Subdiv: Use index instead of pointer for grid to face map

Switch from face pointers to indices, with the following benefits:
- Halve memory usage required for array (saves 16 MB with 1 million quads).
- Allow better code reuse with index-based utilities.
- Ease future replacement of `SubdivCCGFace` with mesh's face offsets.
- Allow future replacement of array with `Mesh::corner_to_face_map()`
- Potentially lower memory bandwidth required during multires evaluation
- Simplify retrieval of index in `BKE_subdiv_ccg_grid_to_face_index`.

For clarity, `gridfaces` was renamed to `grid_to_face_map`.

Pull Request: https://projects.blender.org/blender/blender/pulls/111078

source/blender/blenkernel/BKE_pbvh_api.hh

@@ -210,7 +210,7 @@ void BKE_pbvh_build_grids(PBVH *pbvh,
                           CCGElem **grids,
                           int totgrid,
                           CCGKey *key,
-                          void **gridfaces,
+                          blender::Span<int> grid_to_face_map,
                           DMFlagMat *flagmats,
                           unsigned int **grid_hidden,
                           Mesh *me,
@@ -445,7 +445,7 @@ void BKE_pbvh_redraw_BB(PBVH *pbvh, float bb_min[3], float bb_max[3]);
 void BKE_pbvh_get_grid_updates(PBVH *pbvh, bool clear, void ***r_gridfaces, int *r_totface);
 void BKE_pbvh_grids_update(PBVH *pbvh,
                            CCGElem **grids,
-                           void **gridfaces,
+                           blender::Span<int> grid_to_face_map,
                            DMFlagMat *flagmats,
                            unsigned int **grid_hidden,
                            CCGKey *key);

source/blender/blenkernel/BKE_subdiv_ccg.hh

@@ -160,7 +160,7 @@ struct SubdivCCG {
   int num_faces = -1;
   SubdivCCGFace *faces = nullptr;
   /* Indexed by grid index, points to corresponding face from `faces`. */
-  SubdivCCGFace **grid_faces = nullptr;
+  blender::Array<int> grid_to_face_map;
 
   /* Edges which are adjacent to faces.
    * Used for faster grid stitching, in the cost of extra memory.

source/blender/blenkernel/intern/paint.cc

@@ -2215,7 +2215,7 @@ static PBVH *build_pbvh_from_ccg(Object *ob, SubdivCCG *subdiv_ccg)
                        subdiv_ccg->grids,
                        subdiv_ccg->num_grids,
                        &key,
-                       (void **)subdiv_ccg->grid_faces,
+                       subdiv_ccg->grid_faces,
                        subdiv_ccg->grid_flag_mats,
                        subdiv_ccg->grid_hidden,
                        base_mesh,
@@ -2304,7 +2304,7 @@ void BKE_sculpt_bvh_update_from_ccg(PBVH *pbvh, SubdivCCG *subdiv_ccg)
 
   BKE_pbvh_grids_update(pbvh,
                         subdiv_ccg->grids,
-                        (void **)subdiv_ccg->grid_faces,
+                        subdiv_ccg->grid_faces,
                         subdiv_ccg->grid_flag_mats,
                         subdiv_ccg->grid_hidden,
                         &key);

source/blender/blenkernel/intern/pbvh.cc

@@ -914,7 +914,7 @@ void BKE_pbvh_build_grids(PBVH *pbvh,
                           CCGElem **grids,
                           int totgrid,
                           CCGKey *key,
-                          void **gridfaces,
+                          blender::Span<int> grid_to_face_map,
                           DMFlagMat *flagmats,
                           BLI_bitmap **grid_hidden,
                           Mesh *me,
@@ -924,7 +924,7 @@ void BKE_pbvh_build_grids(PBVH *pbvh,
 
   pbvh->header.type = PBVH_GRIDS;
   pbvh->grids = grids;
-  pbvh->gridfaces = gridfaces;
+  pbvh->grid_to_face_map = grid_to_face_map;
   pbvh->grid_flag_mats = flagmats;
   pbvh->totgrid = totgrid;
   pbvh->gridkey = *key;
@@ -1715,10 +1715,11 @@ void BKE_pbvh_get_grid_updates(PBVH *pbvh, bool clear, void ***r_gridfaces, int
 
   pbvh_iter_begin(&iter, pbvh, {});
 
+  SubdivCCGFace *all_faces = pbvh->subdiv_ccg->faces;
   while ((node = pbvh_iter_next(&iter, PBVH_Leaf))) {
     if (node->flag & PBVH_UpdateNormals) {
       for (const int grid : node->prim_indices) {
-        void *face = pbvh->gridfaces[grid];
+        void *face = &all_faces[pbvh->grid_to_face_map[grid]];
         BLI_gset_add(face_set, face);
       }
 
@@ -2944,14 +2945,14 @@ void BKE_pbvh_draw_debug_cb(PBVH *pbvh,
 
 void BKE_pbvh_grids_update(PBVH *pbvh,
                            CCGElem **grids,
-                           void **gridfaces,
+                           blender::Span<int> grid_to_face_map,
                            DMFlagMat *flagmats,
                            BLI_bitmap **grid_hidden,
                            CCGKey *key)
 {
   pbvh->gridkey = *key;
   pbvh->grids = grids;
-  pbvh->gridfaces = gridfaces;
+  pbvh->grid_to_face_map = grid_to_face_map;
 
   if (flagmats != pbvh->grid_flag_mats || pbvh->grid_hidden != grid_hidden) {
     pbvh->grid_flag_mats = flagmats;

source/blender/blenkernel/intern/pbvh_intern.hh

@@ -175,7 +175,7 @@ struct PBVH {
   /* Grid Data */
   CCGKey gridkey;
   CCGElem **grids;
-  void **gridfaces;
+  blender::Span<int> grid_to_face_map;
   const DMFlagMat *grid_flag_mats;
   int totgrid;
   BLI_bitmap **grid_hidden;

source/blender/blenkernel/intern/subdiv_ccg.cc

@@ -155,8 +155,7 @@ static void subdiv_ccg_alloc_elements(SubdivCCG *subdiv_ccg, Subdiv *subdiv)
   if (num_faces) {
     subdiv_ccg->faces = static_cast<SubdivCCGFace *>(
         MEM_calloc_arrayN(num_faces, sizeof(SubdivCCGFace), "Subdiv CCG faces"));
-    subdiv_ccg->grid_faces = static_cast<SubdivCCGFace **>(
-        MEM_calloc_arrayN(num_grids, sizeof(SubdivCCGFace *), "Subdiv CCG grid faces"));
+    subdiv_ccg->grid_to_face_map.reinitialize(num_grids);
   }
 }
 
@@ -235,7 +234,7 @@ static void subdiv_ccg_eval_regular_grid(CCGEvalGridsData *data, const int face_
   const float grid_size_1_inv = 1.0f / (grid_size - 1);
   const int element_size = element_size_bytes_get(subdiv_ccg);
   SubdivCCGFace *faces = subdiv_ccg->faces;
-  SubdivCCGFace **grid_faces = subdiv_ccg->grid_faces;
+  blender::MutableSpan<int> grid_to_face_map = subdiv_ccg->grid_to_face_map;
   const SubdivCCGFace *face = &faces[face_index];
   for (int corner = 0; corner < face->num_grids; corner++) {
     const int grid_index = face->start_grid_index + corner;
@@ -252,7 +251,7 @@ static void subdiv_ccg_eval_regular_grid(CCGEvalGridsData *data, const int face_
       }
     }
     /* Assign grid's face. */
-    grid_faces[grid_index] = &faces[face_index];
+    grid_to_face_map[grid_index] = face_index;
     /* Assign material flags. */
     subdiv_ccg->grid_flag_mats[grid_index] = data->material_flags_evaluator->eval_material_flags(
         data->material_flags_evaluator, face_index);
@@ -266,7 +265,7 @@ static void subdiv_ccg_eval_special_grid(CCGEvalGridsData *data, const int face_
   const float grid_size_1_inv = 1.0f / (grid_size - 1);
   const int element_size = element_size_bytes_get(subdiv_ccg);
   SubdivCCGFace *faces = subdiv_ccg->faces;
-  SubdivCCGFace **grid_faces = subdiv_ccg->grid_faces;
+  blender::MutableSpan<int> grid_to_face_map = subdiv_ccg->grid_to_face_map;
   const SubdivCCGFace *face = &faces[face_index];
   for (int corner = 0; corner < face->num_grids; corner++) {
     const int grid_index = face->start_grid_index + corner;
@@ -282,7 +281,7 @@ static void subdiv_ccg_eval_special_grid(CCGEvalGridsData *data, const int face_
       }
     }
     /* Assign grid's face. */
-    grid_faces[grid_index] = &faces[face_index];
+    grid_to_face_map[grid_index] = face_index;
     /* Assign material flags. */
     subdiv_ccg->grid_flag_mats[grid_index] = data->material_flags_evaluator->eval_material_flags(
         data->material_flags_evaluator, face_index);
@@ -596,7 +595,6 @@ void BKE_subdiv_ccg_destroy(SubdivCCG *subdiv_ccg)
     BKE_subdiv_free(subdiv_ccg->subdiv);
   }
   MEM_SAFE_FREE(subdiv_ccg->faces);
-  MEM_SAFE_FREE(subdiv_ccg->grid_faces);
   /* Free map of adjacent edges. */
   for (int i = 0; i < subdiv_ccg->num_adjacent_edges; i++) {
     SubdivCCGAdjacentEdge *adjacent_edge = &subdiv_ccg->adjacent_edges[i];
@@ -1452,7 +1450,7 @@ static SubdivCCGCoord coord_step_inside_from_boundary(const SubdivCCG *subdiv_cc
 BLI_INLINE
 int next_grid_index_from_coord(const SubdivCCG *subdiv_ccg, const SubdivCCGCoord *coord)
 {
-  SubdivCCGFace *face = subdiv_ccg->grid_faces[coord->grid_index];
+  const SubdivCCGFace *face = &subdiv_ccg->faces[subdiv_ccg->grid_to_face_map[coord->grid_index]];
   const int face_grid_index = coord->grid_index;
   int next_face_grid_index = face_grid_index + 1 - face->start_grid_index;
   if (next_face_grid_index == face->num_grids) {
@@ -1462,7 +1460,7 @@ int next_grid_index_from_coord(const SubdivCCG *subdiv_ccg, const SubdivCCGCoord
 }
 BLI_INLINE int prev_grid_index_from_coord(const SubdivCCG *subdiv_ccg, const SubdivCCGCoord *coord)
 {
-  SubdivCCGFace *face = subdiv_ccg->grid_faces[coord->grid_index];
+  const SubdivCCGFace *face = &subdiv_ccg->faces[subdiv_ccg->grid_to_face_map[coord->grid_index]];
   const int face_grid_index = coord->grid_index;
   int prev_face_grid_index = face_grid_index - 1 - face->start_grid_index;
   if (prev_face_grid_index < 0) {
@@ -1478,7 +1476,7 @@ static void neighbor_coords_corner_center_get(const SubdivCCG *subdiv_ccg,
                                               const bool include_duplicates,
                                               SubdivCCGNeighbors *r_neighbors)
 {
-  SubdivCCGFace *face = subdiv_ccg->grid_faces[coord->grid_index];
+  const SubdivCCGFace *face = &subdiv_ccg->faces[subdiv_ccg->grid_to_face_map[coord->grid_index]];
   const int num_adjacent_grids = face->num_grids;
 
   subdiv_ccg_neighbors_init(
@@ -1506,7 +1504,7 @@ static int adjacent_vertex_index_from_coord(const SubdivCCG *subdiv_ccg,
   Subdiv *subdiv = subdiv_ccg->subdiv;
   OpenSubdiv_TopologyRefiner *topology_refiner = subdiv->topology_refiner;
 
-  const SubdivCCGFace *face = subdiv_ccg->grid_faces[coord->grid_index];
+  const SubdivCCGFace *face = &subdiv_ccg->faces[subdiv_ccg->grid_to_face_map[coord->grid_index]];
   const int face_index = face - subdiv_ccg->faces;
   const int face_grid_index = coord->grid_index - face->start_grid_index;
   const int num_face_grids = face->num_grids;
@@ -1583,7 +1581,7 @@ static int adjacent_edge_index_from_coord(const SubdivCCG *subdiv_ccg, const Sub
 {
   Subdiv *subdiv = subdiv_ccg->subdiv;
   OpenSubdiv_TopologyRefiner *topology_refiner = subdiv->topology_refiner;
-  SubdivCCGFace *face = subdiv_ccg->grid_faces[coord->grid_index];
+  const SubdivCCGFace *face = &subdiv_ccg->faces[subdiv_ccg->grid_to_face_map[coord->grid_index]];
 
   const int face_grid_index = coord->grid_index - face->start_grid_index;
   const int face_index = face - subdiv_ccg->faces;
@@ -1876,9 +1874,7 @@ void BKE_subdiv_ccg_neighbor_coords_get(const SubdivCCG *subdiv_ccg,
 
 int BKE_subdiv_ccg_grid_to_face_index(const SubdivCCG *subdiv_ccg, const int grid_index)
 {
-  const SubdivCCGFace *face = subdiv_ccg->grid_faces[grid_index];
-  const int face_index = face - subdiv_ccg->faces;
-  return face_index;
+  return subdiv_ccg->grid_to_face_map[grid_index];
 }
 
 const int *BKE_subdiv_ccg_start_face_grid_index_ensure(SubdivCCG *subdiv_ccg)