Don't use bridge flow over dense infill

src/libslic3r/LayerRegion.cpp

@@ -1006,5 +1006,23 @@ void LayerRegion::export_region_fill_surfaces_to_svg_debug(const char *name) con
  this->export_region_fill_surfaces_to_svg(debug_out_path("LayerRegion-fill_surfaces-%s-%d.svg", name, idx ++).c_str());
 }
 
+
+bool LayerRegion::needs_bridge_over_infill() const
+{
+ // Minimum width of voids in sparse infill for the layer above to be treated as a bridge.
+ // Scaled based on layer height, with a 3mm void at 0.2mm layer height as the baseline.
+ const float min_void_width = bridging_flow(frSolidInfill, true).scaled_width() * 3.f * m_layer->height / 0.2;
+
+ const float fill_density = region().config().fill_density.value / 100.0;
+ if (fill_density > 0.0) {
+ // Estimate the infill void width from infill density and extrusion width.
+ const float infill_extrusion_width = flow(frInfill).scaled_width();
+ if (min_void_width > (infill_extrusion_width / fill_density - infill_extrusion_width))
+ return false;
+ }
+
+ return true;
+}
+
 }
 

src/libslic3r/LayerRegion.hpp

@@ -126,6 +126,9 @@ class LayerRegion
 
  // Is there any valid extrusion assigned to this LayerRegion?
  bool has_extrusions() const { return ! this->perimeters().empty() || ! this->fills().empty(); }
+ // True if infill is sparse enough that it requires bridge flow above, false if infill is
+ // dense enough to support normal solid infill layer above.
+ bool needs_bridge_over_infill() const;
 
 protected:
  friend class Layer;

src/libslic3r/PrintObject.cpp

@@ -2499,7 +2499,7 @@ void PrintObject::bridge_over_infill()
  for (const Surface *surface : internal_solids) {
  if (cs.original_surface == surface) {
  Surface tmp{*surface, {}};
- tmp.surface_type = stInternalBridge;
+ tmp.surface_type = region->needs_bridge_over_infill() ? stInternalBridge : stInternalSolid;
  tmp.bridge_angle = cs.bridge_angle;
  for (const ExPolygon &ep : union_ex(cs.new_polys)) {
  new_surfaces.emplace_back(tmp, ep);
@@ -2817,8 +2817,8 @@ void PrintObject::discover_horizontal_shells()
  if (region_config.solid_infill_every_layers.value > 0 && region_config.fill_density.value > 0 &&
  (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;
+ SurfaceType type = (!layerm->needs_bridge_over_infill() || region_config.solid_infill_every_layers == 1) ? stInternalSolid :
+ stInternalBridge;
  for (Surface &surface : layerm->m_fill_surfaces.surfaces)
  if (surface.surface_type == stInternal)
  surface.surface_type = type;