Mesh: Remove redundant custom data pointers

For copy-on-write, we want to share attribute arrays between meshes
where possible. Mutable pointers like `Mesh.mvert` make that difficult
by making ownership vague. They also make code more complex by adding
redundancy.

The simplest solution is just removing them and retrieving layers from
`CustomData` as needed. Similar changes have already been applied to
curves and point clouds (e9f82d3dc7ee, 410a6efb747f). Removing use of
the pointers generally makes code more obvious and more reusable.

Mesh data is now accessed with a C++ API (`Mesh::edges()` or
`Mesh::edges_for_write()`), and a C API (`BKE_mesh_edges(mesh)`).

The CoW changes this commit makes possible are described in T95845
and T95842, and started in D14139 and D14140. The change also simplifies
the ongoing mesh struct-of-array refactors from T95965.

**RNA/Python Access Performance**
Theoretically, accessing mesh elements with the RNA API may become
slower, since the layer needs to be found on every random access.
However, overhead is already high enough that this doesn't make a
noticible differenc, and performance is actually improved in some
cases. Random access can be up to 10% faster, but other situations
might be a bit slower. Generally using `foreach_get/set` are the best
way to improve performance. See the differential revision for more
discussion about Python performance.

Cycles has been updated to use raw pointers and the internal Blender
mesh types, mostly because there is no sense in having this overhead
when it's already compiled with Blender. In my tests this roughly
halves the Cycles mesh creation time (0.19s to 0.10s for a 1 million
face grid).

Differential Revision: https://developer.blender.org/D15488

1 files changed, 18 insertions, 13 deletions

diff --git a/source/blender/editors/object/object_remesh.cc b/source/blender/editors/object/object_remesh.cc
index 812d9bbbc08..56cccd984b6 100644
--- a/source/blender/editors/object/object_remesh.cc
+++ b/source/blender/editors/object/object_remesh.cc
@@ -73,6 +73,9 @@
 
 #include "object_intern.h" /* own include */
 
+using blender::IndexRange;
+using blender::Span;
+
 /* TODO(sebpa): unstable, can lead to unrecoverable errors. */
 // #define USE_MESH_CURVATURE
 
@@ -128,7 +131,8 @@ static int voxel_remesh_exec(bContext *C, wmOperator *op)
   }
 
   /* Output mesh will be all smooth or all flat shading. */
-  const bool smooth_normals = mesh->mpoly[0].flag & ME_SMOOTH;
+  const Span<MPoly> polygons = mesh->polygons();
+  const bool smooth_normals = polygons.first().flag & ME_SMOOTH;
 
   float isovalue = 0.0f;
   if (mesh->flag & ME_REMESH_REPROJECT_VOLUME) {
@@ -678,9 +682,11 @@ static bool mesh_is_manifold_consistent(Mesh *mesh)
    * check that the direction of the faces are consistent and doesn't suddenly
    * flip
    */
+  const Span<MVert> verts = mesh->vertices();
+  const Span<MEdge> edges = mesh->edges();
+  const Span<MLoop> loops = mesh->loops();
 
   bool is_manifold_consistent = true;
-  const MLoop *mloop = mesh->mloop;
   char *edge_faces = (char *)MEM_callocN(mesh->totedge * sizeof(char), "remesh_manifold_check");
   int *edge_vert = (int *)MEM_malloc_arrayN(
       mesh->totedge, sizeof(uint), "remesh_consistent_check");
@@ -689,18 +695,17 @@ static bool mesh_is_manifold_consistent(Mesh *mesh)
     edge_vert[i] = -1;
   }
 
-  for (uint loop_idx = 0; loop_idx < mesh->totloop; loop_idx++) {
-    const MLoop *loop = &mloop[loop_idx];
-    edge_faces[loop->e] += 1;
-    if (edge_faces[loop->e] > 2) {
+  for (const MLoop &loop : loops) {
+    edge_faces[loop.e] += 1;
+    if (edge_faces[loop.e] > 2) {
       is_manifold_consistent = false;
       break;
     }
 
-    if (edge_vert[loop->e] == -1) {
-      edge_vert[loop->e] = loop->v;
+    if (edge_vert[loop.e] == -1) {
+      edge_vert[loop.e] = loop.v;
     }
-    else if (edge_vert[loop->e] == loop->v) {
+    else if (edge_vert[loop.e] == loop.v) {
       /* Mesh has flips in the surface so it is non consistent */
       is_manifold_consistent = false;
       break;
@@ -708,16 +713,16 @@ static bool mesh_is_manifold_consistent(Mesh *mesh)
   }
 
   if (is_manifold_consistent) {
-    for (uint i = 0; i < mesh->totedge; i++) {
+    for (const int i : edges.index_range()) {
       /* Check for wire edges. */
       if (edge_faces[i] == 0) {
         is_manifold_consistent = false;
         break;
       }
       /* Check for zero length edges */
-      MVert *v1 = &mesh->mvert[mesh->medge[i].v1];
-      MVert *v2 = &mesh->mvert[mesh->medge[i].v2];
-      if (compare_v3v3(v1->co, v2->co, 1e-4f)) {
+      const MVert &v1 = verts[edges[i].v1];
+      const MVert &v2 = verts[edges[i].v2];
+      if (compare_v3v3(v1.co, v2.co, 1e-4f)) {
         is_manifold_consistent = false;
         break;
       }