Skeleton modifier for grease pencil opacity.

This comes with utility functions that generate index masks, which can
then be used to filter layers and strokes by layer name, material, or
pass.

@@ -0,0 +109,4 @@
+
+  /* TODO Would be nice to have the checkbox in the same line as the pass button. */
+  row = uiLayoutRow(col, true);
+  uiItemR(row, ptr, "use_layer_pass_filter", UI_ITEM_NONE, "Filter by layer pass", ICON_NONE);

@@ -0,0 +80,4 @@
+}
+
+/* XXX Placeholder for vertex groupn weights. */
+static VArray<float> get_grease_pencil_modifier_vertex_weights(

@@ -0,0 +162,4 @@
+
+    if (use_vgroup_opacity) {
+      /* Use the first stroke point as vertex weight. */
+      const IndexRange points_range = points_by_curve[curve_i];

@@ -0,0 +163,4 @@
+    if (use_vgroup_opacity) {
+      /* Use the first stroke point as vertex weight. */
+      const IndexRange points_range = points_by_curve[curve_i];
+      const float stroke_weight = points_range.is_empty() ? 1.0f :

@@ -0,0 +182,4 @@
+                            const IndexMask &curves_mask)
+{
+  bke::MutableAttributeAccessor attributes = curves.attributes_for_write();
+  bke::SpanAttributeWriter<float> hardnesses = attributes.lookup_or_add_for_write_span<float>(

@@ -0,0 +198,4 @@
+                          const ModifierEvalContext *ctx,
+                          bke::CurvesGeometry &curves)
+{
+  GreasePencilOpacityModifierData *omd = (GreasePencilOpacityModifierData *)md;

@@ -0,0 +122,4 @@
+      else if (use_weight_as_factor) {
+        /* Use vertex group weights as opacity factors. */
+        const float vgroup_weight = vgroup_weights[point_i];
+        const float point_factor = vgroup_weight;

@@ -0,0 +239,4 @@
+      *grease_pencil, omd->influence, mask_memory);
+  Vector<Drawing *> drawings = greasepencil::get_drawings_for_write(
+      *grease_pencil, layer_mask, frame);
+  for (Drawing *drawing : drawings) {

@@ -0,0 +131,4 @@
+        const float vgroup_weight = vgroup_weights[point_i];
+        const float vgroup_influence = invert_vertex_group ? 1.0f - vgroup_weight : vgroup_weight;
+        opacities.span[point_i] = std::clamp(
+            opacities.span[point_i] + omd.color_factor * curve_factor * vgroup_influence - 1.0f,

@@ -46,6 +46,7 @@ class DATA_PT_modifiers(ModifierButtonsPanel, Panel):
 
     def draw(self, _context):
         layout = self.layout
+        ob_type = _context.object.type

@@ -154,2 +155,2 @@
-   * Only set on modifiers in evaluated objects. The flag indicates that the user modified inputs
-   * to the modifier which might invalidate simulation caches.
+   * Only set on modifiers in evaluated objects. The flag indicates that the user modified
+   * inputs to the modifier which might invalidate simulation caches.

@@ -7426,0 +7574,4 @@
+  RNA_def_property_update(prop, 0, "rna_Modifier_update");
+}
+
+static void rna_def_modifier_grease_pencil_vertex_group(StructRNA *srna,

@@ -103,0 +104,4 @@
+     "GREASE_PENCIL_OPACITY",
+     ICON_MOD_OPACITY,
+     "Opacity",
+     "Opacity of the strokes"},

@@ -0,0 +1,338 @@
+/* SPDX-FileCopyrightText: 2005 Blender Authors

@@ -0,0 +57,4 @@
+
+static void copy_data(const ModifierData *md, ModifierData *target, const int flag)
+{
+  const GreasePencilOpacityModifierData *omd =

@@ -0,0 +94,4 @@
+  bke::MutableAttributeAccessor attributes = curves.attributes_for_write();
+  bke::SpanAttributeWriter<float> opacities = attributes.lookup_or_add_for_write_span<float>(
+      "opacity", bke::AttrDomain::Point);
+  const bke::AttributeReader<float> vgroup_weights = attributes.lookup_or_default<float>(