Curves: support deforming curves on surface

Curves that are attached to a surface can now follow the surface when
it is modified using shape keys or modifiers (but not when the original
surface is deformed in edit or sculpt mode).

The surface is allowed to be changed in any way that keeps uv maps
intact. So deformation is allowed, but also some topology changes like
subdivision.

The following features are added:
* A new `Deform Curves on Surface` node, which deforms curves with
  attachment information based on the surface object and uv map set
  in the properties panel.
* A new `Add Rest Position` checkbox in the shape keys panel. When checked,
  a new `rest_position` vector attribute is added to the mesh before shape
  keys and modifiers are applied. This is necessary to support proper
  deformation of the curves, but can also be used for other purposes.
* The `Add > Curve > Empty Hair` operator now sets up a simple geometry
  nodes setup that deforms the hair. It also makes sure that the rest
  position attribute is added to the surface.
* A new `Object (Attach Curves to Surface)` operator in the `Set Parent To`
  (ctrl+P) menu, which attaches existing curves to the surface and sets the
  surface object as parent.

Limitations:
* Sculpting the procedurally deformed curves will be implemented separately.
* The `Deform Curves on Surface` node is not generic and can only be used
  for one specific purpose currently. We plan to generalize this more in the
  future by adding support by exposing more inputs and/or by turning it into
  a node group.

Differential Revision: https://developer.blender.org/D14864

1 files changed, 2 insertions, 1 deletions

diff --git a/source/blender/blenkernel/BKE_mesh.h b/source/blender/blenkernel/BKE_mesh.h
index ff4f4a8dc9e..366083fee7f 100644
--- a/source/blender/blenkernel/BKE_mesh.h
+++ b/source/blender/blenkernel/BKE_mesh.h
@@ -692,7 +692,8 @@ void BKE_mesh_calc_normals_split(struct Mesh *mesh);
  * to split geometry along sharp edges.
  */
 void BKE_mesh_calc_normals_split_ex(struct Mesh *mesh,
-                                    struct MLoopNorSpaceArray *r_lnors_spacearr);
+                                    struct MLoopNorSpaceArray *r_lnors_spacearr,
+                                    float (*r_corner_normals)[3]);
 
 /**
  * Higher level functions hiding most of the code needed around call to