Fix T92561: unstable particle distribution with Alembic files

When enabling or disabling a Mesh Sequence Cache modifier of an Object
with a hair particle system, the hair would switch positions. This is
caused because original coordinates in Blender are expected to be
normalized, and toggling the modifier would cause the usage of different
orco layers: one that is normalized, and the other which isn't.

This bug exposes a few related issues:
- if the Alembic file did not have orco data,
`MOD_deform_mesh_eval_get`, used by the particle system modifier, would
add an orco layer without normalization
- `MOD_deform_mesh_eval_get` would also ignore the presence of an orco
layer (e.g. one that could have been read from Alembic)
- if the Alembic file did have orco data, the data would be read
unnormalized

To fix those various issues, original coordinates are normalized when
read from Alembic and unnormalized when written to Alembic; and a new
utility function `BKE_mesh_orco_ensure` is added to add a normalized
orco layer if none exists on the mesh already, this function derives
from the code used in the particle system.

Reviewed By: brecht

Maniphest Tasks: T92561

Differential Revision: https://developer.blender.org/D13306

1 files changed, 4 insertions, 0 deletions

diff --git a/source/blender/blenkernel/BKE_mesh.h b/source/blender/blenkernel/BKE_mesh.h
index be9b84ccd62..135f8128b9a 100644
--- a/source/blender/blenkernel/BKE_mesh.h
+++ b/source/blender/blenkernel/BKE_mesh.h
@@ -136,6 +136,10 @@ bool BKE_mesh_clear_facemap_customdata(struct Mesh *me);
 
 float (*BKE_mesh_orco_verts_get(struct Object *ob))[3];
 void BKE_mesh_orco_verts_transform(struct Mesh *me, float (*orco)[3], int totvert, int invert);
+
+/* Add a CD_ORCO layer to the Mesh if there is none already. */
+void BKE_mesh_orco_ensure(struct Object *ob, struct Mesh *mesh);
+
 int BKE_mesh_mface_index_validate(struct MFace *mface,
                                   struct CustomData *mfdata,
                                   int mfindex,