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, 4 insertions, 3 deletions

diff --git a/source/blender/gpu/intern/gpu_buffers.c b/source/blender/gpu/intern/gpu_buffers.c
index 951a7102716..a87c7ea4809 100644
--- a/source/blender/gpu/intern/gpu_buffers.c
+++ b/source/blender/gpu/intern/gpu_buffers.c
@@ -233,8 +233,10 @@ void GPU_pbvh_mesh_buffers_update(PBVHGPUFormat *vbo_id,
   GPUAttrRef vcol_refs[MAX_GPU_ATTR];
   GPUAttrRef cd_uvs[MAX_GPU_ATTR];
 
-  const bool *hide_vert = (bool *)CustomData_get_layer_named(
+  const bool *hide_vert = (const bool *)CustomData_get_layer_named(
       &mesh->vdata, CD_PROP_BOOL, ".hide_vert");
+  const int *material_indices = (const int *)CustomData_get_layer_named(
+      &mesh->pdata, CD_PROP_INT32, "material_index");
 
   const CustomDataLayer *actcol = BKE_id_attributes_active_color_get(&mesh->id);
   eAttrDomain actcol_domain = actcol ? BKE_id_attribute_domain(&mesh->id, actcol) :
@@ -449,8 +451,7 @@ void GPU_pbvh_mesh_buffers_update(PBVHGPUFormat *vbo_id,
 
   /* Get material index from the first face of this buffer. */
   const MLoopTri *lt = &buffers->looptri[buffers->face_indices[0]];
-  const MPoly *mp = &buffers->mpoly[lt->poly];
-  buffers->material_index = mp->mat_nr;
+  buffers->material_index = material_indices ? material_indices[lt->poly] : 0;
 
   buffers->show_overlay = !empty_mask || !default_face_set;
   buffers->mvert = mvert;