Curves: new Add brush

This adds a new Add brush for the new curves object type in sculpt mode.
The brush is used to insert new curves (typically hair) on the surface object.

Supported features:
* Add single curve exactly at the cursor position when `Add Amount` is 1.
* Front faces only.
* Independent interpolate shape and interpolate length settings.
* Smooth and flat shading affects curve shape interpolation.
* Spherical and projection brush.

This also adds the `surface_triangle_index` and `surface_triangle_coordinate`
attributes. Those store information about what position on the surface each
added curve is attached to:
* `surface_triangle_index` (`int`): Index of the internal triangle that a curve
  is attached to. `-1` when the curve is not attached to the surface.
* `surface_triangle_coordinate` (`float2`): First two numbers of a barycentric
  coordinate that reference a specific position within the triangle.

Ref T96444.

Differential Revision: https://developer.blender.org/D14340

1 files changed, 18 insertions, 0 deletions

diff --git a/source/blender/blenkernel/BKE_curves.hh b/source/blender/blenkernel/BKE_curves.hh
index ea378c5a0a5..eb4f8f5d5c8 100644
--- a/source/blender/blenkernel/BKE_curves.hh
+++ b/source/blender/blenkernel/BKE_curves.hh
@@ -203,6 +203,24 @@ class CurvesGeometry : public ::CurvesGeometry {
   MutableSpan<float> nurbs_weights();
 
   /**
+   * The index of a triangle (#MLoopTri) that a curve is attached to.
+   * The index is -1, if the curve is not attached.
+   */
+  VArray<int> surface_triangle_indices() const;
+  MutableSpan<int> surface_triangle_indices();
+
+  /**
+   * Barycentric coordinates of the attachment point within a triangle.
+   * Only the first two coordinates are stored. The third coordinate can be derived because the sum
+   * of the three coordinates is 1.
+   *
+   * When the triangle index is -1, this coordinate should be ignored.
+   * The span can be empty, when all triangle indices are -1.
+   */
+  Span<float2> surface_triangle_coords() const;
+  MutableSpan<float2> surface_triangle_coords();
+
+  /**
    * Calculate the largest and smallest position values, only including control points
    * (rather than evaluated points). The existing values of `min` and `max` are taken into account.
    *