Now users could decide to interpolate the initial radius or not. And I moved the code of initialization from ```curves_sculpt_add.cc``` and ```curves_sculpt_density.cc``` into ```add_curves_on_mesh.cc```

@@ -521,7 +521,6 @@ Brush *BKE_brush_add(Main *bmain, const char *name, const eObjectMode ob_mode)
   if (ob_mode == OB_MODE_SCULPT_CURVES) {
     BKE_brush_init_curves_sculpt_settings(brush);
   }
-

@@ -285,6 +285,14 @@ struct DensityAddOperationExecutor {
       selection.finish();
     }
 
+    if (bke::GSpanAttributeWriter radius = attributes.lookup_for_write_span("radius")) {

@@ -286,2 +286,4 @@
     }
 
+    if (bke::GSpanAttributeWriter radius = attributes.lookup_for_write_span("radius")) {
+      blender::GMutableSpan radius_span = radius.span.slice(

@@ -200,6 +200,9 @@ class CurvesGeometry : public ::CurvesGeometry {
   Span<float3> positions() const;
   MutableSpan<float3> positions_for_write();
 
+  Span<float> radiuses() const;

@@ -236,13 +236,99 @@ static void interpolate_position_with_interpolation(CurvesGeometry &curves,
   });
 }
 
+static void interpolate_radius_without_interpolation(CurvesGeometry &curves,

@@ -239,0 +267,4 @@
+  bke::SpanAttributeWriter radius_attr =
+      curves.attributes_for_write().lookup_for_write_span<float>("radius");
+  MutableSpan<float3> positions_cu = curves.positions_for_write();
+  MutableSpan<float> radiuses_cu = radius_attr.span;

@@ -239,0 +279,4 @@
+      const float root_cu = root_radiuses_cu[i];
+
+      if (neighbors.is_empty()) {
+        /* If there are no neighbors, just using uniform radius. */

@@ -350,0 +439,4 @@
+  const bool has_radius_attr = !radius_attr.span.is_empty();  // check if the attribute exists
+  Array<float> root_radius_cu(added_curves_num);
+  if (inputs.interpolate_radius && has_radius_attr) {
+    interpolate_from_neighbors<float>(

@@ -239,0 +245,4 @@
+  bke::SpanAttributeWriter radius_attr =
+      curves.attributes_for_write().lookup_or_add_for_write_span<float>("radius",
+                                                                        bke::AttrDomain::Point);
+  threading::parallel_for(IndexRange(added_curves_num), 256, [&](const IndexRange range) {

@@ -239,0 +287,4 @@
+        continue;
+      }
+
+      radii_cu.slice(points).fill(0);

@@ -239,0 +292,4 @@
+      for (const NeighborCurve &neighbor : neighbors) {
+        const int neighbor_curve_i = neighbor.index;
+        const IndexRange neighbor_points = points_by_curve[neighbor_curve_i];
+        const float3 &neighbor_root_cu = positions_cu[neighbor_points[0]];

@@ -657,2 +657,2 @@
-  BRUSH_CURVES_SCULPT_FLAG_INTERPOLATE_SHAPE = (1 << 3),
-  BRUSH_CURVES_SCULPT_FLAG_INTERPOLATE_POINT_COUNT = (1 << 4),
+  BRUSH_CURVES_SCULPT_FLAG_INTERPOLATE_RADIUS = (1 << 3),
+  BRUSH_CURVES_SCULPT_FLAG_INTERPOLATE_SHAPE = (1 << 4),

@@ -239,0 +240,4 @@
+                                              const IndexRange new_points_range,
+                                              const float radius)
+{
+  const OffsetIndices points_by_curve = curves.points_by_curve();

@@ -239,0 +260,4 @@
+      curves.attributes_for_write().lookup_for_write_span<float>("radius");
+
+  if (!radius_attr) {
+    radius_attr.finish();