Fix T71981: Alembic vertex interpolation can jumble mesh

Add an option to disable Alembic vertex interpolation.
Bump subversion from 5 to 6.

Alembic stores mesh samples at specific time keys; when a frame in
Blender maps to a timecode between two samples, Blender will interpolate
the mesh vertex positions. This interpolation only happens when the mesh
has a constant topology, but sometimes this was not detected properly
when the vertices change order, but the number of mesh elements remains
the same. This would result in a mesh with jumbled up vertices (T71981).
With this patch, users have the ability to disable vertex interpolation.

An alternative would be to have better detection of topology changes,
but that that'll cause a considerable slowdown.

Maniphest Tasks: T71981

Differential Revision: https://developer.blender.org/D9041

1 files changed, 5 insertions, 0 deletions

diff --git a/source/blender/makesrna/intern/rna_modifier.c b/source/blender/makesrna/intern/rna_modifier.c
index b433c938bd6..0e8903a2796 100644
--- a/source/blender/makesrna/intern/rna_modifier.c
+++ b/source/blender/makesrna/intern/rna_modifier.c
@@ -6134,7 +6134,12 @@ static void rna_def_modifier_meshseqcache(BlenderRNA *brna)
   RNA_def_property_enum_sdna(prop, NULL, "read_flag");
   RNA_def_property_enum_items(prop, read_flag_items);
   RNA_def_property_ui_text(prop, "Read Data", "Data to read from the cache");
+  RNA_def_property_update(prop, 0, "rna_Modifier_update");
 
+  prop = RNA_def_property(srna, "use_vertex_interpolation", PROP_BOOLEAN, PROP_NONE);
+  RNA_def_property_boolean_sdna(prop, NULL, "read_flag", MOD_MESHSEQ_INTERPOLATE_VERTICES);
+  RNA_def_property_ui_text(
+      prop, "Vertex Interpolation", "Allow interpolation of vertex positions.");
   RNA_def_property_update(prop, 0, "rna_Modifier_update");
 
   prop = RNA_def_property(srna, "velocity_scale", PROP_FLOAT, PROP_NONE);