Mesh: Move selection flags to generic attributes

Using the attribute name semantics from T97452, this patch moves the
selection status of mesh elements from the `SELECT` of vertices, and
edges, and the `ME_FACE_SEL` of faces to generic boolean attribute
Storing this data as generic attributes can significantly simplify and
improve code, as described in T95965.

The attributes are called `.select_vert`, `.select_edge`, and
`.select_poly`. The `.` prefix means they are "UI attributes",so they
still contain original data edited by users, but they aren't meant to
be accessed procedurally by the user in arbitrary situations. They are
also be hidden in the spreadsheet and the attribute list.

Until 4.0, the attributes are still written to and read from the mesh
in the old way, so neither forward nor backward compatibility are
affected. This means memory requirements will be increased by one byte
per element when selection is used. When the flags are removed
completely, requirements will decrease.

Further notes:
* The `MVert` flag is empty at runtime now, so it can be ignored.
* `BMesh` is unchanged, otherwise the change would be much larger.
* Many tests have slightly different results, since the selection
  attribute uses more generic propagation. Previously you couldn't
  really rely on edit mode selections being propagated procedurally.
  Now it mostly works as expected.

Similar to 2480b55f216c
Ref T95965

Differential Revision: https://developer.blender.org/D15795

1 files changed, 22 insertions, 5 deletions

diff --git a/source/blender/blenkernel/intern/mesh_calc_edges.cc b/source/blender/blenkernel/intern/mesh_calc_edges.cc
index cc315130ad1..038133c33ae 100644
--- a/source/blender/blenkernel/intern/mesh_calc_edges.cc
+++ b/source/blender/blenkernel/intern/mesh_calc_edges.cc
@@ -13,6 +13,7 @@
 #include "BLI_threads.h"
 #include "BLI_timeit.hh"
 
+#include "BKE_attribute.hh"
 #include "BKE_customdata.h"
 #include "BKE_mesh.h"
 
@@ -120,8 +121,7 @@ static void add_polygon_edges_to_hash_maps(Mesh *mesh,
 }
 
 static void serialize_and_initialize_deduplicated_edges(MutableSpan<EdgeMap> edge_maps,
-                                                        MutableSpan<MEdge> new_edges,
-                                                        short new_edge_flag)
+                                                        MutableSpan<MEdge> new_edges)
 {
   /* All edges are distributed in the hash tables now. They have to be serialized into a single
    * array below. To be able to parallelize this, we have to compute edge index offsets for each
@@ -147,7 +147,7 @@ static void serialize_and_initialize_deduplicated_edges(MutableSpan<EdgeMap> edg
         /* Initialize new edge. */
         new_edge.v1 = item.key.v_low;
         new_edge.v2 = item.key.v_high;
-        new_edge.flag = new_edge_flag;
+        new_edge.flag = ME_EDGEDRAW | ME_EDGERENDER;
       }
       item.value.index = new_edge_index;
       new_edge_index++;
@@ -236,8 +236,7 @@ void BKE_mesh_calc_edges(Mesh *mesh, bool keep_existing_edges, const bool select
   /* Create new edges. */
   MutableSpan<MEdge> new_edges{
       static_cast<MEdge *>(MEM_calloc_arrayN(new_totedge, sizeof(MEdge), __func__)), new_totedge};
-  const short new_edge_flag = (ME_EDGEDRAW | ME_EDGERENDER) | (select_new_edges ? SELECT : 0);
-  calc_edges::serialize_and_initialize_deduplicated_edges(edge_maps, new_edges, new_edge_flag);
+  calc_edges::serialize_and_initialize_deduplicated_edges(edge_maps, new_edges);
   calc_edges::update_edge_indices_in_poly_loops(mesh, edge_maps, parallel_mask);
 
   /* Free old CustomData and assign new one. */
@@ -246,6 +245,24 @@ void BKE_mesh_calc_edges(Mesh *mesh, bool keep_existing_edges, const bool select
   CustomData_add_layer(&mesh->edata, CD_MEDGE, CD_ASSIGN, new_edges.data(), new_totedge);
   mesh->totedge = new_totedge;
 
+  if (select_new_edges) {
+    MutableAttributeAccessor attributes = mesh->attributes_for_write();
+    SpanAttributeWriter<bool> select_edge = attributes.lookup_or_add_for_write_span<bool>(
+        ".select_edge", ATTR_DOMAIN_EDGE);
+    if (select_edge) {
+      int new_edge_index = 0;
+      for (const EdgeMap &edge_map : edge_maps) {
+        for (EdgeMap::Item item : edge_map.items()) {
+          if (item.value.original_edge == nullptr) {
+            select_edge.span[new_edge_index] = true;
+          }
+          new_edge_index++;
+        }
+      }
+      select_edge.finish();
+    }
+  }
+
   /* Explicitly clear edge maps, because that way it can be parallelized. */
   clear_hash_tables(edge_maps);
 }