More work for porting PerimeterGenerator to XS

7 files changed, 536 insertions, 51 deletions

diff --git a/xs/src/libslic3r/ExPolygon.cpp b/xs/src/libslic3r/ExPolygon.cpp
index 53a1e9333..83e1929aa 100644
--- a/xs/src/libslic3r/ExPolygon.cpp
+++ b/xs/src/libslic3r/ExPolygon.cpp
@@ -122,6 +122,13 @@ ExPolygon::has_boundary_point(const Point &point) const
     return false;
 }
 
+void
+ExPolygon::simplify_p(double tolerance, Polygons* polygons) const
+{
+    Polygons pp = this->simplify_p(tolerance);
+    polygons->insert(polygons->end(), pp.begin(), pp.end());
+}
+
 Polygons
 ExPolygon::simplify_p(double tolerance) const
 {
diff --git a/xs/src/libslic3r/ExPolygon.hpp b/xs/src/libslic3r/ExPolygon.hpp
index 3d7cd3540..7b7b0b760 100644
--- a/xs/src/libslic3r/ExPolygon.hpp
+++ b/xs/src/libslic3r/ExPolygon.hpp
@@ -27,6 +27,7 @@ class ExPolygon
     bool contains(const Point &point) const;
     bool contains_b(const Point &point) const;
     bool has_boundary_point(const Point &point) const;
+    void simplify_p(double tolerance, Polygons* polygons) const;
     Polygons simplify_p(double tolerance) const;
     ExPolygons simplify(double tolerance) const;
     void simplify(double tolerance, ExPolygons &expolygons) const;
diff --git a/xs/src/libslic3r/ExtrusionEntity.hpp b/xs/src/libslic3r/ExtrusionEntity.hpp
index bcb50155a..0cbabd1b8 100644
--- a/xs/src/libslic3r/ExtrusionEntity.hpp
+++ b/xs/src/libslic3r/ExtrusionEntity.hpp
@@ -52,6 +52,7 @@ class ExtrusionEntity
     virtual Point last_point() const = 0;
     virtual Polygons grow() const = 0;
     virtual double min_mm3_per_mm() const = 0;
+    virtual Polyline as_polyline() const = 0;
 };
 
 typedef std::vector<ExtrusionEntity*> ExtrusionEntitiesPtr;
@@ -83,6 +84,9 @@ class ExtrusionPath : public ExtrusionEntity
     double min_mm3_per_mm() const {
         return this->mm3_per_mm;
     };
+    Polyline as_polyline() const {
+        return this->polyline;
+    };
 
     private:
     void _inflate_collection(const Polylines &polylines, ExtrusionEntityCollection* collection) const;
@@ -97,6 +101,8 @@ class ExtrusionLoop : public ExtrusionEntity
     ExtrusionLoopRole role;
     
     ExtrusionLoop(ExtrusionLoopRole role = elrDefault) : role(role) {};
+    ExtrusionLoop(const ExtrusionPaths &paths, ExtrusionLoopRole role = elrDefault)
+        : paths(paths), role(role) {};
     bool is_loop() const {
         return true;
     };
@@ -120,6 +126,9 @@ class ExtrusionLoop : public ExtrusionEntity
     bool is_solid_infill() const;
     Polygons grow() const;
     double min_mm3_per_mm() const;
+    Polyline as_polyline() const {
+        return this->polygon()->split_at_first_point();
+    };
 };
 
 }
diff --git a/xs/src/libslic3r/ExtrusionEntityCollection.cpp b/xs/src/libslic3r/ExtrusionEntityCollection.cpp
index 79736cbcb..5904252ff 100644
--- a/xs/src/libslic3r/ExtrusionEntityCollection.cpp
+++ b/xs/src/libslic3r/ExtrusionEntityCollection.cpp
@@ -8,9 +8,13 @@ namespace Slic3r {
 ExtrusionEntityCollection::ExtrusionEntityCollection(const ExtrusionEntityCollection& collection)
     : no_sort(collection.no_sort), orig_indices(collection.orig_indices)
 {
-    this->entities.reserve(collection.entities.size());
-    for (ExtrusionEntitiesPtr::const_iterator it = collection.entities.begin(); it != collection.entities.end(); ++it)
-        this->entities.push_back((*it)->clone());
+    this->append(collection);
+}
+
+ExtrusionEntityCollection::ExtrusionEntityCollection(const ExtrusionPaths &paths)
+    : no_sort(false)
+{
+    this->append(paths);
 }
 
 ExtrusionEntityCollection& ExtrusionEntityCollection::operator= (const ExtrusionEntityCollection &other)
@@ -28,6 +32,16 @@ ExtrusionEntityCollection::swap (ExtrusionEntityCollection &c)
     std::swap(this->no_sort, c.no_sort);
 }
 
+ExtrusionEntityCollection::operator ExtrusionPaths() const
+{
+    ExtrusionPaths paths;
+    for (ExtrusionEntitiesPtr::const_iterator it = this->entities.begin(); it != this->entities.end(); ++it) {
+        if (const ExtrusionPath* path = dynamic_cast<const ExtrusionPath*>(*it))
+            paths.push_back(*path);
+    }
+    return paths;
+}
+
 ExtrusionEntityCollection*
 ExtrusionEntityCollection::clone() const
 {
@@ -58,6 +72,33 @@ ExtrusionEntityCollection::last_point() const
 }
 
 void
+ExtrusionEntityCollection::append(const ExtrusionEntity &entity)
+{
+    this->entities.push_back(entity.clone());
+}
+
+void
+ExtrusionEntityCollection::append(const ExtrusionEntityCollection &collection)
+{
+    this->entities.insert(this->entities.end(), collection.entities.begin(), collection.entities.end());
+}
+
+void
+ExtrusionEntityCollection::append(const ExtrusionPaths &paths)
+{
+    for (ExtrusionPaths::const_iterator path = paths.begin(); path != paths.end(); ++path)
+        this->append(*path);
+}
+
+ExtrusionEntityCollection
+ExtrusionEntityCollection::chained_path(bool no_reverse, std::vector<size_t>* orig_indices) const
+{
+    ExtrusionEntityCollection coll;
+    this->chained_path(&coll, no_reverse, orig_indices);
+    return coll;
+}
+
+void
 ExtrusionEntityCollection::chained_path(ExtrusionEntityCollection* retval, bool no_reverse, std::vector<size_t>* orig_indices) const
 {
     if (this->entities.empty()) return;
@@ -145,15 +186,21 @@ ExtrusionEntityCollection::flatten(ExtrusionEntityCollection* retval) const
     for (ExtrusionEntitiesPtr::const_iterator it = this->entities.begin(); it != this->entities.end(); ++it) {
         if ((*it)->is_collection()) {
             ExtrusionEntityCollection* collection = dynamic_cast<ExtrusionEntityCollection*>(*it);
-            ExtrusionEntityCollection contents;
-            collection->flatten(&contents);
-            retval->entities.insert(retval->entities.end(), contents.entities.begin(), contents.entities.end());
+            retval->append(collection->flatten());
         } else {
-            retval->entities.push_back((*it)->clone());
+            retval->append(**it);
         }
     }
 }
 
+ExtrusionEntityCollection
+ExtrusionEntityCollection::flatten() const
+{
+    ExtrusionEntityCollection coll;
+    this->flatten(&coll);
+    return coll;
+}
+
 double
 ExtrusionEntityCollection::min_mm3_per_mm() const
 {
diff --git a/xs/src/libslic3r/ExtrusionEntityCollection.hpp b/xs/src/libslic3r/ExtrusionEntityCollection.hpp
index 24016d638..1cb288d0a 100644
--- a/xs/src/libslic3r/ExtrusionEntityCollection.hpp
+++ b/xs/src/libslic3r/ExtrusionEntityCollection.hpp
@@ -15,7 +15,10 @@ class ExtrusionEntityCollection : public ExtrusionEntity
     bool no_sort;
     ExtrusionEntityCollection(): no_sort(false) {};
     ExtrusionEntityCollection(const ExtrusionEntityCollection &collection);
+    ExtrusionEntityCollection(const ExtrusionPaths &paths);
     ExtrusionEntityCollection& operator= (const ExtrusionEntityCollection &other);
+    operator ExtrusionPaths() const;
+    
     bool is_collection() const {
         return true;
     };
@@ -23,6 +26,10 @@ class ExtrusionEntityCollection : public ExtrusionEntity
         return !this->no_sort;
     };
     void swap (ExtrusionEntityCollection &c);
+    void append(const ExtrusionEntity &entity);
+    void append(const ExtrusionEntityCollection &collection);
+    void append(const ExtrusionPaths &paths);
+    ExtrusionEntityCollection chained_path(bool no_reverse = false, std::vector<size_t>* orig_indices = NULL) const;
     void chained_path(ExtrusionEntityCollection* retval, bool no_reverse = false, std::vector<size_t>* orig_indices = NULL) const;
     void chained_path_from(Point start_near, ExtrusionEntityCollection* retval, bool no_reverse = false, std::vector<size_t>* orig_indices = NULL) const;
     void reverse();
@@ -31,6 +38,7 @@ class ExtrusionEntityCollection : public ExtrusionEntity
     Polygons grow() const;
     size_t items_count() const;
     void flatten(ExtrusionEntityCollection* retval) const;
+    ExtrusionEntityCollection flatten() const;
     double min_mm3_per_mm() const;
 };
 
diff --git a/xs/src/libslic3r/PerimeterGenerator.cpp b/xs/src/libslic3r/PerimeterGenerator.cpp
index 5c054aa6a..a03598db7 100644
--- a/xs/src/libslic3r/PerimeterGenerator.cpp
+++ b/xs/src/libslic3r/PerimeterGenerator.cpp
@@ -5,14 +5,439 @@ namespace Slic3r {
 void
 PerimeterGenerator::process()
 {
-
+    // other perimeters
+    this->_mm3_per_mm           = this->perimeter_flow.mm3_per_mm();
+    coord_t pwidth              = this->perimeter_flow.scaled_width();
+    coord_t pspacing            = this->perimeter_flow.scaled_spacing();
+    
+    // external perimeters
+    this->_ext_mm3_per_mm       = this->ext_perimeter_flow.mm3_per_mm();
+    coord_t = ext_pwidth        = this->ext_perimeter_flow.scaled_width();
+    coord_t = ext_pspacing      = scale_(this->ext_perimeter_flow.spacing_to(this->perimeter_flow));
+    
+    // overhang perimeters
+    this->_mm3_per_mm_overhang  = this->overhang_flow.mm3_per_mm();
+    
+    // solid infill
+    coord_t ispacing            = this->solid_infill_flow->scaled_spacing;
+    coord_t gap_area_threshold  = pwidth * pwidth;
+    
+    // Calculate the minimum required spacing between two adjacent traces.
+    // This should be equal to the nominal flow spacing but we experiment
+    // with some tolerance in order to avoid triggering medial axis when
+    // some squishing might work. Loops are still spaced by the entire
+    // flow spacing; this only applies to collapsing parts.
+    coord_t min_spacing         = pspacing      * (1 - INSET_OVERLAP_TOLERANCE);
+    coord_t ext_min_spacing     = ext_pspacing  * (1 - INSET_OVERLAP_TOLERANCE);
+    
+    // prepare grown lower layer slices for overhang detection
+    if (this->lower_slices != NULL && this->config->overhangs) {
+        // We consider overhang any part where the entire nozzle diameter is not supported by the
+        // lower layer, so we take lower slices and offset them by half the nozzle diameter used 
+        // in the current layer
+        double nozzle_diameter = this->print_config->nozzle_diameter.get_at(this->config->perimeter_extruder-1);
+        
+        this->_lower_slices_p = offset(this->lower_slices, scale_(+nozzle_diameter/2));
+    }
+    
+    // we need to process each island separately because we might have different
+    // extra perimeters for each one
+    for (Surfaces::const_iterator surface = this->slices->surfaces.begin();
+        surface != this->slices->surfaces.end(); ++surface) {
+        // detect how many perimeters must be generated for this island
+        unsigned short loop_number = this->config->perimeters + surface->extra_perimeters;
+        loop_number--;  // 0-indexed loops
+        
+        Polygons gaps;
+        
+        Polygons last = surface->expolygon.simplify_p(SCALED_RESOLUTION);
+        if (loop_number >= 0) {  // no loops = -1
+            
+            std::vector<PerimeterGeneratorLoops> contours(loop_number);    // depth => loops
+            std::vector<PerimeterGeneratorLoops> holes(loop_number);       // depth => loops
+            Polylines thin_walls;
+            
+            // we loop one time more than needed in order to find gaps after the last perimeter was applied
+            for (unsigned short i = 0; i <= loop_number+1; ++i) {  // outer loop is 0
+                Polygons offsets;
+                if (i == 0) {
+                    // the minimum thickness of a single loop is:
+                    // ext_width/2 + ext_spacing/2 + spacing/2 + width/2
+                    if (this->config->thin_walls) {
+                        offsets = offset2(
+                            \@last,
+                            -(0.5*ext_pwidth + 0.5*ext_min_spacing - 1),
+                            +(0.5*ext_min_spacing - 1)
+                        );
+                    } else {
+                        offsets = offset(last, -0.5*ext_pwidth);
+                    }
+                    
+                    // look for thin walls
+                    if (this->config->thin_walls) {
+                        Polygons diff = diff(
+                            last,
+                            offset(offsets, +0.5*ext_pwidth),
+                            true  // medial axis requires non-overlapping geometry
+                        );
+                        
+                        // the following offset2 ensures almost nothing in @thin_walls is narrower than $min_width
+                        // (actually, something larger than that still may exist due to mitering or other causes)
+                        coord_t min_width = ext_pwidth / 2;
+                        ExPolygons expp = offset2(diff, -min_width/2, +min_width/2)};
+                        
+                        // the maximum thickness of our thin wall area is equal to the minimum thickness of a single loop
+                        Polylines pp;
+                        for (ExPolygons::const_iterator ex = expp.begin(); ex != expp.end(); ++ex)
+                            ex->medial_axis(ext_pwidth + ext_pspacing, min_width, &pp);
+                        
+                        double threshold = ext_pwidth * ext_pwidth;
+                        for (Polylines::const_iterator p = pp.begin(); p != pp.end(); ++p) {
+                            if (p->length() > threshold) {
+                                thin_walls.push_back(*p);
+                            }
+                        }
+                        
+                        #ifdef DEBUG
+                        printf("  %zu thin walls detected\n", thin_walls.size());
+                        #endif
+                        
+                        /*
+                        if (false) {
+                            require "Slic3r/SVG.pm";
+                            Slic3r::SVG::output(
+                                "medial_axis.svg",
+                                no_arrows       => 1,
+                                #expolygons      => \@expp,
+                                polylines       => \@thin_walls,
+                            );
+                        }
+                        */
+                    }
+                } else {
+                    coord_t distance = (i == 1) ? ext_pspacing : pspacing;
+                    
+                    if (this->config->thin_walls) {
+                        offsets = offset2(
+                            last,
+                            -(distance + 0.5*min_spacing - 1),
+                            +(0.5*min_spacing - 1),
+                        );
+                    } else {
+                        offsets = offset(
+                            last,
+                            -distance,
+                        );
+                    }
+                    
+                    // look for gaps
+                    if (this->config->gap_fill_speed > 0 && this->config->fill_density > 0) {
+                        // not using safety offset here would "detect" very narrow gaps
+                        // (but still long enough to escape the area threshold) that gap fill
+                        // won't be able to fill but we'd still remove from infill area
+                        ExPolygons diff = diff_ex(
+                            offset(last, -0.5*distance),
+                            offset(offsets, +0.5*distance + 10),  // safety offset
+                        );
+                        for (ExPolygons::const_iterator ex = diff.begin(); ex != diff.end(); ++ex) {
+                            if (fabs(ex->area()) >= gap_area_threshold)
+                                gaps.push_back(*ex);
+                        }
+                    }
+                }
+                
+                if (offsets.empty()) break;
+                if (i > loop_number) break; // we were only looking for gaps this time
+                
+                last = offsets;
+                
+                for (Polygons::const_iterator polygon = offsets.begin(); polygon != offsets.end(); ++polygon) {
+                    PerimeterGeneratorLoop loop(*polygon, i);
+                    loop.is_contour = polygon->is_counter_clockwise();
+                    if (loop.is_contour) {
+                        contours[i].push_back(loop);
+                    } else {
+                        holes[i].push_back(loop);
+                    }
+                }
+            }
+            
+            // nest loops: holes first
+            for (unsigned short d = 0; <= loop_number; ++d) {
+                PerimeterGeneratorLoops &holes_d = holes[d];
+                
+                // loop through all holes having depth == d
+                for (unsigned short i = 0; i < holes_d.size(); ++i) {
+                    const PerimeterGeneratorLoop &loop = holes_d[i];
+                    
+                    // find the hole loop that contains this one, if any
+                    for (unsigned short t = d+1; t <= loop_number; ++t) {
+                        for (unsigned short j = 0; j < holes_d.size(); ++j) {
+                            PerimeterGeneratorLoop &candidate_parent = holes[t][j];
+                            if (candidate_parent.polygon.contains(loop.polygon.first_point())) {
+                                candidate_parent.add_child(loop);
+                                holes_d.erase(holes_d.begin() + i);
+                                --i;
+                                goto NEXT_HOLE;
+                            }
+                        }
+                    }
+                
+                    // if no hole contains this hole, find the contour loop that contains it
+                    for (unsigned short t = loop_number; t >= 0; --t) {
+                        for (unsigned short j = 0; j < contours[t].size(); ++j) {
+                            PerimeterGeneratorLoop &candidate_parent = contours[t][j];
+                            if (candidate_parent.polygon.contains(loop.polygon.first_point())) {
+                                candidate_parent.add_child(loop);
+                                holes_d.erase(holes_d.begin() + i);
+                                --i;
+                                goto NEXT_HOLE;
+                            }
+                        }
+                    }
+                }
+                NEXT_HOLE:
+            }
+        
+            // nest contour loops
+            for (unsigned short d = loop_number; d >= 1; --d) {
+                PerimeterGeneratorLoops &contours_d = contours[d];
+                
+                // loop through all contours having depth == d
+                for (unsigned short i = 0; i < contours_d.size(); ++i) {
+                    const PerimeterGeneratorLoop &loop = contours_d[i];
+                
+                    // find the contour loop that contains it
+                    for (unsigned short t = d-1; t >= 0; --t) {
+                        for (unsigned short j = 0; j < contours_d[t].size(); ++j) {
+                            PerimeterGeneratorLoop &candidate_parent = contours[t][j];
+                            if (candidate_parent.polygon.contains(loop.polygon.first_point())) {
+                                candidate_parent.add_child(loop);
+                                contours_d.erase(contours_d.begin() + i);
+                                --i;
+                                goto NEXT_CONTOUR;
+                            }
+                        }
+                    }
+                    
+                    NEXT_CONTOUR:
+                }
+            }
+        
+            // at this point, all loops should be in contours[0]
+            
+            ExtrusionEntityCollection entities = this->_traverse_loops(contours.front(), thin_walls);
+            
+            // if brim will be printed, reverse the order of perimeters so that
+            // we continue inwards after having finished the brim
+            // TODO: add test for perimeter order
+            if (this->config->external_perimeters_first
+                || (this->layer_id == 0 && this->print_config->brim_width > 0))
+                    entities.reverse();
+        
+            // append perimeters for this slice as a collection
+            if (!entities.empty())
+                this->loops->append(entities);
+        }
+        
+        // fill gaps
+        if (!gaps.empty()) {
+            /*
+            if (false) {
+                require "Slic3r/SVG.pm";
+                Slic3r::SVG::output(
+                    "gaps.svg",
+                    expolygons => union_ex(\@gaps),
+                );
+            }
+            */
+            
+            // where $pwidth < thickness < 2*$pspacing, infill with width = 2*$pwidth
+            // where 0.1*$pwidth < thickness < $pwidth, infill with width = 1*$pwidth
+            std::vector<PerimeterGeneratorGapSize> gap_sizes;
+            gap_sizes.push_back(PerimeterGeneratorGapSize(pwidth, 2*pspacing, unscale(2*pwidth)));
+            gap_sizes.push_back(PerimeterGeneratorGapSize(0.1*pwidth, pwidth, unscale(1*pwidth)));
+            
+            for (std::vector<PerimeterGeneratorGapSize>::const_iterator gap_size = gap_sizes.begin();
+                gap_size != gap_sizes.end(); ++gap_size) {
+                ExtrusionEntityCollection gap_fill = this->_fill_gaps(gap_size.min, gap_size.max, gap_size.width);
+                this->gap_fill->append(gap_fill);
+                
+                // Make sure we don't infill narrow parts that are already gap-filled
+                // (we only consider this surface's gaps to reduce the diff() complexity).
+                // Growing actual extrusions ensures that gaps not filled by medial axis
+                // are not subtracted from fill surfaces (they might be too short gaps
+                // that medial axis skips but infill might join with other infill regions
+                // and use zigzag).
+                double dist = scale_(gap_size->width/2);
+                Polygons filled;
+                for (ExtrusionEntitiesPtr::const_iterator it = gap_fill.entities.begin();
+                    it != gap_fill.entities.end(); ++it)
+                    offset((*it)->as_polyline(), &filled, dist);
+                
+                last = diff(last, filled);
+                gaps = diff(gaps, filled);  // prevent more gap fill here
+            }
+        }
+        
+        // create one more offset to be used as boundary for fill
+        // we offset by half the perimeter spacing (to get to the actual infill boundary)
+        // and then we offset back and forth by half the infill spacing to only consider the
+        // non-collapsing regions
+        coord_t inset = 0;
+        if (loop_number == 0) {
+            // one loop
+            inset += ext_pspacing/2;
+        } else if (loop_number > 0) {
+            // two or more loops
+            inset += pspacing/2;
+        }
+        
+        // only apply infill overlap if we actually have one perimeter
+        if (inset > 0)
+            inset -= this->config->get_abs_value("infill_overlap", inset + ispacing/2);
+        
+        {
+            ExPolygons expp = union_(last);
+            
+            // simplify infill contours according to resolution
+            Polygons pp;
+            for (ExPolygons::const_iterator ex = expp.begin(); ex != expp.end(); ++ex)
+                ex->simplify_p(SCALED_RESOLUTION, &pp);
+            
+            // collapse too narrow infill areas
+            coord_t min_perimeter_infill_spacing = ispacing * (1 - INSET_OVERLAP_TOLERANCE);
+            expp = offset2(
+                pp,
+                -inset -min_perimeter_infill_spacing/2,
+                +min_perimeter_infill_spacing/2,
+            );
+            
+            // append infill areas to fill_surfaces
+            for (ExPolygons::const_iterator ex = expp.begin(); ex != expp.end(); ++ex)
+                this->fill_surfaces->surfaces.push_back(Surface(stInternal, *ex));  // use a bogus surface type
+        }
+    }
 }
 
 ExtrusionEntityCollection
-PerimeterGenerator::_traverse_loops(const std::vector<PerimeterGeneratorLoop> &loops,
+PerimeterGenerator::_traverse_loops(const PerimeterGeneratorLoops &loops,
     const Polylines &thin_walls) const
 {
+    // loops is an arrayref of ::Loop objects
+    // turn each one into an ExtrusionLoop object
+    ExtrusionEntityCollection coll;
+    for (PerimeterGeneratorLoops::const_iterator loop = loops.begin();
+        loop != loops.end(); ++loop) {
+        bool is_external = loop->is_external();
+        
+        ExtrusionRole role;
+        ExtrusionLoopRole loop_role;
+        role = is_external ? erExternalPerimeter : erPerimeter;
+        if (loop->is_internal_contour()) {
+            // Note that we set loop role to ContourInternalPerimeter
+            // also when loop is both internal and external (i.e.
+            // there's only one contour loop).
+            loop_role = elrContourInternalPerimeter;
+        } else {
+            loop_role = elrDefault;
+        }
+        
+        // detect overhanging/bridging perimeters
+        ExtrusionPaths paths;
+        if (this->config->overhangs && this->layer_id > 0
+            && !(this->object_config->support_material && this->object_config->support_material_contact_distance == 0)) {
+            // get non-overhang paths by intersecting this loop with the grown lower slices
+            {
+                Polylines polylines;
+                intersection(loop->polygon(), this->_lower_slices_p, &polylines);
+                
+                for (Polylines::const_iterator polyline = polylines.begin(); polyline != polylines.end(); ++polyline) {
+                    ExtrusionPath path(role);
+                    path.polyline   = *polyline;
+                    path.mm3_per_mm = is_external ? this->_ext_mm3_per_mm           : this->_mm3_per_mm;
+                    path.width      = is_external ? this->ext_perimeter_flow->width : this->perimeter_flow.width;
+                    path.height     = this->layer_height;
+                    paths.push_back(path);
+                }
+            }
+            
+            // get overhang paths by checking what parts of this loop fall 
+            // outside the grown lower slices (thus where the distance between
+            // the loop centerline and original lower slices is >= half nozzle diameter
+            {
+                Polylines polylines;
+                diff(loop->polygon(), this->_lower_slices_p, &polylines);
+                
+                for (Polylines::const_iterator polyline = polylines.begin(); polyline != polylines.end(); ++polyline) {
+                    ExtrusionPath path(erOverhangPerimeter);
+                    path.polyline   = *polyline;
+                    path.mm3_per_mm = this->_mm3_per_mm_overhang;
+                    path.width      = this->overhang_flow.width;
+                    path.height     = this->overhang_flow.height;
+                    paths.push_back(path);
+                }
+            }
+            
+            // reapply the nearest point search for starting point
+            // We allow polyline reversal because Clipper may have randomly
+            // reversed polylines during clipping.
+            paths = ExtrusionEntityCollection(paths).chained_path();
+        } else {
+            ExtrusionPath path(role);
+            path.polyline   = loop.polygon().split_at_first_point();
+            path.mm3_per_mm = is_external ? this->_ext_mm3_per_mm           : this->_mm3_per_mm;
+            path.width      = is_external ? this->ext_perimeter_flow->width : this->perimeter_flow.width;
+            path.height     = this->layer_height;
+        }
+        
+        coll.append(ExtrusionLoop(paths, loop_role));
+    }
+    
+    // append thin walls to the nearest-neighbor search (only for first iteration)
+    if (!thin_walls.empty()) {
+        for (Polylines::const_iterator polyline = thin_walls.begin(); polyline != thin_walls.end(); ++polyline) {
+            ExtrusionPath path(erExternalPerimeter);
+            path.polyline   = *polyline;
+            path.mm3_per_mm = this->_mm3_per_mm;
+            path.width      = this->perimeter_flow.width;
+            path.height     = this->layer_height;
+            coll.append(path);
+        }
+        
+        thin_walls.clear();
+    }
+    
+    // sort entities
+    ExtrusionPathCollection sorted_coll;
+    coll.chained_path(&sorted_coll, false, &sorted_coll.orig_indices);
     
+    // traverse children
+    ExtrusionPathCollection entities;
+    for (unsigned short i = 0; i < sorted_coll.orig_indices.size(); ++i) {
+        size_t idx = sorted_coll.orig_indices[i];
+        if (idx >= loops.size()) {
+            // this is a thin wall
+            // let's get it from the sorted collection as it might have been reversed
+            entities.append(*sorted_coll.entities[i]);
+        } else {
+            PerimeterGeneratorLoop &loop = loops[i];
+            ExtrusionLoop eloop = *coll.entities[idx];
+            
+            ExtrusionEntityCollection children = this->_traverse_loops(loop->children, thin_walls);
+            if (loop->is_contour()) {
+                eloop.make_counter_clockwise();
+                entities.append(children);
+                entities.append(elooop);
+            } else {
+                eloop.make_clockwise();
+                push @entities, $eloop, @children;
+                entities.append(elooop);
+                entities.append(children);
+            }
+        }
+    }
+    return entities;
 }
 
 ExtrusionEntityCollection
@@ -47,44 +472,20 @@ PerimeterGenerator::_fill_gaps(double min, double max, double w,
     
     double mm3_per_mm = flow.mm3_per_mm();
     
-    /*
-    my %path_args = (
-        role        => EXTR_ROLE_GAPFILL,
-        mm3_per_mm  => $flow->mm3_per_mm,
-        width       => $flow->width,
-        height      => $self->layer_height,
-    );
-    */
-    
     for (Polylines::const_iterator p = polylines.begin(); p != polylines.end(); ++p) {
-        /*
-        #if ($polylines[$i]->isa('Slic3r::Polygon')) {
-        #    my $loop = Slic3r::ExtrusionLoop->new;
-        #    $loop->append(Slic3r::ExtrusionPath->new(polyline => $polylines[$i]->split_at_first_point, %path_args));
-        #    $polylines[$i] = $loop;
-        */
+        ExtrusionPath path(erGapFill);
+        path.polyline   = *p;
+        path.mm3_per_mm = mm3_per_mm;
+        path.width      = flow.width;
+        path.height     = this->layer_height;
+        
         if (p->is_valid() && p->first_point().coincides_with(p->last_point())) {
             // since medial_axis() now returns only Polyline objects, detect loops here
-            
-            
             ExtrusionLoop loop;
-            loop.paths.push_back();
-        } else {
-            
-        }
-    }
-    
-    foreach my $polyline (@polylines) {
-        #if ($polylines[$i]->isa('Slic3r::Polygon')) {
-        #    my $loop = Slic3r::ExtrusionLoop->new;
-        #    $loop->append(Slic3r::ExtrusionPath->new(polyline => $polylines[$i]->split_at_first_point, %path_args));
-        #    $polylines[$i] = $loop;
-        if ($polyline->is_valid && $polyline->first_point->coincides_with($polyline->last_point)) {
-            # since medial_axis() now returns only Polyline objects, detect loops here
-            push @entities, my $loop = Slic3r::ExtrusionLoop->new;
-            $loop->append(Slic3r::ExtrusionPath->new(polyline => $polyline, %path_args));
+            loop.paths.push_back(path);
+            coll.append(loop);
         } else {
-            push @entities, Slic3r::ExtrusionPath->new(polyline => $polyline, %path_args);
+            coll.append(path);
         }
     }
     
diff --git a/xs/src/libslic3r/PerimeterGenerator.hpp b/xs/src/libslic3r/PerimeterGenerator.hpp
index d896cd8da..f5a3a6dd9 100644
--- a/xs/src/libslic3r/PerimeterGenerator.hpp
+++ b/xs/src/libslic3r/PerimeterGenerator.hpp
@@ -6,6 +6,7 @@
 namespace Slic3r {
 
 class PerimeterGeneratorLoop;
+typedef std::vector<PerimeterGeneratorLoop> PerimeterGeneratorLoops;
 
 class PerimeterGeneratorLoop {
     public:
@@ -15,7 +16,7 @@ class PerimeterGeneratorLoop {
     std::vector<PerimeterGeneratorLoop> children;
     
     PerimeterGeneratorLoop(Polygon polygon, unsigned short depth)
-        : polygon(polygon), depth(depth)
+        : polygon(polygon), depth(depth), is_contour(false)
         {};
     bool is_external() const;
     bool is_internal_contour() const;
@@ -34,31 +35,42 @@ class PerimeterGenerator {
     PrintRegionConfig* config;
     PrintObjectConfig* object_config;
     PrintConfig* print_config;
-    double _ext_mm3_per_mm;
-    double _mm3_per_mm;
-    double _mm3_per_mm_overhang;
     ExtrusionEntityCollection* loops;
     ExtrusionEntityCollection* gap_fill;
     SurfaceCollection* fill_surfaces;
     
     PerimeterGenerator(SurfaceCollection* slices, double layer_height,
-        ExtrusionEntityCollection* loops, ExtrusionEntityCollection* gap_fill,
-        SurfaceCollection* fill_surfaces)
-        : slices(slices), layer_height(layer_height), layer_id(-1),
-            _ext_mm3_per_mm(-1), _mm3_per_mm(-1), _mm3_per_mm_overhang(-1),
+        PrintRegionConfig* config, PrintObjectConfig* object_config, 
+        PrintConfig* print_config, ExtrusionEntityCollection* loops, 
+        ExtrusionEntityCollection* gap_fill, SurfaceCollection* fill_surfaces)
+        : slices(slices), lower_slices(NULL), layer_height(layer_height), layer_id(-1),
+            config(config), object_config(object_config), print_config(print_config),
             loops(loops), gap_fill(gap_fill), fill_surfaces(fill_surfaces)
+            _ext_mm3_per_mm(-1), _mm3_per_mm(-1), _mm3_per_mm_overhang(-1)
         {};
     void process();
     
     private:
+    double _ext_mm3_per_mm;
+    double _mm3_per_mm;
+    double _mm3_per_mm_overhang;
     Polygons _lower_slices_p;
     
-    ExtrusionEntityCollection _traverse_loops(const std::vector<PerimeterGeneratorLoop> &loops,
+    ExtrusionEntityCollection _traverse_loops(const PerimeterGeneratorLoops &loops,
         const Polylines &thin_walls) const;
     ExtrusionEntityCollection _fill_gaps(double min, double max, double w,
         const Polygons &gaps) const;
 };
 
+class PerimeterGeneratorGapSize {
+    public:
+    coord_t min;
+    coord_t max;
+    coord_t width;
+    PerimeterGeneratorGapSizes(coord_t min, coord_t max, coord_t width)
+        : min(min), max(max), width(width) {};
+};
+
 }
 
 #endif