Subdiv CCG: Add access to first grid index of a face

Is lazily-initialized array owned by the SubdivCCG. Allows to access
index of a first grid of a given face in the flat array of grids.

Currently unused, but is needed for multires bake.

source/blender/blenkernel/BKE_subdiv_ccg.h

@@ -214,6 +214,12 @@ typedef struct SubdivCCG {
     /* Corresponds to MULTIRES_HIDDEN_MODIFIED. */
     bool hidden;
   } dirty;
+
+  /* Cached values, are not supposed to be accessed directly. */
+  struct {
+    /* Indexed by face, indicates index of the first grid which corresponds to the face. */
+    int *start_face_grid_index;
+  } cache_;
 } SubdivCCG;
 
 /* Create CCG representation of subdivision surface.
@@ -307,6 +313,15 @@ 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);
 
+/* Get array which is indexed by face index and contains index of a first grid of the face.
+ *
+ * The "ensure" version allocates the mapping if it's not know yet and stores it in the subdiv_ccg
+ * descriptor. This function is NOT safe for threading.
+ *
+ * The "get" version simply returns cached array. */
+const int *BKE_subdiv_ccg_start_face_grid_index_ensure(SubdivCCG *subdiv_ccg);
+const int *BKE_subdiv_ccg_start_face_grid_index_get(const SubdivCCG *subdiv_ccg);
+
 #ifdef __cplusplus
 }
 #endif

source/blender/blenkernel/intern/subdiv_ccg.c

@@ -655,6 +655,7 @@ void BKE_subdiv_ccg_destroy(SubdivCCG *subdiv_ccg)
     MEM_SAFE_FREE(adjacent_vertex->corner_coords);
   }
   MEM_SAFE_FREE(subdiv_ccg->adjacent_vertices);
+  MEM_SAFE_FREE(subdiv_ccg->cache_.start_face_grid_index);
   MEM_freeN(subdiv_ccg);
 }
 
@@ -1802,4 +1803,35 @@ int BKE_subdiv_ccg_grid_to_face_index(const SubdivCCG *subdiv_ccg, const int gri
   return face_index;
 }
 
+const int *BKE_subdiv_ccg_start_face_grid_index_ensure(SubdivCCG *subdiv_ccg)
+{
+  if (subdiv_ccg->cache_.start_face_grid_index == NULL) {
+    const Subdiv *subdiv = subdiv_ccg->subdiv;
+    OpenSubdiv_TopologyRefiner *topology_refiner = subdiv->topology_refiner;
+    if (topology_refiner == NULL) {
+      return NULL;
+    }
+
+    const int num_coarse_faces = topology_refiner->getNumFaces(topology_refiner);
+
+    subdiv_ccg->cache_.start_face_grid_index = MEM_malloc_arrayN(
+        sizeof(int), num_coarse_faces, "start_face_grid_index");
+
+    int start_grid_index = 0;
+    for (int face_index = 0; face_index < num_coarse_faces; face_index++) {
+      const int num_face_grids = topology_refiner->getNumFaceVertices(topology_refiner,
+                                                                      face_index);
+      subdiv_ccg->cache_.start_face_grid_index[face_index] = start_grid_index;
+      start_grid_index += num_face_grids;
+    }
+  }
+
+  return subdiv_ccg->cache_.start_face_grid_index;
+}
+
+const int *BKE_subdiv_ccg_start_face_grid_index_get(const SubdivCCG *subdiv_ccg)
+{
+  return subdiv_ccg->cache_.start_face_grid_index;
+}
+
 /** \} */