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, 30 insertions, 12 deletions

diff --git a/source/blender/geometry/intern/realize_instances.cc b/source/blender/geometry/intern/realize_instances.cc
index 25ff705385c..8d1c43866eb 100644
--- a/source/blender/geometry/intern/realize_instances.cc
+++ b/source/blender/geometry/intern/realize_instances.cc
@@ -97,6 +97,11 @@ struct MeshElementStartIndices {
 
 struct MeshRealizeInfo {
   const Mesh *mesh = nullptr;
+  Span<MVert> verts;
+  Span<MEdge> edges;
+  Span<MPoly> polys;
+  Span<MLoop> loops;
+
   /** Maps old material indices to new material indices. */
   Array<int> material_index_map;
   /** Matches the order in #AllMeshesInfo.attributes. */
@@ -859,6 +864,10 @@ static AllMeshesInfo preprocess_meshes(const GeometrySet &geometry_set,
     MeshRealizeInfo &mesh_info = info.realize_info[mesh_index];
     const Mesh *mesh = info.order[mesh_index];
     mesh_info.mesh = mesh;
+    mesh_info.verts = mesh->vertices();
+    mesh_info.edges = mesh->edges();
+    mesh_info.polys = mesh->polygons();
+    mesh_info.loops = mesh->loops();
 
     /* Create material index mapping. */
     mesh_info.material_index_map.reinitialize(std::max<int>(mesh->totcol, 1));
@@ -900,30 +909,31 @@ static AllMeshesInfo preprocess_meshes(const GeometrySet &geometry_set,
 static void execute_realize_mesh_task(const RealizeInstancesOptions &options,
                                       const RealizeMeshTask &task,
                                       const OrderedAttributes &ordered_attributes,
-                                      Mesh &dst_mesh,
                                       MutableSpan<GSpanAttributeWriter> dst_attribute_writers,
+                                      MutableSpan<MVert> all_dst_verts,
+                                      MutableSpan<MEdge> all_dst_edges,
+                                      MutableSpan<MPoly> all_dst_polys,
+                                      MutableSpan<MLoop> all_dst_loops,
                                       MutableSpan<int> all_dst_vertex_ids,
                                       MutableSpan<int> all_dst_material_indices)
 {
   const MeshRealizeInfo &mesh_info = *task.mesh_info;
   const Mesh &mesh = *mesh_info.mesh;
 
-  const Span<MVert> src_verts{mesh.mvert, mesh.totvert};
-  const Span<MEdge> src_edges{mesh.medge, mesh.totedge};
-  const Span<MLoop> src_loops{mesh.mloop, mesh.totloop};
-  const Span<MPoly> src_polys{mesh.mpoly, mesh.totpoly};
+  const Span<MVert> src_verts = mesh_info.verts;
+  const Span<MEdge> src_edges = mesh_info.edges;
+  const Span<MPoly> src_polys = mesh_info.polys;
+  const Span<MLoop> src_loops = mesh_info.loops;
 
   const IndexRange dst_vert_range(task.start_indices.vertex, src_verts.size());
   const IndexRange dst_edge_range(task.start_indices.edge, src_edges.size());
   const IndexRange dst_poly_range(task.start_indices.poly, src_polys.size());
   const IndexRange dst_loop_range(task.start_indices.loop, src_loops.size());
 
-  MutableSpan dst_verts = MutableSpan(dst_mesh.mvert, dst_mesh.totvert).slice(dst_vert_range);
-  MutableSpan dst_edges = MutableSpan(dst_mesh.medge, dst_mesh.totedge).slice(dst_edge_range);
-  MutableSpan dst_polys = MutableSpan(dst_mesh.mpoly, dst_mesh.totpoly).slice(dst_poly_range);
-  MutableSpan dst_loops = MutableSpan(dst_mesh.mloop, dst_mesh.totloop).slice(dst_loop_range);
-
-  const Span<int> material_index_map = mesh_info.material_index_map;
+  MutableSpan<MVert> dst_verts = all_dst_verts.slice(dst_vert_range);
+  MutableSpan<MEdge> dst_edges = all_dst_edges.slice(dst_edge_range);
+  MutableSpan<MPoly> dst_polys = all_dst_polys.slice(dst_poly_range);
+  MutableSpan<MLoop> dst_loops = all_dst_loops.slice(dst_loop_range);
 
   threading::parallel_for(src_verts.index_range(), 1024, [&](const IndexRange vert_range) {
     for (const int i : vert_range) {
@@ -960,6 +970,7 @@ static void execute_realize_mesh_task(const RealizeInstancesOptions &options,
     }
   });
   if (!all_dst_material_indices.is_empty()) {
+    const Span<int> material_index_map = mesh_info.material_index_map;
     MutableSpan<int> dst_material_indices = all_dst_material_indices.slice(dst_poly_range);
     if (mesh.totcol == 0) {
       /* The material index map contains the index of the null material in the result. */
@@ -1035,6 +1046,10 @@ static void execute_realize_mesh_tasks(const RealizeInstancesOptions &options,
   MeshComponent &dst_component = r_realized_geometry.get_component_for_write<MeshComponent>();
   dst_component.replace(dst_mesh);
   bke::MutableAttributeAccessor dst_attributes = bke::mesh_attributes_for_write(*dst_mesh);
+  MutableSpan<MVert> dst_verts = dst_mesh->vertices_for_write();
+  MutableSpan<MEdge> dst_edges = dst_mesh->edges_for_write();
+  MutableSpan<MPoly> dst_polys = dst_mesh->polygons_for_write();
+  MutableSpan<MLoop> dst_loops = dst_mesh->loops_for_write();
 
   /* Copy settings from the first input geometry set with a mesh. */
   const RealizeMeshTask &first_task = tasks.first();
@@ -1079,8 +1094,11 @@ static void execute_realize_mesh_tasks(const RealizeInstancesOptions &options,
       execute_realize_mesh_task(options,
                                 task,
                                 ordered_attributes,
-                                *dst_mesh,
                                 dst_attribute_writers,
+                                dst_verts,
+                                dst_edges,
+                                dst_polys,
+                                dst_loops,
                                 vertex_ids.span,
                                 material_indices.span);
     }