[layer_id]), stInternal);
+            m_layers[layer_id]->regions()[region_id]->m_slices.append(std::move(by_layer[layer_id]), stInternal);
     }
     region_slices.clear();
     
@@ -754,14 +763,14 @@ void PrintObject::slice_volumes()
 	                    LayerRegion *layerm = layer->m_regions.front();
 	                    if (elfoot > 0) {
 		                    // Apply the elephant foot compensation and store the 1st layer slices without the Elephant foot compensation applied.
-		                    lslices_1st_layer = to_expolygons(std::move(layerm->slices.surfaces));
+		                    lslices_1st_layer = to_expolygons(std::move(layerm->m_slices.surfaces));
 		                    float delta = xy_compensation_scaled;
 	                        if (delta > elfoot) {
 	                            delta -= elfoot;
 	                            elfoot = 0.f;
 	                        } else if (delta > 0)
 	                            elfoot -= delta;
-							layerm->slices.set(
+							layerm->m_slices.set(
 								union_ex(
 									Slic3r::elephant_foot_compensation(
 										(delta == 0.f) ? lslices_1st_layer : offset_ex(lslices_1st_layer, delta), 
@@ -771,8 +780,8 @@ void PrintObject::slice_volumes()
 								lslices_1st_layer = offset_ex(std::move(lslices_1st_layer), xy_compensation_scaled);
 	                    } else if (xy_compensation_scaled < 0.f) {
 	                        // Apply the XY compensation.
-	                        layerm->slices.set(
-                                offset_ex(to_expolygons(std::move(layerm->slices.surfaces)), xy_compensation_scaled),
+	                        layerm->m_slices.set(
+                                offset_ex(to_expolygons(std::move(layerm->m_slices.surfaces)), xy_compensation_scaled),
 	                            stInternal);
 	                    }
 	                } else {
diff --git a/src/libslic3r/SLA/DefaultSupportTree.cpp b/src/libslic3r/SLA/DefaultSupportTree.cpp
index 8634fc3bb..9e21fca45 100644
--- a/src/libslic3r/SLA/DefaultSupportTree.cpp
+++ b/src/libslic3r/SLA/DefaultSupportTree.cpp
@@ -180,7 +180,7 @@ bool DefaultSupportTree::interconnect(const Pillar &pillar,
     Vec3d eupper = pillar.endpoint();
     Vec3d elower = nextpillar.endpoint();
 
-    double zmin = ground_level(this->m_sm) + m_sm.cfg.base_height_mm;
+    double zmin = ground_level(m_sm) + m_sm.cfg.base_height_mm;
     eupper.z() = std::max(eupper.z(), zmin);
     elower.z() = std::max(elower.z(), zmin);
 
diff --git a/src/libslic3r/SupportMaterial.cpp b/src/libslic3r/SupportMaterial.cpp
index c4a4efbf2..7242bee3e 100644
--- a/src/libslic3r/SupportMaterial.cpp
+++ b/src/libslic3r/SupportMaterial.cpp
@@ -650,14 +650,14 @@ Polygons collect_region_slices_by_type(const Layer &layer, SurfaceType surface_t
     // 1) Count the new polygons first.
     size_t n_polygons_new = 0;
     for (const LayerRegion *region : layer.regions())
-        for (const Surface &surface : region->slices.surfaces)
+        for (const Surface &surface : region->slices())
             if (surface.surface_type == surface_type)
                 n_polygons_new += surface.expolygon.holes.size() + 1;
     // 2) Collect the new polygons.
     Polygons out;
     out.reserve(n_polygons_new);
     for (const LayerRegion *region : layer.regions())
-        for (const Surface &surface : region->slices.surfaces)
+        for (const Surface &surface : region->slices())
             if (surface.surface_type == surface_type)
                 polygons_append(out, surface.expolygon);
     return out;
@@ -1213,9 +1213,9 @@ namespace SupportMaterialInternal {
     static bool has_bridging_extrusions(const Layer &layer) 
     {
         for (const LayerRegion *region : layer.regions()) {
-            if (SupportMaterialInternal::has_bridging_perimeters(region->perimeters))
+            if (SupportMaterialInternal::has_bridging_perimeters(region->perimeters()))
                 return true;
-            if (region->fill_surfaces.has(stBottomBridge) && has_bridging_fills(region->fills))
+            if (region->fill_surfaces().has(stBottomBridge) && has_bridging_fills(region->fills()))
                 return true;
         }
         return false;
@@ -1287,7 +1287,7 @@ namespace SupportMaterialInternal {
             // Trim the perimeters of this layer by the lower layer to get the unsupported pieces of perimeters.
             overhang_perimeters = diff_pl(overhang_perimeters, lower_grown_slices);
         #else
-            Polylines overhang_perimeters = diff_pl(layerm.perimeters.as_polylines(), lower_grown_slices);
+            Polylines overhang_perimeters = diff_pl(layerm.perimeters().as_polylines(), lower_grown_slices);
         #endif
             
             // only consider straight overhangs
@@ -1311,7 +1311,7 @@ namespace SupportMaterialInternal {
                     bool  supported[2] = { false, false };
                     for (size_t i = 0; i < lower_layer.lslices.size() && ! (supported[0] && supported[1]); ++ i)
                         for (int j = 0; j < 2; ++ j)
-                            if (! supported[j] && lower_layer.lslices_bboxes[i].contains(pts[j]) && lower_layer.lslices[i].contains(pts[j]))
+                            if (! supported[j] && lower_layer.lslices_ex[i].bbox.contains(pts[j]) && lower_layer.lslices[i].contains(pts[j]))
                                 supported[j] = true;
                     if (supported[0] && supported[1])
                         // Offset a polyline into a thick line.
@@ -1321,7 +1321,7 @@ namespace SupportMaterialInternal {
         }
         // remove the entire bridges and only support the unsupported edges
         //FIXME the brided regions are already collected as layerm.bridged. Use it?
-        for (const Surface &surface : layerm.fill_surfaces.surfaces)
+        for (const Surface &surface : layerm.fill_surfaces())
             if (surface.surface_type == stBottomBridge && surface.bridge_angle != -1)
                 polygons_append(bridges, surface.expolygon);
         //FIXME add the gap filled areas. Extrude the gaps with a bridge flow?
@@ -1329,14 +1329,14 @@ namespace SupportMaterialInternal {
         //FIXME add supports at regular intervals to support long bridges!
         bridges = diff(bridges,
                 // Offset unsupported edges into polygons.
-                offset(layerm.unsupported_bridge_edges, scale_(SUPPORT_MATERIAL_MARGIN), SUPPORT_SURFACES_OFFSET_PARAMETERS));
+                offset(layerm.unsupported_bridge_edges(), scale_(SUPPORT_MATERIAL_MARGIN), SUPPORT_SURFACES_OFFSET_PARAMETERS));
         // Remove bridged areas from the supported areas.
         contact_polygons = diff(contact_polygons, bridges, ApplySafetyOffset::Yes);
 
         #ifdef SLIC3R_DEBUG
             static int iRun = 0;
             SVG::export_expolygons(debug_out_path("support-top-contacts-remove-bridges-run%d.svg", iRun ++),
-                { { { union_ex(offset(layerm.unsupported_bridge_edges, scale_(SUPPORT_MATERIAL_MARGIN), SUPPORT_SURFACES_OFFSET_PARAMETERS)) }, { "unsupported_bridge_edges", "orange", 0.5f } },
+                { { { union_ex(offset(layerm.unsupported_bridge_edges(), scale_(SUPPORT_MATERIAL_MARGIN), SUPPORT_SURFACES_OFFSET_PARAMETERS)) }, { "unsupported_bridge_edges", "orange", 0.5f } },
                   { { union_ex(contact_polygons) },            { "contact_polygons",           "blue",   0.5f } },
                   { { union_ex(bridges) },                     { "bridges",                    "red",    "black", "", scaled<coord_t>(0.1f), 0.5f } } });
         #endif /* SLIC3R_DEBUG */
@@ -1487,7 +1487,7 @@ static inline std::tuple<Polygons, Polygons, Polygons, float> detect_overhangs(
                     0.5f * fw);
             // Overhang polygons for this layer and region.
             Polygons diff_polygons;
-            Polygons layerm_polygons = to_polygons(layerm->slices.surfaces);
+            Polygons layerm_polygons = to_polygons(layerm->slices().surfaces);
             if (lower_layer_offset == 0.f) {
                 // Support everything.
                 diff_polygons = diff(layerm_polygons, lower_layer_polygons);
@@ -2858,10 +2858,10 @@ void PrintObjectSupportMaterial::trim_support_layers_by_object(
                                 break;
                             some_region_overlaps = true;
                             polygons_append(polygons_trimming, 
-                                offset(region->fill_surfaces.filter_by_type(stBottomBridge), gap_xy_scaled, SUPPORT_SURFACES_OFFSET_PARAMETERS));
+                                offset(region->fill_surfaces().filter_by_type(stBottomBridge), gap_xy_scaled, SUPPORT_SURFACES_OFFSET_PARAMETERS));
                             if (region->region().config().overhangs.value)
                                 // Add bridging perimeters.
-                                SupportMaterialInternal::collect_bridging_perimeter_areas(region->perimeters, gap_xy_scaled, polygons_trimming);
+                                SupportMaterialInternal::collect_bridging_perimeter_areas(region->perimeters(), gap_xy_scaled, polygons_trimming);
                         }
                         if (! some_region_overlaps)
                             break;
diff --git a/src/libslic3r/SupportSpotsGenerator.cpp b/src/libslic3r/SupportSpotsGenerator.cpp
index 09b2bc5ae..8dd4a013d 100644
--- a/src/libslic3r/SupportSpotsGenerator.cpp
+++ b/src/libslic3r/SupportSpotsGenerator.cpp
@@ -1039,12 +1039,12 @@ std::tuple<Issues, std::vector<LayerIslands>> check_extrusions_and_build_graph(c
 // PREPARE BASE LAYER
     const Layer *layer = po->layers()[0];
     for (const LayerRegion *layer_region : layer->regions()) {
-        for (const ExtrusionEntity *ex_entity : layer_region->perimeters.entities) {
+        for (const ExtrusionEntity *ex_entity : layer_region->perimeters()) {
             for (const ExtrusionEntity *perimeter : static_cast<const ExtrusionEntityCollection*>(ex_entity)->entities) {
                 push_lines(perimeter, layer_lines);
             } // perimeter
         } // ex_entity
-        for (const ExtrusionEntity *ex_entity : layer_region->fills.entities) {
+        for (const ExtrusionEntity *ex_entity : layer_region->fills()) {
             for (const ExtrusionEntity *fill : static_cast<const ExtrusionEntityCollection*>(ex_entity)->entities) {
                 push_lines(fill, layer_lines);
             } // fill
@@ -1083,13 +1083,13 @@ std::tuple<Issues, std::vector<LayerIslands>> check_extrusions_and_build_graph(c
     for (size_t layer_idx = 1; layer_idx < po->layer_count(); ++layer_idx) {
         const Layer *layer = po->layers()[layer_idx];
         for (const LayerRegion *layer_region : layer->regions()) {
-            for (const ExtrusionEntity *ex_entity : layer_region->perimeters.entities) {
+            for (const ExtrusionEntity *ex_entity : layer_region->perimeters()) {
                 for (const ExtrusionEntity *perimeter : static_cast<const ExtrusionEntityCollection*>(ex_entity)->entities) {
                     check_extrusion_entity_stability(perimeter, layer_lines, layer->slice_z, layer_region,
                             external_lines, issues, params);
                 } // perimeter
             } // ex_entity
-            for (const ExtrusionEntity *ex_entity : layer_region->fills.entities) {
+            for (const ExtrusionEntity *ex_entity : layer_region->fills()) {
                 for (const ExtrusionEntity *fill : static_cast<const ExtrusionEntityCollection*>(ex_entity)->entities) {
                     if (fill->role() == ExtrusionRole::erGapFill
                             || fill->role() == ExtrusionRole::erBridgeInfill) {
diff --git a/src/libslic3r/Surface.hpp b/src/libslic3r/Surface.hpp
index ef1de30e9..4afcdf6f5 100644
--- a/src/libslic3r/Surface.hpp
+++ b/src/libslic3r/Surface.hpp
@@ -104,7 +104,7 @@ public:
 };
 
 typedef std::vector<Surface> Surfaces;
-typedef std::vector<Surface*> SurfacesPtr;
+typedef std::vector<const Surface*> SurfacesPtr;
 
 inline Polygons to_polygons(const Surface &surface)
 {
@@ -221,6 +221,7 @@ inline void polygons_append(Polygons &dst, const SurfacesPtr &src)
     }
 }
 
+/*
 inline void polygons_append(Polygons &dst, SurfacesPtr &&src) 
 { 
     dst.reserve(dst.size() + number_polygons(src));
@@ -230,6 +231,7 @@ inline void polygons_append(Polygons &dst, SurfacesPtr &&src)
         (*it)->expolygon.holes.clear();
     }
 }
+*/
 
 // Append a vector of Surfaces at the end of another vector of polygons.
 inline void surfaces_append(Surfaces &dst, const ExPolygons &src, SurfaceType surfaceType) 
diff --git a/src/libslic3r/SurfaceCollection.cpp b/src/libslic3r/SurfaceCollection.cpp
index 2ec071f7d..10a12b683 100644
--- a/src/libslic3r/SurfaceCollection.cpp
+++ b/src/libslic3r/SurfaceCollection.cpp
@@ -22,46 +22,45 @@ void SurfaceCollection::simplify(double tolerance)
 }
 
 /* group surfaces by common properties */
-void SurfaceCollection::group(std::vector<SurfacesPtr> *retval)
+void SurfaceCollection::group(std::vector<SurfacesPtr> *retval) const
 {
-    for (Surfaces::iterator it = this->surfaces.begin(); it != this->surfaces.end(); ++it) {
+    for (const Surface &surface : this->surfaces) {
         // find a group with the same properties
-        SurfacesPtr* group = NULL;
+        SurfacesPtr *group = nullptr;
         for (std::vector<SurfacesPtr>::iterator git = retval->begin(); git != retval->end(); ++git)
-            if (! git->empty() && surfaces_could_merge(*git->front(), *it)) {
+            if (! git->empty() && surfaces_could_merge(*git->front(), surface)) {
                 group = &*git;
                 break;
             }
         // if no group with these properties exists, add one
-        if (group == NULL) {
+        if (group == nullptr) {
             retval->resize(retval->size() + 1);
             group = &retval->back();
         }
         // append surface to group
-        group->push_back(&*it);
+        group->push_back(&surface);
     }
 }
 
-SurfacesPtr SurfaceCollection::filter_by_type(const SurfaceType type)
+SurfacesPtr SurfaceCollection::filter_by_type(const SurfaceType type) const
 {
     SurfacesPtr ss;
-    for (Surfaces::iterator surface = this->surfaces.begin(); surface != this->surfaces.end(); ++surface) {
-        if (surface->surface_type == type) ss.push_back(&*surface);
-    }
+    for (const Surface &surface : this->surfaces)
+        if (surface.surface_type == type)
+            ss.push_back(&surface);
     return ss;
 }
 
-SurfacesPtr SurfaceCollection::filter_by_types(const SurfaceType *types, int ntypes)
+SurfacesPtr SurfaceCollection::filter_by_types(const SurfaceType *types, int ntypes) const
 {
     SurfacesPtr ss;
-    for (Surfaces::iterator surface = this->surfaces.begin(); surface != this->surfaces.end(); ++surface) {
+    for (const Surface &surface : this->surfaces)
         for (int i = 0; i < ntypes; ++ i) {
-            if (surface->surface_type == types[i]) {
-                ss.push_back(&*surface);
+            if (surface.surface_type == types[i]) {
+                ss.push_back(&surface);
                 break;
             }
         }
-    }
     return ss;
 }
 
diff --git a/src/libslic3r/SurfaceCollection.hpp b/src/libslic3r/SurfaceCollection.hpp
index 8c27a507b..b81808b32 100644
--- a/src/libslic3r/SurfaceCollection.hpp
+++ b/src/libslic3r/SurfaceCollection.hpp
@@ -17,7 +17,7 @@ public:
     SurfaceCollection(Surfaces &&surfaces) : surfaces(std::move(surfaces)) {};
 
     void simplify(double tolerance);
-    void group(std::vector<SurfacesPtr> *retval);
+    void group(std::vector<SurfacesPtr> *retval) const;
     template <class T> bool any_internal_contains(const T &item) const {
         for (const Surface &surface : this->surfaces) if (surface.is_internal() && surface.expolygon.contains(item)) return true;
         return false;
@@ -26,8 +26,8 @@ public:
         for (const Surface &surface : this->surfaces) if (surface.is_bottom() && surface.expolygon.contains(item)) return true;
         return false;
     }
-    SurfacesPtr filter_by_type(const SurfaceType type);
-    SurfacesPtr filter_by_types(const SurfaceType *types, int ntypes);
+    SurfacesPtr filter_by_type(const SurfaceType type) const;
+    SurfacesPtr filter_by_types(const SurfaceType *types, int ntypes) const;
     void keep_type(const SurfaceType type);
     void keep_types(const SurfaceType *types, int ntypes);
     void remove_type(const SurfaceType type);
@@ -48,6 +48,13 @@ public:
         return false;
     }
 
+    Surfaces::const_iterator    cbegin() const { return this->surfaces.cbegin(); }
+    Surfaces::const_iterator    cend()   const { return this->surfaces.cend(); }
+    Surfaces::const_iterator    begin()  const { return this->surfaces.cbegin(); }
+    Surfaces::const_iterator    end()    const { return this->surfaces.cend(); }
+    Surfaces::iterator          begin()        { return this->surfaces.begin(); }
+    Surfaces::iterator          end()          { return this->surfaces.end(); }
+
     void set(const SurfaceCollection &coll) { surfaces = coll.surfaces; }
     void set(SurfaceCollection &&coll) { surfaces = std::move(coll.surfaces); }
     void set(const ExPolygons &src, SurfaceType surfaceType) { clear(); this->append(src, surfaceType); }
diff --git a/src/libslic3r/TriangleMesh.cpp b/src/libslic3r/TriangleMesh.cpp
index df820fac9..97175ea89 100644
--- a/src/libslic3r/TriangleMesh.cpp
+++ b/src/libslic3r/TriangleMesh.cpp
@@ -284,7 +284,7 @@ void TriangleMesh::rotate(float angle, const Axis &axis)
         case Z:  its_rotate_z(this->its, angle); break;
         default: assert(false);                  return;
         }
-        update_bounding_box(this->its, this->m_stats);
+        update_bounding_box(this->its, m_stats);
     }
 }
 
@@ -295,7 +295,7 @@ void TriangleMesh::rotate(float angle, const Vec3d& axis)
         Transform3d m = Transform3d::Identity();
         m.rotate(Eigen::AngleAxisd(angle, axis_norm));
         its_transform(its, m);
-        update_bounding_box(this->its, this->m_stats);
+        update_bounding_box(this->its, m_stats);
     }
 }
 
@@ -334,7 +334,7 @@ void TriangleMesh::transform(const Transform3d& t, bool fix_left_handed)
         det = -det;
     }
     m_stats.volume *= det;
-    update_bounding_box(this->its, this->m_stats);
+    update_bounding_box(this->its, m_stats);
 }
 
 void TriangleMesh::transform(const Matrix3d& m, bool fix_left_handed)
@@ -346,7 +346,7 @@ void TriangleMesh::transform(const Matrix3d& m, bool fix_left_handed)
         det = -det;
     }
     m_stats.volume *= det;
-    update_bounding_box(this->its, this->m_stats);
+    update_bounding_box(this->its, m_stats);
 }
 
 void TriangleMesh::flip_triangles()
@@ -512,7 +512,7 @@ std::vector<ExPolygons> TriangleMesh::slice(const std::vector<double> &z) const
 
 size_t TriangleMesh::memsize() const
 {
-    size_t memsize = 8 + this->its.memsize() + sizeof(this->m_stats);
+    size_t memsize = 8 + this->its.memsize() + sizeof(m_stats);
     return memsize;
 }
 
diff --git a/src/libslic3r/TriangleMesh.hpp b/src/libslic3r/TriangleMesh.hpp
index f30776307..6be4c4173 100644
--- a/src/libslic3r/TriangleMesh.hpp
+++ b/src/libslic3r/TriangleMesh.hpp
@@ -92,7 +92,7 @@ public:
     TriangleMesh(std::vector<Vec3f> &&vertices, const std::vector<Vec3i> &&faces);
     explicit TriangleMesh(const indexed_triangle_set &M);
     explicit TriangleMesh(indexed_triangle_set &&M, const RepairedMeshErrors& repaired_errors = RepairedMeshErrors());
-    void clear() { this->its.clear(); this->m_stats.clear(); }
+    void clear() { this->its.clear(); m_stats.clear(); }
     bool ReadSTLFile(const char* input_file, bool repair = true);
     bool write_ascii(const char* output_file);
     bool write_binary(const char* output_file);
diff --git a/src/libslic3r/TriangleSelector.cpp b/src/libslic3r/TriangleSelector.cpp
index 820c26acc..90f77d738 100644
--- a/src/libslic3r/TriangleSelector.cpp
+++ b/src/libslic3r/TriangleSelector.cpp
@@ -371,7 +371,7 @@ std::pair<std::vector<Vec3i>, std::vector<Vec3i>> TriangleSelector::precompute_a
 {
     std::vector<Vec3i> neighbors(m_triangles.size(), Vec3i(-1, -1, -1));
     std::vector<Vec3i> neighbors_propagated(m_triangles.size(), Vec3i(-1, -1, -1));
-    for (int facet_idx = 0; facet_idx < this->m_orig_size_indices; ++facet_idx) {
+    for (int facet_idx = 0; facet_idx < m_orig_size_indices; ++facet_idx) {
         neighbors[facet_idx]            = m_neighbors[facet_idx];
         neighbors_propagated[facet_idx] = neighbors[facet_idx];
         assert(this->verify_triangle_neighbors(m_triangles[facet_idx], neighbors[facet_idx]));
@@ -1480,7 +1480,7 @@ void TriangleSelector::get_facets_split_by_tjoints(const Vec3i &vertices, const
 
 std::vector<Vec2i> TriangleSelector::get_seed_fill_contour() const {
     std::vector<Vec2i> edges_out;
-    for (int facet_idx = 0; facet_idx < this->m_orig_size_indices; ++facet_idx) {
+    for (int facet_idx = 0; facet_idx < m_orig_size_indices; ++facet_idx) {
         const Vec3i neighbors = m_neighbors[facet_idx];
         assert(this->verify_triangle_neighbors(m_triangles[facet_idx], neighbors));
         this->get_seed_fill_contour_recursive(facet_idx, neighbors, neighbors, edges_out);
diff --git a/src/slic3r/GUI/GLCanvas3D.cpp b/src/slic3r/GUI/GLCanvas3D.cpp
index f77eed369..c1e740672 100644
--- a/src/slic3r/GUI/GLCanvas3D.cpp
+++ b/src/slic3r/GUI/GLCanvas3D.cpp
@@ -7165,7 +7165,7 @@ void GLCanvas3D::_load_print_object_toolpaths(const PrintObject& print_object, c
             if (is_selected_separate_extruder) {
                 bool at_least_one_has_correct_extruder = false;
                 for (const LayerRegion* layerm : layer->regions()) {
-                    if (layerm->slices.surfaces.empty())
+                    if (layerm->slices().empty())
                         continue;
                     const PrintRegionConfig& cfg = layerm->region().config();
                     if (cfg.perimeter_extruder.value    == m_selected_extruder ||
@@ -7208,14 +7208,14 @@ void GLCanvas3D::_load_print_object_toolpaths(const PrintObject& print_object, c
                     }
                     if (ctxt.has_perimeters)
 #if ENABLE_LEGACY_OPENGL_REMOVAL
-                        _3DScene::extrusionentity_to_verts(layerm->perimeters, float(layer->print_z), copy,
+                        _3DScene::extrusionentity_to_verts(layerm->perimeters(), float(layer->print_z), copy,
                             select_geometry(idx_layer, layerm->region().config().perimeter_extruder.value, 0));
 #else
                         _3DScene::extrusionentity_to_verts(layerm->perimeters, float(layer->print_z), copy,
                         	volume(idx_layer, layerm->region().config().perimeter_extruder.value, 0));
 #endif // ENABLE_LEGACY_OPENGL_REMOVAL
                     if (ctxt.has_infill) {
-                        for (const ExtrusionEntity *ee : layerm->fills.entities) {
+                        for (const ExtrusionEntity *ee : layerm->fills()) {
                             // fill represents infill extrusions of a single island.
                             const auto *fill = dynamic_cast<const ExtrusionEntityCollection*>(ee);
                             if (! fill->entities.empty())
diff --git a/src/slic3r/GUI/GUI_App.cpp b/src/slic3r/GUI/GUI_App.cpp
index b03010b17..c5f4d9672 100644
--- a/src/slic3r/GUI/GUI_App.cpp
+++ b/src/slic3r/GUI/GUI_App.cpp
@@ -3244,7 +3244,7 @@ void GUI_App::app_updater(bool from_user)
     app_data.target_path =dwnld_dlg.get_download_path();
 
     // start download
-    this->plater_->get_notification_manager()->push_download_progress_notification(_utf8("Download"), std::bind(&AppUpdater::cancel_callback, this->m_app_updater.get()));
+    this->plater_->get_notification_manager()->push_download_progress_notification(_utf8("Download"), std::bind(&AppUpdater::cancel_callback, m_app_updater.get()));
     app_data.start_after = dwnld_dlg.run_after_download();
     m_app_updater->set_app_data(std::move(app_data));
     m_app_updater->sync_download();
diff --git a/src/slic3r/GUI/Gizmos/GLGizmoPainterBase.cpp b/src/slic3r/GUI/Gizmos/GLGizmoPainterBase.cpp
index 62611ac25..966c36b27 100644
--- a/src/slic3r/GUI/Gizmos/GLGizmoPainterBase.cpp
+++ b/src/slic3r/GUI/Gizmos/GLGizmoPainterBase.cpp
@@ -1158,18 +1158,18 @@ void TriangleSelectorGUI::update_render_data()
 #if !ENABLE_LEGACY_OPENGL_REMOVAL
 void GLPaintContour::render() const
 {
-    assert(this->m_contour_VBO_id != 0);
-    assert(this->m_contour_EBO_id != 0);
+    assert(m_contour_VBO_id != 0);
+    assert(m_contour_EBO_id != 0);
 
     glsafe(::glLineWidth(4.0f));
 
-    glsafe(::glBindBuffer(GL_ARRAY_BUFFER, this->m_contour_VBO_id));
+    glsafe(::glBindBuffer(GL_ARRAY_BUFFER, m_contour_VBO_id));
     glsafe(::glVertexPointer(3, GL_FLOAT, 3 * sizeof(float), nullptr));
 
     glsafe(::glEnableClientState(GL_VERTEX_ARRAY));
 
     if (this->contour_indices_size > 0) {
-        glsafe(::glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, this->m_contour_EBO_id));
+        glsafe(::glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, m_contour_EBO_id));
         glsafe(::glDrawElements(GL_LINES, GLsizei(this->contour_indices_size), GL_UNSIGNED_INT, nullptr));
         glsafe(::glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0));
     }
@@ -1181,20 +1181,20 @@ void GLPaintContour::render() const
 
 void GLPaintContour::finalize_geometry()
 {
-    assert(this->m_contour_VBO_id == 0);
-    assert(this->m_contour_EBO_id == 0);
+    assert(m_contour_VBO_id == 0);
+    assert(m_contour_EBO_id == 0);
 
     if (!this->contour_vertices.empty()) {
-        glsafe(::glGenBuffers(1, &this->m_contour_VBO_id));
-        glsafe(::glBindBuffer(GL_ARRAY_BUFFER, this->m_contour_VBO_id));
+        glsafe(::glGenBuffers(1, &m_contour_VBO_id));
+        glsafe(::glBindBuffer(GL_ARRAY_BUFFER, m_contour_VBO_id));
         glsafe(::glBufferData(GL_ARRAY_BUFFER, this->contour_vertices.size() * sizeof(float), this->contour_vertices.data(), GL_STATIC_DRAW));
         glsafe(::glBindBuffer(GL_ARRAY_BUFFER, 0));
         this->contour_vertices.clear();
     }
 
     if (!this->contour_indices.empty()) {
-        glsafe(::glGenBuffers(1, &this->m_contour_EBO_id));
-        glsafe(::glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, this->m_contour_EBO_id));
+        glsafe(::glGenBuffers(1, &m_contour_EBO_id));
+        glsafe(::glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, m_contour_EBO_id));
         glsafe(::glBufferData(GL_ELEMENT_ARRAY_BUFFER, this->contour_indices.size() * sizeof(unsigned int), this->contour_indices.data(), GL_STATIC_DRAW));
         glsafe(::glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0));
         this->contour_indices.clear();
@@ -1203,13 +1203,13 @@ void GLPaintContour::finalize_geometry()
 
 void GLPaintContour::release_geometry()
 {
-    if (this->m_contour_VBO_id) {
-        glsafe(::glDeleteBuffers(1, &this->m_contour_VBO_id));
-        this->m_contour_VBO_id = 0;
+    if (m_contour_VBO_id) {
+        glsafe(::glDeleteBuffers(1, &m_contour_VBO_id));
+        m_contour_VBO_id = 0;
     }
-    if (this->m_contour_EBO_id) {
-        glsafe(::glDeleteBuffers(1, &this->m_contour_EBO_id));
-        this->m_contour_EBO_id = 0;
+    if (m_contour_EBO_id) {
+        glsafe(::glDeleteBuffers(1, &m_contour_EBO_id));
+        m_contour_EBO_id = 0;
     }
     this->clear();
 }
diff --git a/src/slic3r/GUI/Gizmos/GLGizmoPainterBase.hpp b/src/slic3r/GUI/Gizmos/GLGizmoPainterBase.hpp
index 04f24b20d..94aca4fdb 100644
--- a/src/slic3r/GUI/Gizmos/GLGizmoPainterBase.hpp
+++ b/src/slic3r/GUI/Gizmos/GLGizmoPainterBase.hpp
@@ -41,7 +41,7 @@ public:
 
     void render() const;
 
-    inline bool has_VBO() const { return this->m_contour_EBO_id != 0; }
+    inline bool has_VBO() const { return m_contour_EBO_id != 0; }
 
     // Release the geometry data, release OpenGL VBOs.
     void release_geometry();
diff --git a/src/slic3r/GUI/OptionsGroup.cpp b/src/slic3r/GUI/OptionsGroup.cpp
index 1855b6a4e..cfa2ef15a 100644
--- a/src/slic3r/GUI/OptionsGroup.cpp
+++ b/src/slic3r/GUI/OptionsGroup.cpp
@@ -101,7 +101,7 @@ const t_field& OptionsGroup::build_field(const t_config_option_key& id, const Co
 			this->back_to_initial_value(opt_id);
 	};
 	field->m_back_to_sys_value = [this](std::string opt_id) {
-		if (!this->m_disabled)
+		if (!m_disabled)
 			this->back_to_sys_value(opt_id);
 	};
 
diff --git a/src/slic3r/Utils/AppUpdater.cpp b/src/slic3r/Utils/AppUpdater.cpp
index 27f2e34bc..a17adea8a 100644
--- a/src/slic3r/Utils/AppUpdater.cpp
+++ b/src/slic3r/Utils/AppUpdater.cpp
@@ -168,7 +168,7 @@ bool  AppUpdater::priv::http_get_file(const std::string& url, size_t size_limit,
 		.size_limit(size_limit)
 		.on_progress([&, progress_fn](Http::Progress progress, bool& cancel) {
 			// progress function returns true as success (to continue) 
-			cancel = (this->m_cancel ? true : !progress_fn(std::move(progress)));
+			cancel = (m_cancel ? true : !progress_fn(std::move(progress)));
 			if (cancel) {
 				error_message = GUI::format("Error getting: `%1%`: Download was canceled.", //lm:typo //dk: am i blind? :)
 					url);
@@ -259,7 +259,7 @@ boost::filesystem::path AppUpdater::priv::download_file(const DownloadAppData& d
 	);
 	if (!res)
 	{
-		if (this->m_cancel)
+		if (m_cancel)
 		{
 			BOOST_LOG_TRIVIAL(info) << error_message; //lm:Is this an error? // dk: changed to info
 			wxCommandEvent* evt = new wxCommandEvent(EVT_SLIC3R_APP_DOWNLOAD_FAILED); // FAILED with empty msg only closes progress notification
diff --git a/tests/fff_print/test_perimeters.cpp b/tests/fff_print/test_perimeters.cpp
index 71ab3e675..54cf37067 100644
--- a/tests/fff_print/test_perimeters.cpp
+++ b/tests/fff_print/test_perimeters.cpp
@@ -459,8 +459,8 @@ SCENARIO("Some weird coverage test", "[Perimeters]")
     object->slice();
     Layer       *layer = object->get_layer(1);
     LayerRegion *layerm = layer->get_region(0);
-    layerm->slices.clear();
-    layerm->slices.append({ expolygon }, stInternal);
+    layerm->m_slices.clear();
+    layerm->m_slices.append({ expolygon }, stInternal);
     
     // make perimeters
     layer->make_perimeters();
@@ -472,22 +472,22 @@ SCENARIO("Some weird coverage test", "[Perimeters]")
     Polygons covered_by_infill;
     {
         Polygons acc;
-        for (const ExtrusionEntity *ee : layerm->perimeters.entities)
+        for (const ExtrusionEntity *ee : layerm->perimeters())
             for (const ExtrusionEntity *ee : dynamic_cast<const ExtrusionEntityCollection*>(ee)->entities)
                 append(acc, offset(dynamic_cast<const ExtrusionLoop*>(ee)->polygon().split_at_first_point(), float(pflow.scaled_width() / 2.f + SCALED_EPSILON)));
         covered_by_perimeters = union_(acc);
     }
     {
         Polygons acc;
-        for (const Surface &surface : layerm->fill_surfaces.surfaces)
-            append(acc, to_polygons(surface.expolygon));
-        for (const ExtrusionEntity *ee : layerm->thin_fills.entities)
+        for (const ExPolygon &expolygon : layerm->fill_expolygons())
+            append(acc, to_polygons(expolygon));
+        for (const ExtrusionEntity *ee : layerm->thin_fills().entities)
             append(acc, offset(dynamic_cast<const ExtrusionPath*>(ee)->polyline, float(iflow.scaled_width() / 2.f + SCALED_EPSILON)));
         covered_by_infill = union_(acc);
     }
     
     // compute the non covered area
-    ExPolygons non_covered = diff_ex(to_polygons(layerm->slices.surfaces), union_(covered_by_perimeters, covered_by_infill));
+    ExPolygons non_covered = diff_ex(to_polygons(layerm->slices().surfaces), union_(covered_by_perimeters, covered_by_infill));
     
     /*
     if (0) {
@@ -506,7 +506,8 @@ SCENARIO("Some weird coverage test", "[Perimeters]")
     }
     */
     THEN("no gap between perimeters and infill") {
-        size_t num_non_convered = std::count_if(non_covered.begin(), non_covered.end(), [&iflow](const ExPolygon &ex){ return ex.area() > sqr(double(iflow.scaled_width())); });
+        size_t num_non_convered = std::count_if(non_covered.begin(), non_covered.end(), 
+            [&iflow](const ExPolygon &ex){ return ex.area() > sqr(double(iflow.scaled_width())); });
         REQUIRE(num_non_convered == 0);
     }
 }
diff --git a/tests/fff_print/test_print.cpp b/tests/fff_print/test_print.cpp
index d8ab5a3fa..204f3f8e0 100644
--- a/tests/fff_print/test_print.cpp
+++ b/tests/fff_print/test_print.cpp
@@ -20,11 +20,11 @@ SCENARIO("PrintObject: Perimeter generation", "[PrintObject]") {
             }
             THEN("Every layer in region 0 has 1 island of perimeters") {
                 for (const Layer *layer : object.layers())
-                    REQUIRE(layer->regions().front()->perimeters.entities.size() == 1);
+                    REQUIRE(layer->regions().front()->perimeters().size() == 1);
             }
             THEN("Every layer in region 0 has 3 paths in its perimeters list.") {
                 for (const Layer *layer : object.layers())
-                    REQUIRE(layer->regions().front()->perimeters.items_count() == 3);
+                    REQUIRE(layer->regions().front()->perimeters().items_count() == 3);
             }
         }
     }
@@ -66,7 +66,7 @@ SCENARIO("Print: Changing number of solid surfaces does not cause all surfaces t
 		    // iterate over all of the regions in the layer
 		    for (const LayerRegion *region : layer.regions()) {
 		        // for each region, iterate over the fill surfaces
-		        for (const Surface &surface : region->fill_surfaces.surfaces)
+		        for (const Surface &surface : region->fill_surfaces())
 		            CHECK(surface.is_solid());
 		    }
 		};
diff --git a/xs/xsp/Layer.xsp b/xs/xsp/Layer.xsp
index e42486985..da2962c1e 100644
--- a/xs/xsp/Layer.xsp
+++ b/xs/xsp/Layer.xsp
@@ -13,15 +13,15 @@
         %code%{ RETVAL = &THIS->region(); %};
 
     Ref<SurfaceCollection> slices()
-        %code%{ RETVAL = &THIS->slices; %};
+        %code%{ RETVAL = const_cast<SurfaceCollection*>(&THIS->slices()); %};
     Ref<ExtrusionEntityCollection> thin_fills()
-        %code%{ RETVAL = &THIS->thin_fills; %};
+        %code%{ RETVAL = const_cast<ExtrusionEntityCollection*>(&THIS->thin_fills()); %};
     Ref<SurfaceCollection> fill_surfaces()
-        %code%{ RETVAL = &THIS->fill_surfaces; %};
+        %code%{ RETVAL = const_cast<SurfaceCollection*>(&THIS->fill_surfaces()); %};
     Ref<ExtrusionEntityCollection> perimeters()
-        %code%{ RETVAL = &THIS->perimeters; %};
+        %code%{ RETVAL = const_cast<ExtrusionEntityCollection*>(&THIS->perimeters()); %};
     Ref<ExtrusionEntityCollection> fills()
-        %code%{ RETVAL = &THIS->fills; %};
+        %code%{ RETVAL = const_cast<ExtrusionEntityCollection*>(&THIS->fills()); %};
     
     void prepare_fill_surfaces();
     void make_perimeters(SurfaceCollection* slices, SurfaceCollection* fill_surfaces)