Don't use bridge flow over dense infill

Make infill bridging use the same void threshold check as other bridging
spans (previously bridge flow applied to all non-solid infill.)

src/libslic3r/PrintObject.cpp

@@ -1418,10 +1418,6 @@ void PrintObject::bridge_over_infill()
  for (size_t region_id = 0; region_id < this->num_printing_regions(); ++ region_id) {
  const PrintRegion &region = this->printing_region(region_id);
 
- // skip bridging in case there are no voids
- if (region.config().fill_density.value == 100)
- continue;
-
  for (LayerPtrs::iterator layer_it = m_layers.begin(); layer_it != m_layers.end(); ++ layer_it) {
  // skip first layer
  if (layer_it == m_layers.begin())
@@ -1431,6 +1427,18 @@ void PrintObject::bridge_over_infill()
  LayerRegion *layerm = layer->m_regions[region_id];
  Flow bridge_flow = layerm->bridging_flow(frSolidInfill);
 
+ // Minimum width of a void for the extrusion above to be treated as a bridge.
+ const float min_void_width = float(bridge_flow.scaled_width()) * 3.f;
+
+ // Skip bridging if the infill voids are small enough.
+ 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 = layerm->flow(frInfill).scaled_width();
+ if (min_void_width > (infill_extrusion_width / fill_density - infill_extrusion_width))
+ continue;
+ }
+
  // extract the stInternalSolid surfaces that might be transformed into bridges
  Polygons internal_solid;
  layerm->fill_surfaces.filter_by_type(stInternalSolid, &internal_solid);
@@ -1463,10 +1471,7 @@ void PrintObject::bridge_over_infill()
  //FIXME Vojtech: The offset2 function is not a geometric offset, 
  // therefore it may create 1) gaps, and 2) sharp corners, which are outside the original contour.
  // The gaps will be filled by a separate region, which makes the infill less stable and it takes longer.
- {
- float min_width = float(bridge_flow.scaled_width()) * 3.f;
- to_bridge_pp = opening(to_bridge_pp, min_width);
- }
+ to_bridge_pp = opening(to_bridge_pp, min_void_width);
 
  if (to_bridge_pp.empty()) continue;
 
@@ -1799,11 +1804,9 @@ void PrintObject::discover_horizontal_shells()
  const PrintRegionConfig &region_config = layerm->region().config();
  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;
  for (Surface &surface : layerm->fill_surfaces.surfaces)
  if (surface.surface_type == stInternal)
- surface.surface_type = type;
+ surface.surface_type = stInternalSolid;
  }
 
  // If ensure_vertical_shell_thickness, then the rest has already been performed by discover_vertical_shells().