Add internal_infill_min_width setting

src/libslic3r/Preset.cpp

@@ -511,6 +511,7 @@ static std::vector<std::string> s_Preset_print_options {
     "wall_distribution_count", "min_feature_size", "min_bead_width",
     "top_one_perimeter_type", "only_one_perimeter_first_layer",
     "automatic_extrusion_widths", "automatic_infill_combination", "automatic_infill_combination_max_layer_height",
+    "internal_infill_min_width",
 };
 
 static std::vector<std::string> s_Preset_filament_options {

src/libslic3r/PrintConfig.cpp

@@ -2876,6 +2876,18 @@ void PrintConfigDef::init_fff_params()
     def->mode = comExpert;
     def->set_default_value(new ConfigOptionFloat(70));
 
+    def = this->add("internal_infill_min_width", coFloat);
+    def->label = L("Internal infill minimum width");
+    def->category = L("Infill");
+    def->tooltip = L("Force solid infill for regions that would be internal infill but are surrounded by solid infill "
+                     "and are thinner than this. This can be used to avoid unneeded patches of internal infill which "
+                     "can slow the print down due to excessive direction changes, or to get a cleaner top surface for "
+                     "e.g. signs with raised text.");
+    def->sidetext = L("mm");
+    def->min = 0;
+    def->mode = comExpert;
+    def->set_default_value(new ConfigOptionFloat(0));
+
     def = this->add("solid_infill_extruder", coInt);
     def->label = L("Solid infill extruder");
     def->category = L("Extruders");

src/libslic3r/PrintConfig.hpp

@@ -720,6 +720,7 @@ PRINT_CONFIG_CLASS_DEFINE(
     ((ConfigOptionInt,                  infill_every_layers))
     ((ConfigOptionFloatOrPercent,       infill_overlap))
     ((ConfigOptionFloat,                infill_speed))
+    ((ConfigOptionFloat,                internal_infill_min_width))
     // Ironing options
     ((ConfigOptionBool,                 ironing))
     ((ConfigOptionEnum<IroningType>,    ironing_type))

src/libslic3r/PrintObject.cpp

@@ -1131,7 +1131,38 @@ void PrintObject::detect_surfaces_type()
                     {
                         Polygons topbottom = to_polygons(top);
                         polygons_append(topbottom, to_polygons(bottom));
-                        surfaces_append(surfaces_out, diff_ex(surfaces_prev, topbottom), stInternal);
+
+                        Surfaces internal;
+                        surfaces_append(internal, diff_ex(surfaces_prev, topbottom), stInternal);
+
+                        float internal_infill_min_width = layerm->region().config().internal_infill_min_width.value;
+                        if (internal_infill_min_width > 0) {
+                            // convert thin patches of internal infill surrounded by other surfaces into top surfaces
+
+                            float shrink_by = scale_(internal_infill_min_width) * 0.5;
+
+                            // use extrusion width as basis for a heuristic for what should be considered a significant surrounding area
+                            float edge_band_width = layerm->flow(frExternalPerimeter).scaled_width() * 10;
+
+                            for (Surface &surface : internal) {
+                                // keep as internal if this surface is not thin enough
+                                if (!offset_ex(surface.expolygon, -shrink_by).empty()) {
+                                    continue;
+                                }
+
+                                // calculate an edge band around this surface to probe for adjacent surfaces
+                                ExPolygons edge_band = diff_ex(offset_ex(surface.expolygon, edge_band_width), {surface.expolygon});
+
+                                // keep as internal if this surface is not sufficiently surrounded by adjacent surfaces
+                                if (area(intersection_ex(surfaces_prev, edge_band)) < area(edge_band) * 0.25) {
+                                    continue;
+                                }
+
+                                surface.surface_type = stTop;
+                            }
+                        }
+
+                        surfaces_append(surfaces_out, std::move(internal));
                     }
 
                     surfaces_append(surfaces_out, std::move(top));

src/slic3r/GUI/Tab.cpp

@@ -1511,6 +1511,7 @@ void TabPrint::build()
         optgroup->append_single_option_line("solid_infill_every_layers", category_path + "solid-infill-every-x-layers");
         optgroup->append_single_option_line("fill_angle", category_path + "fill-angle");
         optgroup->append_single_option_line("solid_infill_below_area", category_path + "solid-infill-threshold-area");
+        optgroup->append_single_option_line("internal_infill_min_width", category_path + "internal-infill-min-width");
         optgroup->append_single_option_line("bridge_angle");
         optgroup->append_single_option_line("only_retract_when_crossing_perimeters");
         optgroup->append_single_option_line("infill_first");