Fix default weight in grease pencil modifiers when vertex group not set

Grease pencil modifiers were using a default weight of 1.0 if the
influence vertex group is not found. Whether or not that is correct
depends on whether a vertex group is set (name not empty). In case the
name is set but the group is not found a weight of zero should be used.
This can happen in GP3 when no vertex is assigned to a vertex group,
since each drawing only has a subset of all the vertex groups.

A utility function was added to make this consistent and less error
prone.

Pull Request: https://projects.blender.org/blender/blender/pulls/117671

source/blender/modifiers/intern/MOD_grease_pencil_offset.cc

@@ -134,8 +134,8 @@ static void modify_stroke_random(const Object &ob,
   bke::SpanAttributeWriter<float> radii = attributes.lookup_or_add_for_write_span<float>(
       "radius", bke::AttrDomain::Point);
   const MutableSpan<float3> positions = curves.positions_for_write();
-  const VArray<float> vgroup_weights = *attributes.lookup_or_default<float>(
-      omd.influence.vertex_group_name, bke::AttrDomain::Point, 1.0f);
+  const VArray<float> vgroup_weights = modifier::greasepencil::get_influence_vertex_weights(
+      curves, omd.influence);
 
   /* Make sure different modifiers get different seeds. */
   const int seed = omd.seed + BLI_hash_string(ob.id.name + 2) + BLI_hash_string(omd.modifier.name);
@@ -204,8 +204,8 @@ static void modify_stroke_by_index(const GreasePencilOffsetModifierData &omd,
   bke::SpanAttributeWriter<float> radii = attributes.lookup_or_add_for_write_span<float>(
       "radius", bke::AttrDomain::Point);
   const MutableSpan<float3> positions = curves.positions_for_write();
-  const VArray<float> vgroup_weights = *attributes.lookup_or_default<float>(
-      omd.influence.vertex_group_name, bke::AttrDomain::Point, 1.0f);
+  const VArray<float> vgroup_weights = modifier::greasepencil::get_influence_vertex_weights(
+      curves, omd.influence);
 
   curves_mask.foreach_index(GrainSize(512), [&](const int64_t curve_i) {
     const IndexRange points = points_by_curve[curve_i];
@@ -237,8 +237,8 @@ static void modify_stroke_by_material(const Object &ob,
   bke::SpanAttributeWriter<float> radii = attributes.lookup_or_add_for_write_span<float>(
       "radius", bke::AttrDomain::Point);
   const MutableSpan<float3> positions = curves.positions_for_write();
-  const VArray<float> vgroup_weights = *attributes.lookup_or_default<float>(
-      omd.influence.vertex_group_name, bke::AttrDomain::Point, 1.0f);
+  const VArray<float> vgroup_weights = modifier::greasepencil::get_influence_vertex_weights(
+      curves, omd.influence);
   const VArray<int> stroke_materials = *attributes.lookup_or_default<int>(
       "material_index", bke::AttrDomain::Curve, 0);
 
@@ -270,8 +270,8 @@ static void modify_stroke_by_layer(const GreasePencilOffsetModifierData &omd,
   bke::SpanAttributeWriter<float> radii = attributes.lookup_or_add_for_write_span<float>(
       "radius", bke::AttrDomain::Point);
   const MutableSpan<float3> positions = curves.positions_for_write();
-  const VArray<float> vgroup_weights = *attributes.lookup_or_default<float>(
-      omd.influence.vertex_group_name, bke::AttrDomain::Point, 1.0f);
+  const VArray<float> vgroup_weights = modifier::greasepencil::get_influence_vertex_weights(
+      curves, omd.influence);
 
   const float factor = get_factor_from_index(omd, layers_num, layer_index);
 

source/blender/modifiers/intern/MOD_grease_pencil_opacity.cc

@@ -87,10 +87,8 @@ static void modify_stroke_color(const GreasePencilOpacityModifierData &omd,
   bke::MutableAttributeAccessor attributes = curves.attributes_for_write();
   bke::SpanAttributeWriter<float> opacities = attributes.lookup_or_add_for_write_span<float>(
       "opacity", bke::AttrDomain::Point);
-  const VArray<float> vgroup_weights =
-      attributes
-          .lookup_or_default<float>(omd.influence.vertex_group_name, bke::AttrDomain::Point, 1.0f)
-          .varray;
+  const VArray<float> vgroup_weights = modifier::greasepencil::get_influence_vertex_weights(
+      curves, omd.influence);
 
   curves_mask.foreach_index(GrainSize(512), [&](const int64_t curve_i) {
     const IndexRange points = points_by_curve[curve_i];
@@ -138,9 +136,8 @@ static void modify_fill_color(const GreasePencilOpacityModifierData &omd,
   /* Fill color opacity per stroke. */
   bke::SpanAttributeWriter<float> fill_opacities = attributes.lookup_or_add_for_write_span<float>(
       "fill_opacity", bke::AttrDomain::Curve);
-  const StringRef vgroup_name = omd.influence.vertex_group_name;
-  const VArray<float> vgroup_weights =
-      attributes.lookup_or_default<float>(vgroup_name, bke::AttrDomain::Point, 1.0f).varray;
+  const VArray<float> vgroup_weights = modifier::greasepencil::get_influence_vertex_weights(
+      curves, omd.influence);
 
   curves_mask.foreach_index(GrainSize(512), [&](int64_t curve_i) {
     if (use_vgroup_opacity) {

source/blender/modifiers/intern/MOD_grease_pencil_tint.cc

@@ -174,8 +174,8 @@ static void modify_stroke_color(Object &ob,
   bke::AttributeAccessor attributes = curves.attributes();
   const VArray<int> stroke_materials = *attributes.lookup_or_default<int>(
       "material_index", bke::AttrDomain::Curve, 0);
-  const VArray<float> vgroup_weights = *attributes.lookup_or_default<float>(
-      tmd.influence.vertex_group_name, bke::AttrDomain::Point, 1.0f);
+  const VArray<float> vgroup_weights = modifier::greasepencil::get_influence_vertex_weights(
+      curves, tmd.influence);
 
   /* Common input color and base factor calculation. */
   auto get_material_color = [&](const int64_t curve_i) {
@@ -261,8 +261,8 @@ static void modify_fill_color(Object &ob,
                                                                bke::AttrDomain::Curve);
   const VArray<int> stroke_materials = *attributes.lookup_or_default<int>(
       "material_index", bke::AttrDomain::Curve, 0);
-  const VArray<float> vgroup_weights = *attributes.lookup_or_default<float>(
-      tmd.influence.vertex_group_name, bke::AttrDomain::Point, 1.0f);
+  const VArray<float> vgroup_weights = modifier::greasepencil::get_influence_vertex_weights(
+      curves, tmd.influence);
 
   /* Common input color and base factor calculation. */
   auto get_material_color = [&](const int64_t curve_i) {

source/blender/modifiers/intern/MOD_grease_pencil_util.cc

@@ -309,6 +309,18 @@ IndexMask get_filtered_stroke_mask(const Object *ob,
       memory);
 }
 
+VArray<float> get_influence_vertex_weights(const bke::CurvesGeometry &curves,
+                                           const GreasePencilModifierInfluenceData &influence_data)
+{
+  if (influence_data.vertex_group_name[0] == '\0') {
+    /* If vertex group is not set, use full weight for all vertices. */
+    return VArray<float>::ForSingle(1.0f, curves.point_num);
+  }
+  /* Vertex group weights, with zero weight as fallback. */
+  return *curves.attributes().lookup_or_default<float>(
+      influence_data.vertex_group_name, bke::AttrDomain::Point, 0.0f);
+}
+
 Vector<bke::greasepencil::Drawing *> get_drawings_for_write(GreasePencil &grease_pencil,
                                                             const IndexMask &layer_mask,
                                                             const int frame)

source/blender/modifiers/intern/MOD_grease_pencil_util.hh

@@ -59,6 +59,9 @@ IndexMask get_filtered_stroke_mask(const Object *ob,
                                    const GreasePencilModifierInfluenceData &influence_data,
                                    IndexMaskMemory &memory);
 
+VArray<float> get_influence_vertex_weights(
+    const bke::CurvesGeometry &curves, const GreasePencilModifierInfluenceData &influence_data);
+
 Vector<bke::greasepencil::Drawing *> get_drawings_for_write(GreasePencil &grease_pencil,
                                                             const IndexMask &layer_mask,
                                                             int frame);