Mesh: Move hide flags to generic attributes

This commit moves the hide status of mesh vertices, edges, and faces
from the `ME_FLAG` to optional generic boolean attributes. Storing this
data as generic attributes can significantly simplify and improve code,
as described in T95965.

The attributes are called `.hide_vert`, `.hide_edge`, and `.hide_poly`,
using the attribute name semantics discussed in T97452. 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 by default,

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 the hide status is used. When the flags are removed
completely, requirements will decrease when hiding is unused.

Further notes:
 * Some code can be further simplified to skip some processing when the
   hide attributes don't exist.
 * The data is still stored in flags for `BMesh`, necessitating some
   complexity in the conversion to and from `Mesh`.
 * Access to the "hide" property of mesh elements in RNA is slower.
   The separate boolean arrays should be used where possible.

Ref T95965

Differential Revision: https://developer.blender.org/D14685

1 files changed, 10 insertions, 0 deletions

diff --git a/source/blender/blenkernel/BKE_mesh_legacy_convert.h b/source/blender/blenkernel/BKE_mesh_legacy_convert.h
index 909fd0e0dea..bbc61d5af5e 100644
--- a/source/blender/blenkernel/BKE_mesh_legacy_convert.h
+++ b/source/blender/blenkernel/BKE_mesh_legacy_convert.h
@@ -18,6 +18,16 @@ struct Mesh;
 struct MFace;
 
 /**
+ * Convert the hidden element attributes to the old flag format for writing.
+ */
+void BKE_mesh_legacy_convert_hide_layers_to_flags(struct Mesh *mesh);
+/**
+ * Convert the old hide flags (#ME_HIDE) to the hidden element attribute for reading.
+ * Only add the attributes when there are any elements in each domain hidden.
+ */
+void BKE_mesh_legacy_convert_flags_to_hide_layers(struct Mesh *mesh);
+
+/**
  * Recreate #MFace Tessellation.
  *
  * \note This doesn't use multi-threading like #BKE_mesh_recalc_looptri since