Proof-of-concept: fractional support z-distance.

Instead of half-height layers in each area that could potentially be affected by non-layer-height modulo support z distance, actually calculate the proper z-offset that is needed for the fractional layer.

Proof of concept mostly in the sense that the quality of the code added may not reflect our standards, and would also be missing functionality (for instance; what if we print without support-interface, or tree-support at all).

part of CURA-11041, which is the proof-of-concept for CURA-10407

src/FffGcodeWriter.cpp

@@ -2287,7 +2287,7 @@ bool FffGcodeWriter::processInsets(
         if (mesh_group_settings.get<bool>("support_enable"))
         {
             const coord_t z_distance_top = mesh.settings.get<coord_t>("support_top_distance");
-            const size_t z_distance_top_layers = round_up_divide(z_distance_top, layer_height) + 1;
+            const size_t z_distance_top_layers = round_up_divide(z_distance_top, layer_height); // Previously '... +1', but now there is an extra fractional layer on top.
             const int support_layer_nr = gcode_layer.getLayerNr() - z_distance_top_layers;
 
             if (support_layer_nr > 0)
@@ -2596,7 +2596,7 @@ void FffGcodeWriter::processTopBottom(
     {
         const coord_t layer_height = mesh_config.inset0_config.getLayerThickness();
         const coord_t z_distance_top = mesh.settings.get<coord_t>("support_top_distance");
-        const size_t z_distance_top_layers = round_up_divide(z_distance_top, layer_height) + 1;
+        const size_t z_distance_top_layers = round_up_divide(z_distance_top, layer_height); // Previously '... +1', but now there is an extra fractional layer on top.
         support_layer_nr = layer_nr - z_distance_top_layers;
     }
 
@@ -3327,7 +3327,9 @@ bool FffGcodeWriter::addSupportRoofsToGCode(const SliceDataStorage& storage, Lay
         support_roof_line_distance *= roof_extruder.settings.get<Ratio>("initial_layer_line_width_factor");
     }
 
-    const auto half_layer_height = Application::getInstance().current_slice->scene.current_mesh_group->settings.get<coord_t>("layer_height") / 2;
+    const auto layer_height = Application::getInstance().current_slice->scene.current_mesh_group->settings.get<coord_t>("layer_height");
+    const auto support_top_distance = Application::getInstance().current_slice->scene.current_mesh_group->settings.get<coord_t>("support_top_distance");
+    const coord_t leftover_support_distance = support_top_distance % layer_height;
 
     auto infill_outlines = { support_layer.support_roof.difference(support_layer.support_fractional_roof_top), support_layer.support_fractional_roof_top };
     auto current_roof_config = gcode_layer.configs_storage.support_roof_config;  // copy!
@@ -3417,8 +3419,8 @@ bool FffGcodeWriter::addSupportRoofsToGCode(const SliceDataStorage& storage, Lay
             gcode_layer.configs_storage.support_roof_config,
             (pattern == EFillMethod::ZIG_ZAG) ? SpaceFillType::PolyLines : SpaceFillType::Lines);
 
-        current_roof_config.z_offset = -half_layer_height;
-        current_roof_config.flow /= 2.0;
+        current_roof_config.z_offset = -leftover_support_distance;
+        current_roof_config.flow *= Ratio(layer_height - leftover_support_distance, layer_height);
     }
     return generated_something;
 }

src/support.cpp

@@ -794,7 +794,7 @@ void AreaSupport::generateOverhangAreasForMesh(SliceDataStorage& storage, SliceM
     // Don't generate overhang areas if the Z distance is higher than the objects we're generating support for.
     const coord_t layer_height = Application::getInstance().current_slice->scene.current_mesh_group->settings.get<coord_t>("layer_height");
     const coord_t z_distance_top = mesh.settings.get<coord_t>("support_top_distance");
-    const size_t z_distance_top_layers = round_up_divide(z_distance_top, layer_height) + 1; // Support must always be 1 layer below overhang.
+    const size_t z_distance_top_layers = round_up_divide(z_distance_top, layer_height); // Previously '... +1', but now there is an extra fractional layer on top.
     if (z_distance_top_layers + 1 > storage.print_layer_count)
     {
         return;
@@ -1059,7 +1059,7 @@ void AreaSupport::generateSupportAreasForMesh(
     // early out
     const coord_t layer_thickness = mesh_group_settings.get<coord_t>("layer_height");
     const coord_t z_distance_top = ((mesh.settings.get<bool>("support_roof_enable")) ? roof_settings : infill_settings).get<coord_t>("support_top_distance");
-    const size_t layer_z_distance_top = round_up_divide(z_distance_top, layer_thickness) + 1; // support must always be 1 layer below overhang
+    const size_t layer_z_distance_top = round_up_divide(z_distance_top, layer_thickness); // Previously '... +1', but now there is an extra fractional layer on top.
     if (layer_z_distance_top + 1 > layer_count)
     {
         return;
@@ -1805,7 +1805,7 @@ void AreaSupport::generateSupportRoof(SliceDataStorage& storage, const SliceMesh
     const double minimum_roof_area = mesh.settings.get<double>("minimum_roof_area");
 
     std::vector<SupportLayer>& support_layers = storage.support.supportLayers;
-    for (LayerIndex layer_idx = static_cast<int>(support_layers.size() - z_distance_top) - 1; layer_idx >= 0; --layer_idx)
+    for (LayerIndex layer_idx = static_cast<int>(support_layers.size() - z_distance_top); layer_idx >= 0; --layer_idx)
     {
         const LayerIndex top_layer_idx_above{
             std::min(LayerIndex{ support_layers.size() - 1 }, LayerIndex{ layer_idx + roof_layer_count + z_distance_top })