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

13 files changed, 111 insertions, 70 deletions

diff --git a/source/blender/io/alembic/exporter/abc_writer_mesh.cc b/source/blender/io/alembic/exporter/abc_writer_mesh.cc
index 06c511db326..83f970d3965 100644
--- a/source/blender/io/alembic/exporter/abc_writer_mesh.cc
+++ b/source/blender/io/alembic/exporter/abc_writer_mesh.cc
@@ -12,6 +12,7 @@
 #include "BLI_math_vector.h"
 
 #include "BKE_attribute.h"
+#include "BKE_attribute.hh"
 #include "BKE_customdata.h"
 #include "BKE_lib_id.h"
 #include "BKE_material.h"
@@ -390,12 +391,12 @@ void ABCGenericMeshWriter::get_geo_groups(Object *object,
                                           struct Mesh *mesh,
                                           std::map<std::string, std::vector<int32_t>> &geo_groups)
 {
-  const int num_poly = mesh->totpoly;
-  MPoly *polygons = mesh->mpoly;
+  const bke::AttributeAccessor attributes = bke::mesh_attributes(*mesh);
+  const VArraySpan<int> material_indices = attributes.lookup_or_default<int>(
+      "material_index", ATTR_DOMAIN_FACE, 0);
 
-  for (int i = 0; i < num_poly; i++) {
-    MPoly &current_poly = polygons[i];
-    short mnr = current_poly.mat_nr;
+  for (const int i : material_indices.index_range()) {
+    short mnr = material_indices[i];
 
     Material *mat = BKE_object_material_get(object, mnr + 1);
 
diff --git a/source/blender/io/alembic/intern/abc_reader_mesh.cc b/source/blender/io/alembic/intern/abc_reader_mesh.cc
index e18770f2bef..16e5ee968a3 100644
--- a/source/blender/io/alembic/intern/abc_reader_mesh.cc
+++ b/source/blender/io/alembic/intern/abc_reader_mesh.cc
@@ -25,7 +25,7 @@
 #include "BLI_listbase.h"
 #include "BLI_math_geom.h"
 
-#include "BKE_attribute.h"
+#include "BKE_attribute.hh"
 #include "BKE_main.h"
 #include "BKE_material.h"
 #include "BKE_mesh.h"
@@ -766,7 +766,11 @@ Mesh *AbcMeshReader::read_mesh(Mesh *existing_mesh,
     size_t num_polys = new_mesh->totpoly;
     if (num_polys > 0) {
       std::map<std::string, int> mat_map;
-      assign_facesets_to_mpoly(sample_sel, new_mesh->mpoly, num_polys, mat_map);
+      bke::MutableAttributeAccessor attributes = bke::mesh_attributes_for_write(*new_mesh);
+      bke::SpanAttributeWriter<int> material_indices =
+          attributes.lookup_or_add_for_write_only_span<int>("material_index", ATTR_DOMAIN_FACE);
+      assign_facesets_to_material_indices(sample_sel, material_indices.span, mat_map);
+      material_indices.finish();
     }
 
     return new_mesh;
@@ -775,10 +779,9 @@ Mesh *AbcMeshReader::read_mesh(Mesh *existing_mesh,
   return existing_mesh;
 }
 
-void AbcMeshReader::assign_facesets_to_mpoly(const ISampleSelector &sample_sel,
-                                             MPoly *mpoly,
-                                             int totpoly,
-                                             std::map<std::string, int> &r_mat_map)
+void AbcMeshReader::assign_facesets_to_material_indices(const ISampleSelector &sample_sel,
+                                                        MutableSpan<int> material_indices,
+                                                        std::map<std::string, int> &r_mat_map)
 {
   std::vector<std::string> face_sets;
   m_schema.getFaceSetNames(face_sets);
@@ -811,13 +814,12 @@ void AbcMeshReader::assign_facesets_to_mpoly(const ISampleSelector &sample_sel,
     for (size_t l = 0; l < num_group_faces; l++) {
       size_t pos = (*group_faces)[l];
 
-      if (pos >= totpoly) {
+      if (pos >= material_indices.size()) {
         std::cerr << "Faceset overflow on " << faceset.getName() << '\n';
         break;
       }
 
-      MPoly &poly = mpoly[pos];
-      poly.mat_nr = assigned_mat - 1;
+      material_indices[pos] = assigned_mat - 1;
     }
   }
 }
@@ -825,7 +827,11 @@ void AbcMeshReader::assign_facesets_to_mpoly(const ISampleSelector &sample_sel,
 void AbcMeshReader::readFaceSetsSample(Main *bmain, Mesh *mesh, const ISampleSelector &sample_sel)
 {
   std::map<std::string, int> mat_map;
-  assign_facesets_to_mpoly(sample_sel, mesh->mpoly, mesh->totpoly, mat_map);
+  bke::MutableAttributeAccessor attributes = bke::mesh_attributes_for_write(*mesh);
+  bke::SpanAttributeWriter<int> material_indices =
+      attributes.lookup_or_add_for_write_only_span<int>("material_index", ATTR_DOMAIN_FACE);
+  assign_facesets_to_material_indices(sample_sel, material_indices.span, mat_map);
+  material_indices.finish();
   utils::assign_materials(bmain, m_object, mat_map);
 }
 
diff --git a/source/blender/io/alembic/intern/abc_reader_mesh.h b/source/blender/io/alembic/intern/abc_reader_mesh.h
index f97525297b7..151f4d82226 100644
--- a/source/blender/io/alembic/intern/abc_reader_mesh.h
+++ b/source/blender/io/alembic/intern/abc_reader_mesh.h
@@ -5,6 +5,8 @@
  * \ingroup balembic
  */
 
+#include "BLI_span.hh"
+
 #include "abc_customdata.h"
 #include "abc_reader_object.h"
 
@@ -38,10 +40,9 @@ class AbcMeshReader final : public AbcObjectReader {
                           Mesh *mesh,
                           const Alembic::AbcGeom::ISampleSelector &sample_sel);
 
-  void assign_facesets_to_mpoly(const Alembic::Abc::ISampleSelector &sample_sel,
-                                MPoly *mpoly,
-                                int totpoly,
-                                std::map<std::string, int> &r_mat_map);
+  void assign_facesets_to_material_indices(const Alembic::Abc::ISampleSelector &sample_sel,
+                                           MutableSpan<int> material_indices,
+                                           std::map<std::string, int> &r_mat_map);
 };
 
 class AbcSubDReader final : public AbcObjectReader {
diff --git a/source/blender/io/collada/GeometryExporter.cpp b/source/blender/io/collada/GeometryExporter.cpp
index 7e2a24aeb41..1a3a68923e3 100644
--- a/source/blender/io/collada/GeometryExporter.cpp
+++ b/source/blender/io/collada/GeometryExporter.cpp
@@ -17,6 +17,7 @@
 
 #include "BLI_utildefines.h"
 
+#include "BKE_attribute.hh"
 #include "BKE_customdata.h"
 #include "BKE_global.h"
 #include "BKE_lib_id.h"
@@ -284,15 +285,18 @@ static bool collect_vertex_counts_per_poly(Mesh *me,
                                            int material_index,
                                            std::vector<unsigned long> &vcount_list)
 {
+  const blender::bke::AttributeAccessor attributes = blender::bke::mesh_attributes(*me);
+  const blender::VArray<int> material_indices = attributes.lookup_or_default<int>(
+      "material_index", ATTR_DOMAIN_FACE, 0);
   MPoly *mpolys = me->mpoly;
   int totpolys = me->totpoly;
   bool is_triangulated = true;
 
   int i;
-  /* Expecting that p->mat_nr is always 0 if the mesh has no materials assigned */
+  /* Expecting that the material index is always 0 if the mesh has no materials assigned */
   for (i = 0; i < totpolys; i++) {
-    MPoly *p = &mpolys[i];
-    if (p->mat_nr == material_index) {
+    if (material_indices[i] == material_index) {
+      MPoly *p = &mpolys[i];
       int vertex_count = p->totloop;
       vcount_list.push_back(vertex_count);
       if (vertex_count != 3) {
@@ -397,13 +401,17 @@ void GeometryExporter::create_mesh_primitive_list(short material_index,
   /* performs the actual writing */
   prepareToAppendValues(is_triangulated, *primitive_list, vcount_list);
 
+  const blender::bke::AttributeAccessor attributes = blender::bke::mesh_attributes(*me);
+  const blender::VArray<int> material_indices = attributes.lookup_or_default<int>(
+      "material_index", ATTR_DOMAIN_FACE, 0);
+
   /* <p> */
   int texindex = 0;
   for (int i = 0; i < totpolys; i++) {
     MPoly *p = &mpolys[i];
     int loop_count = p->totloop;
 
-    if (p->mat_nr == material_index) {
+    if (material_indices[i] == material_index) {
       MLoop *l = &mloops[p->loopstart];
       BCPolygonNormalsIndices normal_indices = norind[i];
 
diff --git a/source/blender/io/collada/MeshImporter.cpp b/source/blender/io/collada/MeshImporter.cpp
index 34792fd6bb4..fab53908d5a 100644
--- a/source/blender/io/collada/MeshImporter.cpp
+++ b/source/blender/io/collada/MeshImporter.cpp
@@ -615,6 +615,9 @@ void MeshImporter::read_polys(COLLADAFW::Mesh *collada_mesh, Mesh *me)
 
   MaterialIdPrimitiveArrayMap mat_prim_map;
 
+  int *material_indices = (int *)CustomData_add_layer_named(
+      &me->pdata, CD_PROP_INT32, CD_SET_DEFAULT, nullptr, me->totpoly, "material_index");
+
   COLLADAFW::MeshPrimitiveArray &prim_arr = collada_mesh->getMeshPrimitives();
   COLLADAFW::MeshVertexData &nor = collada_mesh->getNormals();
 
@@ -633,7 +636,7 @@ void MeshImporter::read_polys(COLLADAFW::Mesh *collada_mesh, Mesh *me)
     int collada_meshtype = mp->getPrimitiveType();
 
     /* since we cannot set mpoly->mat_nr here, we store a portion of me->mpoly in Primitive */
-    Primitive prim = {mpoly, 0};
+    Primitive prim = {mpoly, material_indices, 0};
 
     /* If MeshPrimitive is TRIANGLE_FANS we split it into triangles
      * The first triangle-fan vertex will be the first vertex in every triangle
@@ -663,6 +666,9 @@ void MeshImporter::read_polys(COLLADAFW::Mesh *collada_mesh, Mesh *me)
           }
 
           mpoly++;
+          if (material_indices) {
+            material_indices++;
+          }
           mloop += 3;
           loop_index += 3;
           prim.totpoly++;
@@ -1007,10 +1013,9 @@ void MeshImporter::assign_material_to_geom(
 
     for (it = prims.begin(); it != prims.end(); it++) {
       Primitive &prim = *it;
-      MPoly *mpoly = prim.mpoly;
 
-      for (int i = 0; i < prim.totpoly; i++, mpoly++) {
-        mpoly->mat_nr = mat_index;
+      for (int i = 0; i < prim.totpoly; i++) {
+        prim.material_indices[i] = mat_index;
       }
     }
   }
diff --git a/source/blender/io/collada/MeshImporter.h b/source/blender/io/collada/MeshImporter.h
index 92b387a4bfe..1def84e8f99 100644
--- a/source/blender/io/collada/MeshImporter.h
+++ b/source/blender/io/collada/MeshImporter.h
@@ -80,6 +80,7 @@ class MeshImporter : public MeshImporterBase {
    * (<triangles>, <polylist>, etc.) */
   struct Primitive {
     MPoly *mpoly;
+    int *material_indices;
     unsigned int totpoly;
   };
   typedef std::map<COLLADAFW::MaterialId, std::vector<Primitive>> MaterialIdPrimitiveArrayMap;
diff --git a/source/blender/io/usd/intern/usd_reader_mesh.cc b/source/blender/io/usd/intern/usd_reader_mesh.cc
index 94ff24421e2..89a98097780 100644
--- a/source/blender/io/usd/intern/usd_reader_mesh.cc
+++ b/source/blender/io/usd/intern/usd_reader_mesh.cc
@@ -6,6 +6,7 @@
 #include "usd_reader_mesh.h"
 #include "usd_reader_material.h"
 
+#include "BKE_attribute.hh"
 #include "BKE_customdata.h"
 #include "BKE_main.h"
 #include "BKE_material.h"
@@ -319,7 +320,6 @@ void USDMeshReader::read_mpolys(Mesh *mesh)
     MPoly &poly = mpolys[i];
     poly.loopstart = loop_index;
     poly.totloop = face_size;
-    poly.mat_nr = 0;
 
     /* Polygons are always assumed to be smooth-shaded. If the mesh should be flat-shaded,
      * this is encoded in custom loop normals. */
@@ -735,10 +735,9 @@ void USDMeshReader::read_mesh_sample(ImportSettings *settings,
   }
 }
 
-void USDMeshReader::assign_facesets_to_mpoly(double motionSampleTime,
-                                             MPoly *mpoly,
-                                             const int /* totpoly */,
-                                             std::map<pxr::SdfPath, int> *r_mat_map)
+void USDMeshReader::assign_facesets_to_material_indices(double motionSampleTime,
+                                                        MutableSpan<int> material_indices,
+                                                        std::map<pxr::SdfPath, int> *r_mat_map)
 {
   if (r_mat_map == nullptr) {
     return;
@@ -778,9 +777,8 @@ void USDMeshReader::assign_facesets_to_mpoly(double motionSampleTime,
       pxr::VtIntArray indices;
       indicesAttribute.Get(&indices, motionSampleTime);
 
-      for (int i = 0; i < indices.size(); i++) {
-        MPoly &poly = mpoly[indices[i]];
-        poly.mat_nr = mat_idx;
+      for (const int i : indices) {
+        material_indices[i] = mat_idx;
       }
     }
   }
@@ -805,7 +803,12 @@ void USDMeshReader::readFaceSetsSample(Main *bmain, Mesh *mesh, const double mot
   }
 
   std::map<pxr::SdfPath, int> mat_map;
-  assign_facesets_to_mpoly(motionSampleTime, mesh->mpoly, mesh->totpoly, &mat_map);
+
+  bke::MutableAttributeAccessor attributes = bke::mesh_attributes_for_write(*mesh);
+  bke::SpanAttributeWriter<int> material_indices =
+      attributes.lookup_or_add_for_write_only_span<int>("material_index", ATTR_DOMAIN_FACE);
+  this->assign_facesets_to_material_indices(motionSampleTime, material_indices.span, &mat_map);
+  material_indices.finish();
   /* Build material name map if it's not built yet. */
   if (this->settings_->mat_name_to_mat.empty()) {
     utils::build_mat_map(bmain, &this->settings_->mat_name_to_mat);
@@ -911,7 +914,11 @@ Mesh *USDMeshReader::read_mesh(Mesh *existing_mesh,
     size_t num_polys = active_mesh->totpoly;
     if (num_polys > 0 && import_params_.import_materials) {
       std::map<pxr::SdfPath, int> mat_map;
-      assign_facesets_to_mpoly(motionSampleTime, active_mesh->mpoly, num_polys, &mat_map);
+      bke::MutableAttributeAccessor attributes = bke::mesh_attributes_for_write(*active_mesh);
+      bke::SpanAttributeWriter<int> material_indices =
+          attributes.lookup_or_add_for_write_only_span<int>("material_index", ATTR_DOMAIN_FACE);
+      assign_facesets_to_material_indices(motionSampleTime, material_indices.span, &mat_map);
+      material_indices.finish();
     }
   }
 
diff --git a/source/blender/io/usd/intern/usd_reader_mesh.h b/source/blender/io/usd/intern/usd_reader_mesh.h
index 5e33ce8b5e8..9a3b0565668 100644
--- a/source/blender/io/usd/intern/usd_reader_mesh.h
+++ b/source/blender/io/usd/intern/usd_reader_mesh.h
@@ -3,6 +3,8 @@
  * Modifications Copyright 2021 Tangent Animation and. NVIDIA Corporation. All rights reserved. */
 #pragma once
 
+#include "BLI_span.hh"
+
 #include "usd.h"
 #include "usd_reader_geom.h"
 
@@ -61,10 +63,9 @@ class USDMeshReader : public USDGeomReader {
   /** Set USD uniform (per-face) normals as Blender loop normals. */
   void process_normals_uniform(Mesh *mesh);
   void readFaceSetsSample(Main *bmain, Mesh *mesh, double motionSampleTime);
-  void assign_facesets_to_mpoly(double motionSampleTime,
-                                struct MPoly *mpoly,
-                                int totpoly,
-                                std::map<pxr::SdfPath, int> *r_mat_map);
+  void assign_facesets_to_material_indices(double motionSampleTime,
+                                           MutableSpan<int> material_indices,
+                                           std::map<pxr::SdfPath, int> *r_mat_map);
 
   void read_mpolys(Mesh *mesh);
   void read_uvs(Mesh *mesh, double motionSampleTime, bool load_uvs = false);
diff --git a/source/blender/io/usd/intern/usd_writer_mesh.cc b/source/blender/io/usd/intern/usd_writer_mesh.cc
index b76f74cfd3d..ade75fdb365 100644
--- a/source/blender/io/usd/intern/usd_writer_mesh.cc
+++ b/source/blender/io/usd/intern/usd_writer_mesh.cc
@@ -11,6 +11,7 @@
 #include "BLI_math_vector.h"
 
 #include "BKE_attribute.h"
+#include "BKE_attribute.hh"
 #include "BKE_customdata.h"
 #include "BKE_lib_id.h"
 #include "BKE_material.h"
@@ -255,7 +256,15 @@ static void get_loops_polys(const Mesh *mesh, USDMeshData &usd_mesh_data)
 {
   /* Only construct face groups (a.k.a. geometry subsets) when we need them for material
    * assignments. */
-  bool construct_face_groups = mesh->totcol > 1;
+  const bke::AttributeAccessor attributes = bke::mesh_attributes(*mesh);
+  const VArray<int> material_indices = attributes.lookup_or_default<int>(
+      "material_index", ATTR_DOMAIN_FACE, 0);
+  if (!material_indices.is_single() && mesh->totcol > 1) {
+    const VArraySpan<int> indices_span(material_indices);
+    for (const int i : indices_span.index_range()) {
+      usd_mesh_data.face_groups[indices_span[i]].push_back(i);
+    }
+  }
 
   usd_mesh_data.face_vertex_counts.reserve(mesh->totpoly);
   usd_mesh_data.face_indices.reserve(mesh->totloop);
@@ -268,10 +277,6 @@ static void get_loops_polys(const Mesh *mesh, USDMeshData &usd_mesh_data)
     for (int j = 0; j < mpoly->totloop; ++j, ++loop) {
       usd_mesh_data.face_indices.push_back(loop->v);
     }
-
-    if (construct_face_groups) {
-      usd_mesh_data.face_groups[mpoly->mat_nr].push_back(i);
-    }
   }
 }
 
diff --git a/source/blender/io/wavefront_obj/exporter/obj_export_file_writer.cc b/source/blender/io/wavefront_obj/exporter/obj_export_file_writer.cc
index 9bcc061caf7..902f801ee5b 100644
--- a/source/blender/io/wavefront_obj/exporter/obj_export_file_writer.cc
+++ b/source/blender/io/wavefront_obj/exporter/obj_export_file_writer.cc
@@ -374,10 +374,14 @@ void OBJWriter::write_poly_elements(FormatHandler &fh,
       }
     }
 
+    const bke::AttributeAccessor attributes = bke::mesh_attributes(*obj_mesh_data.get_mesh());
+    const VArray<int> material_indices = attributes.lookup_or_default<int>(
+        "material_index", ATTR_DOMAIN_FACE, 0);
+
     /* Write material name and material group if different from previous. */
     if (export_params_.export_materials && obj_mesh_data.tot_materials() > 0) {
-      const int16_t prev_mat = idx == 0 ? NEGATIVE_INIT : obj_mesh_data.ith_poly_matnr(prev_i);
-      const int16_t mat = obj_mesh_data.ith_poly_matnr(i);
+      const int16_t prev_mat = idx == 0 ? NEGATIVE_INIT : std::max(0, material_indices[prev_i]);
+      const int16_t mat = std::max(0, material_indices[i]);
       if (mat != prev_mat) {
         if (mat == NOT_FOUND) {
           buf.write_obj_usemtl(MATERIAL_GROUP_DISABLED);
diff --git a/source/blender/io/wavefront_obj/exporter/obj_export_mesh.cc b/source/blender/io/wavefront_obj/exporter/obj_export_mesh.cc
index 815163ad19e..a888c6d11a2 100644
--- a/source/blender/io/wavefront_obj/exporter/obj_export_mesh.cc
+++ b/source/blender/io/wavefront_obj/exporter/obj_export_mesh.cc
@@ -6,6 +6,7 @@
 /* Silence warnings from copying deprecated fields. Needed for an Object copy constructor use. */
 #define DNA_DEPRECATED_ALLOW
 
+#include "BKE_attribute.hh"
 #include "BKE_customdata.h"
 #include "BKE_deform.h"
 #include "BKE_lib_id.h"
@@ -199,16 +200,23 @@ void OBJMesh::calc_smooth_groups(const bool use_bitflags)
 
 void OBJMesh::calc_poly_order()
 {
-  const int tot_polys = tot_polygons();
-  poly_order_.resize(tot_polys);
-  for (int i = 0; i < tot_polys; ++i) {
+  const bke::AttributeAccessor attributes = bke::mesh_attributes(*export_mesh_eval_);
+  const VArray<int> material_indices = attributes.lookup_or_default<int>(
+      "material_index", ATTR_DOMAIN_FACE, 0);
+  if (material_indices.is_single() && material_indices.get_internal_single() == 0) {
+    return;
+  }
+  const VArraySpan<int> material_indices_span(material_indices);
+
+  poly_order_.resize(material_indices_span.size());
+  for (const int i : material_indices_span.index_range()) {
     poly_order_[i] = i;
   }
-  const MPoly *mpolys = export_mesh_eval_->mpoly;
+
   /* Sort polygons by their material index. */
   blender::parallel_sort(poly_order_.begin(), poly_order_.end(), [&](int a, int b) {
-    int mat_a = mpolys[a].mat_nr;
-    int mat_b = mpolys[b].mat_nr;
+    int mat_a = material_indices_span[a];
+    int mat_b = material_indices_span[b];
     if (mat_a != mat_b) {
       return mat_a < mat_b;
     }
@@ -234,13 +242,6 @@ bool OBJMesh::is_ith_poly_smooth(const int poly_index) const
   return export_mesh_eval_->mpoly[poly_index].flag & ME_SMOOTH;
 }
 
-int16_t OBJMesh::ith_poly_matnr(const int poly_index) const
-{
-  BLI_assert(poly_index < export_mesh_eval_->totpoly);
-  const int16_t r_mat_nr = export_mesh_eval_->mpoly[poly_index].mat_nr;
-  return r_mat_nr >= 0 ? r_mat_nr : NOT_FOUND;
-}
-
 const char *OBJMesh::get_object_name() const
 {
   return export_object_eval_.id.name + 2;
diff --git a/source/blender/io/wavefront_obj/exporter/obj_export_mesh.hh b/source/blender/io/wavefront_obj/exporter/obj_export_mesh.hh
index ee2e6227700..db29f5651ed 100644
--- a/source/blender/io/wavefront_obj/exporter/obj_export_mesh.hh
+++ b/source/blender/io/wavefront_obj/exporter/obj_export_mesh.hh
@@ -130,11 +130,6 @@ class OBJMesh : NonCopyable {
    * Return mat_nr-th material of the object. The given index should be zero-based.
    */
   const Material *get_object_material(int16_t mat_nr) const;
-  /**
-   * Returns a zero-based index of a polygon's material indexing into
-   * the Object's material slots.
-   */
-  int16_t ith_poly_matnr(int poly_index) const;
 
   void ensure_mesh_normals() const;
   void ensure_mesh_edges() const;
diff --git a/source/blender/io/wavefront_obj/importer/obj_import_mesh.cc b/source/blender/io/wavefront_obj/importer/obj_import_mesh.cc
index 2a0676b72ff..7f7fda8d8f1 100644
--- a/source/blender/io/wavefront_obj/importer/obj_import_mesh.cc
+++ b/source/blender/io/wavefront_obj/importer/obj_import_mesh.cc
@@ -8,7 +8,7 @@
 #include "DNA_mesh_types.h"
 #include "DNA_scene_types.h"
 
-#include "BKE_attribute.h"
+#include "BKE_attribute.hh"
 #include "BKE_customdata.h"
 #include "BKE_deform.h"
 #include "BKE_material.h"
@@ -184,6 +184,10 @@ void MeshFromGeometry::create_polys_loops(Mesh *mesh, bool use_vertex_groups)
         CustomData_add_layer(&mesh->vdata, CD_MDEFORMVERT, CD_SET_DEFAULT, nullptr, total_verts));
   }
 
+  bke::SpanAttributeWriter<int> material_indices =
+      bke::mesh_attributes_for_write(*mesh).lookup_or_add_for_write_only_span<int>(
+          "material_index", ATTR_DOMAIN_FACE);
+
   const int64_t tot_face_elems{mesh->totpoly};
   int tot_loop_idx = 0;
 
@@ -201,11 +205,11 @@ void MeshFromGeometry::create_polys_loops(Mesh *mesh, bool use_vertex_groups)
     if (curr_face.shaded_smooth) {
       mpoly.flag |= ME_SMOOTH;
     }
-    mpoly.mat_nr = curr_face.material_index;
+    material_indices.span[poly_idx] = curr_face.material_index;
     /* Importing obj files without any materials would result in negative indices, which is not
      * supported. */
-    if (mpoly.mat_nr < 0) {
-      mpoly.mat_nr = 0;
+    if (material_indices.span[poly_idx] < 0) {
+      material_indices.span[poly_idx] = 0;
     }
 
     for (int idx = 0; idx < curr_face.corner_count_; ++idx) {
@@ -223,6 +227,8 @@ void MeshFromGeometry::create_polys_loops(Mesh *mesh, bool use_vertex_groups)
       dw->weight = 1.0f;
     }
   }
+
+  material_indices.finish();
 }
 
 void MeshFromGeometry::create_vertex_groups(Object *obj)