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

2 files changed, 20 insertions, 9 deletions

diff --git a/source/blender/nodes/geometry/nodes/node_geo_material_selection.cc b/source/blender/nodes/geometry/nodes/node_geo_material_selection.cc
index 31e2092a6d7..9822e0ea0d6 100644
--- a/source/blender/nodes/geometry/nodes/node_geo_material_selection.cc
+++ b/source/blender/nodes/geometry/nodes/node_geo_material_selection.cc
@@ -23,19 +23,30 @@ static void node_declare(NodeDeclarationBuilder &b)
 static void select_mesh_by_material(const Mesh &mesh,
                                     const Material *material,
                                     const IndexMask mask,
-                                    const MutableSpan<bool> r_selection)
+                                    MutableSpan<bool> r_selection)
 {
   BLI_assert(mesh.totpoly >= r_selection.size());
-  Vector<int> material_indices;
+  Vector<int> slots;
   for (const int i : IndexRange(mesh.totcol)) {
     if (mesh.mat[i] == material) {
-      material_indices.append(i);
+      slots.append(i);
     }
   }
+  const AttributeAccessor attributes = bke::mesh_attributes(mesh);
+  const VArray<int> material_indices = attributes.lookup_or_default<int>(
+      "material_index", ATTR_DOMAIN_FACE, 0);
+  if (material != nullptr && material_indices.is_single() &&
+      material_indices.get_internal_single() == 0) {
+    r_selection.fill_indices(mask, false);
+    return;
+  }
+
+  const VArraySpan<int> material_indices_span(material_indices);
+
   threading::parallel_for(mask.index_range(), 1024, [&](IndexRange range) {
     for (const int i : range) {
       const int face_index = mask[i];
-      r_selection[i] = material_indices.contains(mesh.mpoly[face_index].mat_nr);
+      r_selection[i] = slots.contains(material_indices_span[face_index]);
     }
   });
 }
diff --git a/source/blender/nodes/geometry/nodes/node_geo_set_material.cc b/source/blender/nodes/geometry/nodes/node_geo_set_material.cc
index e7b95506068..c6a2f89220a 100644
--- a/source/blender/nodes/geometry/nodes/node_geo_set_material.cc
+++ b/source/blender/nodes/geometry/nodes/node_geo_set_material.cc
@@ -49,11 +49,11 @@ static void assign_material_to_faces(Mesh &mesh, const IndexMask selection, Mate
     BKE_id_material_eval_assign(&mesh.id, new_material_index + 1, material);
   }
 
-  mesh.mpoly = (MPoly *)CustomData_duplicate_referenced_layer(&mesh.pdata, CD_MPOLY, mesh.totpoly);
-  for (const int i : selection) {
-    MPoly &poly = mesh.mpoly[i];
-    poly.mat_nr = new_material_index;
-  }
+  MutableAttributeAccessor attributes = bke::mesh_attributes_for_write(mesh);
+  SpanAttributeWriter<int> material_indices = attributes.lookup_or_add_for_write_span<int>(
+      "material_index", ATTR_DOMAIN_FACE);
+  material_indices.span.fill_indices(selection, new_material_index);
+  material_indices.finish();
 }
 
 static void node_geo_exec(GeoNodeExecParams params)