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

3 files changed, 35 insertions, 19 deletions

diff --git a/source/blender/modifiers/intern/MOD_screw.c b/source/blender/modifiers/intern/MOD_screw.c
index b2ebf2ffeec..d8b11c0e89e 100644
--- a/source/blender/modifiers/intern/MOD_screw.c
+++ b/source/blender/modifiers/intern/MOD_screw.c
@@ -947,6 +947,9 @@ static Mesh *modifyMesh(ModifierData *md, const ModifierEvalContext *ctx, Mesh *
   /* more of an offset in this case */
   edge_offset = totedge + (totvert * (step_tot - (close ? 0 : 1)));
 
+  const int *src_material_index = BKE_mesh_material_indices(mesh);
+  int *dst_material_index = BKE_mesh_material_indices_for_write(result);
+
   for (i = 0; i < totedge; i++, med_new_firstloop++) {
     const uint step_last = step_tot - (close ? 1 : 2);
     const uint mpoly_index_orig = totpoly ? edge_poly_map[i] : UINT_MAX;
@@ -959,14 +962,14 @@ static Mesh *modifyMesh(ModifierData *md, const ModifierEvalContext *ctx, Mesh *
     };
     const bool has_mloop_orig = mloop_index_orig[0] != UINT_MAX;
 
-    short mat_nr;
+    int mat_nr;
 
     /* for each edge, make a cylinder of quads */
     i1 = med_new_firstloop->v1;
     i2 = med_new_firstloop->v2;
 
     if (has_mpoly_orig) {
-      mat_nr = mpoly_orig[mpoly_index_orig].mat_nr;
+      mat_nr = src_material_index == NULL ? 0 : src_material_index[mpoly_index_orig];
     }
     else {
       mat_nr = 0;
@@ -992,8 +995,8 @@ static Mesh *modifyMesh(ModifierData *md, const ModifierEvalContext *ctx, Mesh *
       }
       else {
         origindex[mpoly_index] = ORIGINDEX_NONE;
+        dst_material_index[mpoly_index] = mat_nr;
         mp_new->flag = mpoly_flag;
-        mp_new->mat_nr = mat_nr;
       }
       mp_new->loopstart = mpoly_index * 4;
       mp_new->totloop = 4;
diff --git a/source/blender/modifiers/intern/MOD_solidify_extrude.c b/source/blender/modifiers/intern/MOD_solidify_extrude.c
index 15e8bd25997..aa8c49ee0b8 100644
--- a/source/blender/modifiers/intern/MOD_solidify_extrude.c
+++ b/source/blender/modifiers/intern/MOD_solidify_extrude.c
@@ -425,6 +425,8 @@ Mesh *MOD_solidify_extrude_modifyMesh(ModifierData *md, const ModifierEvalContex
   } \
   (void)0
 
+  int *dst_material_index = BKE_mesh_material_indices_for_write(result);
+
   /* flip normals */
 
   if (do_shell) {
@@ -462,8 +464,8 @@ Mesh *MOD_solidify_extrude_modifyMesh(ModifierData *md, const ModifierEvalContex
 #endif
 
       if (mat_ofs) {
-        mp->mat_nr += mat_ofs;
-        CLAMP(mp->mat_nr, 0, mat_nr_max);
+        dst_material_index[mp - mpoly] += mat_ofs;
+        CLAMP(dst_material_index[mp - mpoly], 0, mat_nr_max);
       }
 
       e = ml2[0].e;
@@ -1151,8 +1153,8 @@ Mesh *MOD_solidify_extrude_modifyMesh(ModifierData *md, const ModifierEvalContex
 
       /* use the next material index if option enabled */
       if (mat_ofs_rim) {
-        mp->mat_nr += mat_ofs_rim;
-        CLAMP(mp->mat_nr, 0, mat_nr_max);
+        dst_material_index[mp - mpoly] += mat_ofs_rim;
+        CLAMP(dst_material_index[mp - mpoly], 0, mat_nr_max);
       }
       if (crease_outer) {
         /* crease += crease_outer; without wrapping */
diff --git a/source/blender/modifiers/intern/MOD_solidify_nonmanifold.c b/source/blender/modifiers/intern/MOD_solidify_nonmanifold.c
index 9205083e836..29adbd70198 100644
--- a/source/blender/modifiers/intern/MOD_solidify_nonmanifold.c
+++ b/source/blender/modifiers/intern/MOD_solidify_nonmanifold.c
@@ -2108,6 +2108,9 @@ Mesh *MOD_solidify_nonmanifold_modifyMesh(ModifierData *md,
   }
 #endif
 
+  const int *src_material_index = BKE_mesh_material_indices(mesh);
+  int *dst_material_index = BKE_mesh_material_indices_for_write(result);
+
   /* Make boundary edges/faces. */
   {
     gs_ptr = orig_vert_groups_arr;
@@ -2224,7 +2227,7 @@ Mesh *MOD_solidify_nonmanifold_modifyMesh(ModifierData *md,
 
               /* Loop data. */
               int *loops = MEM_malloc_arrayN(j, sizeof(*loops), "loops in solidify");
-              /* The #mat_nr is from consensus. */
+              /* The result material index is from consensus. */
               short most_mat_nr = 0;
               uint most_mat_nr_face = 0;
               uint most_mat_nr_count = 0;
@@ -2235,16 +2238,20 @@ Mesh *MOD_solidify_nonmanifold_modifyMesh(ModifierData *md,
                 for (EdgeGroup *g3 = g2; g3->valid && k < j; g3++) {
                   if ((do_rim && !g3->is_orig_closed) || (do_shell && g3->split)) {
                     /* Check both far ends in terms of faces of an edge group. */
-                    if (g3->edges[0]->faces[0]->face->mat_nr == l) {
+                    if ((src_material_index ? src_material_index[g3->edges[0]->faces[0]->index] :
+                                              0) == l) {
                       face = g3->edges[0]->faces[0]->index;
                       count++;
                     }
                     NewEdgeRef *le = g3->edges[g3->edges_len - 1];
-                    if (le->faces[1] && le->faces[1]->face->mat_nr == l) {
+                    if (le->faces[1] &&
+                        (src_material_index ? src_material_index[le->faces[1]->index] : 0) == l) {
                       face = le->faces[1]->index;
                       count++;
                     }
-                    else if (!le->faces[1] && le->faces[0]->face->mat_nr == l) {
+                    else if (!le->faces[1] &&
+                             (src_material_index ? src_material_index[le->faces[0]->index] : 0) ==
+                                 l) {
                       face = le->faces[0]->index;
                       count++;
                     }
@@ -2264,9 +2271,9 @@ Mesh *MOD_solidify_nonmanifold_modifyMesh(ModifierData *md,
               }
               mpoly[poly_index].loopstart = (int)loop_index;
               mpoly[poly_index].totloop = (int)j;
-              mpoly[poly_index].mat_nr = most_mat_nr +
-                                         (g->is_orig_closed || !do_rim ? 0 : mat_ofs_rim);
-              CLAMP(mpoly[poly_index].mat_nr, 0, mat_nr_max);
+              dst_material_index[poly_index] = most_mat_nr +
+                                               (g->is_orig_closed || !do_rim ? 0 : mat_ofs_rim);
+              CLAMP(dst_material_index[poly_index], 0, mat_nr_max);
               mpoly[poly_index].flag = orig_mpoly[most_mat_nr_face].flag;
               poly_index++;
 
@@ -2334,13 +2341,15 @@ Mesh *MOD_solidify_nonmanifold_modifyMesh(ModifierData *md,
           continue;
         }
 
-        MPoly *face = (*new_edges)->faces[0]->face;
+        const uint orig_face_index = (*new_edges)->faces[0]->index;
+        MPoly *face = &orig_mpoly[orig_face_index];
         CustomData_copy_data(
             &mesh->pdata, &result->pdata, (int)(*new_edges)->faces[0]->index, (int)poly_index, 1);
         mpoly[poly_index].loopstart = (int)loop_index;
         mpoly[poly_index].totloop = 4 - (int)(v1_singularity || v2_singularity);
-        mpoly[poly_index].mat_nr = face->mat_nr + mat_ofs_rim;
-        CLAMP(mpoly[poly_index].mat_nr, 0, mat_nr_max);
+        dst_material_index[poly_index] =
+            (src_material_index ? src_material_index[orig_face_index] : 0) + mat_ofs_rim;
+        CLAMP(dst_material_index[poly_index], 0, mat_nr_max);
         mpoly[poly_index].flag = face->flag;
         poly_index++;
 
@@ -2530,8 +2539,10 @@ Mesh *MOD_solidify_nonmanifold_modifyMesh(ModifierData *md,
           CustomData_copy_data(&mesh->pdata, &result->pdata, (int)(i / 2), (int)poly_index, 1);
           mpoly[poly_index].loopstart = (int)loop_index;
           mpoly[poly_index].totloop = (int)k;
-          mpoly[poly_index].mat_nr = fr->face->mat_nr + (fr->reversed != do_flip ? mat_ofs : 0);
-          CLAMP(mpoly[poly_index].mat_nr, 0, mat_nr_max);
+          dst_material_index[poly_index] = (src_material_index ? src_material_index[fr->index] :
+                                                                 0) +
+                                           (fr->reversed != do_flip ? mat_ofs : 0);
+          CLAMP(dst_material_index[poly_index], 0, mat_nr_max);
           mpoly[poly_index].flag = fr->face->flag;
           if (fr->reversed != do_flip) {
             for (int l = (int)k - 1; l >= 0; l--) {