Mesh: Avoid creating incorrect original index layers

Currently, whenever any BMesh is converted to a Mesh (except for edit
mode switching), original index (`CD_ORIGINDEX`) layers are added.
This is incorrect, because many operations just convert some Mesh into
a BMesh and then back, but they shouldn't make any assumption about
where their input mesh came from. It might even come from a primitive
in geometry nodes, where there are no original indices at all.

Conceptually, mesh original indices should be filled by the modifier
stack when first creating the evaluated mesh. So that's where they're
moved in this patch. A separate function now fills the indices with their
default (0,1,2,3...) values. The way the mesh wrapper system defers
the BMesh to Mesh conversion makes this a bit less obvious though.

The old behavior is incorrect, but it's also slower, because three
arrays the size of the mesh's vertices, edges, and faces had to be
allocated and filled during the BMesh to Mesh conversion, which just
ends up putting more pressure on the cache. In the many cases where
original indices aren't used, I measured an **8% speedup** for the
conversion (from 76.5ms to 70.7ms).

Generally there is an assumption that BMesh is "original" and Mesh is
"evaluated". After this patch, that assumption isn't quite as strong,
but it still exists for two reasons. First, original indices are added
whenever converting a BMesh "wrapper" to a Mesh. Second, original
indices are not added to the BMesh at the beginning of evaluation,
which assumes that every BMesh in the viewport is original and doesn't
need the mapping.

Differential Revision: https://developer.blender.org/D14018

3 files changed, 35 insertions, 0 deletions

diff --git a/source/blender/blenkernel/intern/DerivedMesh.cc b/source/blender/blenkernel/intern/DerivedMesh.cc
index 00a6fa6d178..79a1d8a88e4 100644
--- a/source/blender/blenkernel/intern/DerivedMesh.cc
+++ b/source/blender/blenkernel/intern/DerivedMesh.cc
@@ -708,6 +708,7 @@ static Mesh *create_orco_mesh(Object *ob, Mesh *me, BMEditMesh *em, int layer)
 
   if (em) {
     mesh = BKE_mesh_from_bmesh_for_eval_nomain(em->bm, nullptr, me);
+    BKE_mesh_ensure_default_orig_index_customdata(mesh);
   }
   else {
     mesh = BKE_mesh_copy_for_eval(me, true);
@@ -1521,6 +1522,12 @@ static void editbmesh_calc_modifiers(struct Depsgraph *depsgraph,
         em_input, &final_datamask, nullptr, mesh_input);
   }
 
+  /* The mesh from edit mode should not have any original index layers already, since those
+   * are added during evaluation when necessary and are redundant on an original mesh. */
+  BLI_assert(CustomData_get_layer(&em_input->bm->pdata, CD_ORIGINDEX) == nullptr &&
+             CustomData_get_layer(&em_input->bm->edata, CD_ORIGINDEX) == nullptr &&
+             CustomData_get_layer(&em_input->bm->pdata, CD_ORIGINDEX) == nullptr);
+
   /* Clear errors before evaluation. */
   BKE_modifiers_clear_errors(ob);
 
@@ -1559,6 +1566,7 @@ static void editbmesh_calc_modifiers(struct Depsgraph *depsgraph,
       else if (isPrevDeform && mti->dependsOnNormals && mti->dependsOnNormals(md)) {
         if (mesh_final == nullptr) {
           mesh_final = BKE_mesh_from_bmesh_for_eval_nomain(em_input->bm, nullptr, mesh_input);
+          BKE_mesh_ensure_default_orig_index_customdata(mesh_final);
           ASSERT_IS_VALID_MESH(mesh_final);
         }
         BLI_assert(deformed_verts != nullptr);
diff --git a/source/blender/blenkernel/intern/mesh.cc b/source/blender/blenkernel/intern/mesh.cc
index 09d9b19330a..61a02e10063 100644
--- a/source/blender/blenkernel/intern/mesh.cc
+++ b/source/blender/blenkernel/intern/mesh.cc
@@ -1199,6 +1199,23 @@ Mesh *BKE_mesh_from_bmesh_for_eval_nomain(BMesh *bm,
   return mesh;
 }
 
+static void ensure_orig_index_layer(CustomData &data, const int size)
+{
+  if (CustomData_has_layer(&data, CD_ORIGINDEX)) {
+    return;
+  }
+  int *indices = (int *)CustomData_add_layer(&data, CD_ORIGINDEX, CD_DEFAULT, nullptr, size);
+  range_vn_i(indices, size, 0);
+}
+
+void BKE_mesh_ensure_default_orig_index_customdata(Mesh *mesh)
+{
+  BLI_assert(mesh->runtime.wrapper_type == ME_WRAPPER_TYPE_MDATA);
+  ensure_orig_index_layer(mesh->vdata, mesh->totvert);
+  ensure_orig_index_layer(mesh->edata, mesh->totedge);
+  ensure_orig_index_layer(mesh->pdata, mesh->totpoly);
+}
+
 BoundBox *BKE_mesh_boundbox_get(Object *ob)
 {
   /* This is Object-level data access,
diff --git a/source/blender/blenkernel/intern/mesh_wrapper.c b/source/blender/blenkernel/intern/mesh_wrapper.c
index 3b6e7b73b4f..71a753d8d30 100644
--- a/source/blender/blenkernel/intern/mesh_wrapper.c
+++ b/source/blender/blenkernel/intern/mesh_wrapper.c
@@ -129,6 +129,16 @@ static void mesh_wrapper_ensure_mdata_isolated(void *userdata)
       BMEditMesh *em = me->edit_mesh;
       BM_mesh_bm_to_me_for_eval(em->bm, me, &me->runtime.cd_mask_extra);
 
+      /* Adding original index layers assumes that all BMesh mesh wrappers are created from
+       * original edit mode meshes (the only case where adding original indices makes sense).
+       * If that assumption is broken, the layers might be incorrect in that they might not
+       * actually be "original".
+       *
+       * There is also a performance aspect, where this also assumes that original indices are
+       * always needed when converting an edit mesh to a mesh. That might be wrong, but it's not
+       * harmful. */
+      BKE_mesh_ensure_default_orig_index_customdata(me);
+
       EditMeshData *edit_data = me->runtime.edit_data;
       if (edit_data->vertexCos) {
         BKE_mesh_vert_coords_apply(me, edit_data->vertexCos);