Also possible to exclude bottom from small skin behaviour.

Small skin areas replace the normal fill pattern and use walls. Previously in this branch, the possibility was added to not do that to areas exposed to air. It turns out this means in general, not just on the buildplate (also, bottoms can happen away from the buildplate as well of course). As such, take skin bottoms into account of when to exclude areas for consideration when 'Small Top/Bottom On Surface' is off.

part of CURA-10829

include/sliceDataStorage.h

@@ -174,6 +174,13 @@ class SliceLayer
      */
     TopSurface top_surface;
 
+    /*!
+     * \brief The parts of the model that are exposed at the bottom(s) of the model.
+     *
+     * Note: Filled only when needed.
+     */
+    Polygons bottom_surface;
+
     /*!
      * Get the all outlines of all layer parts in this layer.
      * 

src/FffGcodeWriter.cpp

@@ -2764,7 +2764,8 @@ void FffGcodeWriter::processSkinPrintFeature(
     constexpr int zag_skip_count = 0;
     constexpr coord_t pocket_size = 0;
     const bool small_areas_on_surface = mesh.settings.get<bool>("small_skin_on_surface");
-    const auto& exposed_to_air = mesh.layers[gcode_layer.getLayerNr()].top_surface.areas;
+    const auto& current_layer = mesh.layers[gcode_layer.getLayerNr()];
+    const auto& exposed_to_air = current_layer.top_surface.areas.unionPolygons(current_layer.bottom_surface);
 
     Infill infill_comp(
         pattern,

src/FffPolygonGenerator.cpp

@@ -830,6 +830,16 @@ void FffPolygonGenerator::processSkinsAndInfill(SliceMeshStorage& mesh, const La
         // Generate the top surface to iron over.
         mesh.layers[layer_nr].top_surface.setAreasFromMeshAndLayerNumber(mesh, layer_nr);
     }
+
+    if (layer_nr >= 0 && ! mesh.settings.get<bool>("small_skin_on_surface"))
+    {
+        // Generate the bottom surface.
+        mesh.layers[layer_nr].bottom_surface = mesh.layers[layer_nr].getOutlines();
+        if (layer_nr > 0)
+        {
+            mesh.layers[layer_nr].bottom_surface = mesh.layers[layer_nr].bottom_surface.difference(mesh.layers[layer_nr - 1].getOutlines());
+        }
+    }
 }
 
 void FffPolygonGenerator::computePrintHeightStatistics(SliceDataStorage& storage)