Mesh: Move material indices to a generic attribute

This patch moves material indices from the mesh `MPoly` struct to a
generic integer attribute. The builtin material index was already
exposed in geometry nodes, but this makes it a "proper" attribute
accessible with Python and visible in the "Attributes" panel.

The goals of the refactor are code simplification and memory and
performance improvements, mainly because the attribute doesn't have
to be stored and processed if there are no materials. However, until
4.0, material indices will still be read and written in the old
format, meaning there may be a temporary increase in memory usage.

Further notes:
* Completely removing the `MPoly.mat_nr` after 4.0 may require
  changes to DNA or introducing a new `MPoly` type.
* Geometry nodes regression tests didn't look at material indices,
  so the change reveals a bug in the realize instances node that I fixed.
* Access to material indices from the RNA `MeshPolygon` type is slower
  with this patch. The `material_index` attribute can be used instead.
* Cycles is changed to read from the attribute instead.
* BMesh isn't changed in this patch. Theoretically it could be though,
  to save 2 bytes per face when less than two materials are used.
* Eventually we could use a 16 bit integer attribute type instead.

Ref T95967

Differential Revision: https://developer.blender.org/D15675

1 files changed, 10 insertions, 0 deletions

diff --git a/source/blender/blenkernel/BKE_mesh_legacy_convert.h b/source/blender/blenkernel/BKE_mesh_legacy_convert.h
index bbc61d5af5e..11ee86c62a7 100644
--- a/source/blender/blenkernel/BKE_mesh_legacy_convert.h
+++ b/source/blender/blenkernel/BKE_mesh_legacy_convert.h
@@ -28,6 +28,16 @@ void BKE_mesh_legacy_convert_hide_layers_to_flags(struct Mesh *mesh);
 void BKE_mesh_legacy_convert_flags_to_hide_layers(struct Mesh *mesh);
 
 /**
+ * Move material indices from a generic attribute to #MPoly.
+ */
+void BKE_mesh_legacy_convert_material_indices_to_mpoly(struct Mesh *mesh);
+/**
+ * Move material indices from the #MPoly struct to a generic attributes.
+ * Only add the attribute when the indices are not all zero.
+ */
+void BKE_mesh_legacy_convert_mpoly_to_material_indices(struct Mesh *mesh);
+
+/**
  * Recreate #MFace Tessellation.
  *
  * \note This doesn't use multi-threading like #BKE_mesh_recalc_looptri since