GPv3: Implement the alpha threshold setting for the Fill tool

The fill tool can take opacity into account ("Advanced" brush settings).
In that case any point with a total opacity lower than the threshold is
rendered as a transparent boundary.

This was still a hardcoded setting in the fill tool, now uses brush
settings.

Pull Request: https://projects.blender.org/blender/blender/pulls/123469

source/blender/editors/include/ED_grease_pencil.hh

@@ -443,6 +443,7 @@ enum FillToolFitMethod {
  * \param boundary_layers: Layers that are purely for boundaries, regular strokes are not rendered.
  * \param src_drawings: Drawings to include as boundary strokes.
  * \param invert: Construct boundary around empty areas instead.
+ * \param alpha_threshold: Render transparent stroke where opacity is below the threshold.
  * \param fill_point: Point from which to start the bucket fill.
  * \param fit_method: View fitting method to include all strokes.
  * \param stroke_material_index: Material index to use for the new strokes.
@@ -455,6 +456,7 @@ bke::CurvesGeometry fill_strokes(const ViewContext &view_context,
                                  const VArray<bool> &boundary_layers,
                                  Span<DrawingInfo> src_drawings,
                                  bool invert,
+                                 const std::optional<float> alpha_threshold,
                                  const float2 &fill_point,
                                  FillToolFitMethod fit_method,
                                  int stroke_material_index,

source/blender/editors/sculpt_paint/grease_pencil_draw_ops.cc

@@ -59,6 +59,7 @@
 #include "grease_pencil_intern.hh"
 #include "paint_intern.hh"
 #include "wm_event_types.hh"
+#include <optional>
 
 namespace blender::ed::sculpt_paint {
 
@@ -727,6 +728,10 @@ static bool grease_pencil_apply_fill(bContext &C, wmOperator &op, const wmEvent
   const Brush &brush = *BKE_paint_brush(&ts.gp_paint->paint);
   const float2 mouse_position = float2(event.mval);
   const int simplify_levels = brush.gpencil_settings->fill_simplylvl;
+  const std::optional<float> alpha_threshold =
+      (brush.gpencil_settings->flag & GP_BRUSH_FILL_HIDE) ?
+          std::nullopt :
+          std::make_optional(brush.gpencil_settings->fill_threshold);
 
   if (!grease_pencil.has_active_layer()) {
     return false;
@@ -748,6 +753,7 @@ static bool grease_pencil_apply_fill(bContext &C, wmOperator &op, const wmEvent
                                                    boundary_layers,
                                                    info.sources,
                                                    op_data.invert,
+                                                   alpha_threshold,
                                                    mouse_position,
                                                    fit_method,
                                                    op_data.material_index,

source/blender/editors/sculpt_paint/grease_pencil_fill.cc

@@ -803,10 +803,9 @@ static VArray<ColorGeometry4f> stroke_colors(const Object &object,
                                              const VArray<float> &opacities,
                                              const VArray<int> materials,
                                              const ColorGeometry4f &tint_color,
-                                             const float alpha_threshold,
-                                             const bool brush_fill_hide)
+                                             const std::optional<float> alpha_threshold)
 {
-  if (brush_fill_hide) {
+  if (!alpha_threshold) {
     return VArray<ColorGeometry4f>::ForSingle(tint_color, curves.points_num());
   }
 
@@ -820,7 +819,7 @@ static VArray<ColorGeometry4f> stroke_colors(const Object &object,
                                        1.0f;
       const IndexRange points = curves.points_by_curve()[curve_i];
       for (const int point_i : points) {
-        const float alpha = (material_alpha * opacities[point_i] > alpha_threshold ? 1.0f : 0.0f);
+        const float alpha = (material_alpha * opacities[point_i] > *alpha_threshold ? 1.0f : 0.0f);
         colors[point_i] = ColorGeometry4f(tint_color.r, tint_color.g, tint_color.b, alpha);
       }
     }
@@ -987,6 +986,7 @@ bke::CurvesGeometry fill_strokes(const ViewContext &view_context,
                                  const VArray<bool> &boundary_layers,
                                  const Span<DrawingInfo> src_drawings,
                                  const bool invert,
+                                 const std::optional<float> alpha_threshold,
                                  const float2 &fill_point,
                                  const FillToolFitMethod fit_method,
                                  const int stroke_material_index,
@@ -1046,8 +1046,6 @@ bke::CurvesGeometry fill_strokes(const ViewContext &view_context,
   GPU_depth_mask(true);
   image_render::set_viewmat(view_context, scene, image_size, zoom, offset);
 
-  const float alpha_threshold = 0.2f;
-  const bool brush_fill_hide = false;
   const bool use_xray = false;
 
   const float4x4 layer_to_world = layer.to_world_space(object);
@@ -1081,8 +1079,7 @@ bke::CurvesGeometry fill_strokes(const ViewContext &view_context,
                                                          opacities,
                                                          materials,
                                                          draw_boundary_color,
-                                                         alpha_threshold,
-                                                         brush_fill_hide);
+                                                         alpha_threshold);
 
     image_render::draw_grease_pencil_strokes(rv3d,
                                              image_size,