Fix of "Bridging and gap fill are parsed incorrectly when infill is set to 0" #1476

The gap fill was disabled for zero infill.
Now the gap fill is enabled in between the perimeters, but disabled between
the inner-most perimeter and infill in case the infill is set to zero.

Also in case there are multiple infill regions inside a perimeter,
the mutliple infills are considered as non-zero if at least one infill
is non-zero, therefore the gap fill will be added inside the inner-most
perimeter.

src/libslic3r/Layer.cpp

@@ -121,7 +121,6 @@ void Layer::make_perimeters()
         for (LayerRegionPtrs::const_iterator it = layerm + 1; it != m_regions.end(); ++it) {
             LayerRegion* other_layerm = *it;
             const PrintRegionConfig &other_config = other_layerm->region()->config();
-            
             if (config.perimeter_extruder   == other_config.perimeter_extruder
                 && config.perimeters        == other_config.perimeters
                 && config.perimeter_speed   == other_config.perimeter_speed
@@ -130,7 +129,8 @@ void Layer::make_perimeters()
                 && config.overhangs         == other_config.overhangs
                 && config.opt_serialize("perimeter_extrusion_width") == other_config.opt_serialize("perimeter_extrusion_width")
                 && config.thin_walls        == other_config.thin_walls
-                && config.external_perimeters_first == other_config.external_perimeters_first) {
+                && config.external_perimeters_first == other_config.external_perimeters_first
+                && config.infill_overlap    == other_config.infill_overlap) {
                 layerms.push_back(other_layerm);
                 done[it - m_regions.begin()] = true;
             }
@@ -142,12 +142,17 @@ void Layer::make_perimeters()
             (*layerm)->fill_expolygons = to_expolygons((*layerm)->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.
+            LayerRegion *layerm_config = layerms.front();
             {
                 // group slices (surfaces) according to number of extra perimeters
                 std::map<unsigned short, Surfaces> slices;  // extra_perimeters => [ surface, surface... ]
-                for (LayerRegion *layerm : layerms)
+                for (LayerRegion *layerm : layerms) {
                     for (Surface &surface : layerm->slices.surfaces)
                         slices[surface.extra_perimeters].emplace_back(surface);
+                    if (layerm->region()->config().fill_density > layerm_config->region()->config().fill_density)
+                    	layerm_config = layerm;
+                }
                 // merge the surfaces assigned to each group
                 for (std::pair<const unsigned short,Surfaces> &surfaces_with_extra_perimeters : slices)
                     new_slices.append(union_ex(surfaces_with_extra_perimeters.second, true), surfaces_with_extra_perimeters.second.front());
@@ -155,7 +160,7 @@ void Layer::make_perimeters()
             
             // make perimeters
             SurfaceCollection fill_surfaces;
-            (*layerm)->make_perimeters(new_slices, &fill_surfaces);
+            layerm_config->make_perimeters(new_slices, &fill_surfaces);
 
             // assign fill_surfaces to each layer
             if (!fill_surfaces.surfaces.empty()) { 

src/libslic3r/PerimeterGenerator.cpp

@@ -37,7 +37,8 @@ void PerimeterGenerator::process()
     // internal flow which is unrelated.
     coord_t min_spacing         = perimeter_spacing      * (1 - INSET_OVERLAP_TOLERANCE);
     coord_t ext_min_spacing     = ext_perimeter_spacing  * (1 - INSET_OVERLAP_TOLERANCE);
-    
+    bool    has_gap_fill 		= this->config->gap_fill_speed.value > 0;
+
     // 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
@@ -105,7 +106,7 @@ void PerimeterGenerator::process()
                         // leads to overflows, as in prusa3d/Slic3r GH #32
                         offset_ex(last, - distance);
                     // look for gaps
-                    if (this->config->gap_fill_speed.value > 0 && this->config->fill_density.value > 0)
+                    if (has_gap_fill)
                         // 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
@@ -132,6 +133,11 @@ void PerimeterGenerator::process()
                     }
                 }
                 last = std::move(offsets);
+                if (i == loop_number && (! has_gap_fill || this->config->fill_density.value == 0)) {
+                	// The last run of this loop is executed to collect gaps for gap fill.
+                	// As the gap fill is either disabled or not 
+                	break;
+                }
             }
 
             // nest loops: holes first

src/slic3r/GUI/ConfigManipulation.cpp

@@ -243,7 +243,8 @@ void ConfigManipulation::toggle_print_fff_options(DynamicPrintConfig* config)
                     "infill_speed", "bridge_speed" })
         toggle_field(el, have_infill || have_solid_infill);
 
-    toggle_field("gap_fill_speed", have_perimeters && have_infill);
+    // Gap fill is newly allowed in between perimeter lines even for empty infill (see GH #1476).
+    toggle_field("gap_fill_speed", have_perimeters);
 
     bool have_top_solid_infill = config->opt_int("top_solid_layers") > 0;
     for (auto el : { "top_infill_extrusion_width", "top_solid_infill_speed" })