Add setting to control when bridge infill is used

Adds bridge_infill_threshold setting to control when bridge infill is
used. Bridge infill will be applied only if the average area of the
voids between infill lines (computed from the infill density and infill
extrusion width) is greater than the supplied threshold value.

src/libslic3r/Layer.hpp

@@ -57,6 +57,8 @@ class LayerRegion
     void    make_perimeters(const SurfaceCollection &slices, SurfaceCollection* fill_surfaces);
     void    process_external_surfaces(const Layer *lower_layer, const Polygons *lower_layer_covered);
     double  infill_area_threshold() const;
+    // True if infill voids (computed from infill_density) are larger than bridge_infill_threshold.
+    bool    needs_bridge_over_infill() const;
     // Trim surfaces by trimming polygons. Used by the elephant foot compensation at the 1st layer.
     void    trim_surfaces(const Polygons &trimming_polygons);
     // Single elephant foot compensation step, used by the elephant foor compensation at the 1st layer.

src/libslic3r/LayerRegion.cpp

@@ -397,6 +397,17 @@ double LayerRegion::infill_area_threshold() const
     return ss*ss;
 }
 
+bool LayerRegion::needs_bridge_over_infill() const
+{
+    auto region_config = region()->config();
+    if (region_config.fill_density <= 0)
+        return true;
+    const double infill_extrusion_width = flow(frInfill).width;
+    // Compute the unsupported area assuming a grid, which is pragmatically good enough for all infill types.
+    const double bridging_area = pow<double>(2.0 * 100.0 * infill_extrusion_width / region_config.fill_density - infill_extrusion_width, 2);
+    return bridging_area > region_config.bridge_infill_threshold;
+}
+
 void LayerRegion::trim_surfaces(const Polygons &trimming_polygons)
 {
 #ifndef NDEBUG

src/libslic3r/Print.cpp

@@ -74,6 +74,7 @@ bool Print::invalidate_state_by_config_options(const std::vector<t_config_option
         "before_layer_gcode",
         "between_objects_gcode",
         "bridge_acceleration",
+        "bridge_infill_threshold",
         "bridge_fan_speed",
         "colorprint_heights",
         "cooling",

src/libslic3r/PrintConfig.cpp

@@ -238,6 +238,16 @@ void PrintConfigDef::init_fff_params()
     def->mode = comAdvanced;
     def->set_default_value(new ConfigOptionFloat(1));
 
+    def = this->add("bridge_infill_threshold", coFloat);
+    def->label = L("Bridge infill threshold");
+    def->tooltip = L("Forces bridge settings for the layer above infill where voids are larger than the specified area. "
+                    "The infill void area is computed from the infill density and infill extrusion width. "
+                    "Set to zero to force bridge settings for any layer above non-solid infill.");
+    def->sidetext = L("mm²");
+    def->min = 0;
+    def->mode = comExpert;
+    def->set_default_value(new ConfigOptionFloat(25.));
+
     def = this->add("bridge_speed", coFloat);
     def->label = L("Bridges");
     def->category = L("Speed");

src/libslic3r/PrintConfig.hpp

@@ -490,6 +490,7 @@ class PrintRegionConfig : public StaticPrintConfig
     ConfigOptionFloat               bottom_solid_min_thickness;
     ConfigOptionFloat               bridge_flow_ratio;
     ConfigOptionFloat               bridge_speed;
+    ConfigOptionFloat               bridge_infill_threshold;
     ConfigOptionBool                ensure_vertical_shell_thickness;
     ConfigOptionEnum<InfillPattern> top_fill_pattern;
     ConfigOptionEnum<InfillPattern> bottom_fill_pattern;
@@ -540,6 +541,7 @@ class PrintRegionConfig : public StaticPrintConfig
         OPT_PTR(bottom_solid_layers);
         OPT_PTR(bottom_solid_min_thickness);
         OPT_PTR(bridge_flow_ratio);
+        OPT_PTR(bridge_infill_threshold);
         OPT_PTR(bridge_speed);
         OPT_PTR(ensure_vertical_shell_thickness);
         OPT_PTR(top_fill_pattern);

src/libslic3r/PrintObject.cpp

@@ -1408,9 +1408,16 @@ void PrintObject::bridge_over_infill()
 
                     // iterate through regions and collect internal surfaces
                     Polygons lower_internal;
-                    for (LayerRegion *lower_layerm : lower_layer->m_regions)
-                        lower_layerm->fill_surfaces.filter_by_type(stInternal, &lower_internal);
-
+                    for (LayerRegion* lower_layerm : lower_layer->m_regions) {
+                        auto surfaces = lower_layerm->fill_surfaces.surfaces;
+                        for (Surfaces::iterator surface = surfaces.begin(); surface != surfaces.end(); ++surface) {
+                            if (surface->surface_type == stInternal && layerm->needs_bridge_over_infill()) {
+                                Polygons pp = surface->expolygon;
+                                lower_internal.insert(lower_internal.end(), pp.begin(), pp.end());
+                            }
+                        }
+                    }
+
                     // intersect such lower internal surfaces with the candidate solid surfaces
                     to_bridge_pp = intersection(to_bridge_pp, lower_internal);
                 }

src/slic3r/GUI/Preset.cpp

@@ -407,7 +407,7 @@ const std::vector<std::string>& Preset::print_options()
         "infill_every_layers", "infill_only_where_needed", "solid_infill_every_layers", "fill_angle", "bridge_angle",
         "solid_infill_below_area", "only_retract_when_crossing_perimeters", "infill_first", 
     	"ironing", "ironing_type", "ironing_flowrate", "ironing_speed", "ironing_spacing",
-        "max_print_speed", "max_volumetric_speed",
+        "max_print_speed", "max_volumetric_speed", "bridge_infill_threshold",
 #ifdef HAS_PRESSURE_EQUALIZER
         "max_volumetric_extrusion_rate_slope_positive", "max_volumetric_extrusion_rate_slope_negative",
 #endif /* HAS_PRESSURE_EQUALIZER */

src/slic3r/GUI/Tab.cpp

@@ -1345,6 +1345,7 @@ void TabPrint::build()
         optgroup->append_single_option_line("fill_angle");
         optgroup->append_single_option_line("solid_infill_below_area");
         optgroup->append_single_option_line("bridge_angle");
+        optgroup->append_single_option_line("bridge_infill_threshold");
         optgroup->append_single_option_line("only_retract_when_crossing_perimeters");
         optgroup->append_single_option_line("infill_first");