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, 11 insertions, 6 deletions

diff --git a/source/blender/io/alembic/intern/abc_reader_mesh.cc b/source/blender/io/alembic/intern/abc_reader_mesh.cc
index ead908e87c2..e69c0edfec8 100644
--- a/source/blender/io/alembic/intern/abc_reader_mesh.cc
+++ b/source/blender/io/alembic/intern/abc_reader_mesh.cc
@@ -155,8 +155,8 @@ static void read_mverts(CDStreamConfig &config, const AbcMeshData &mesh_data)
   MVert *mverts = config.mvert;
   const P3fArraySamplePtr &positions = mesh_data.positions;
 
-  if (config.weight != 0.0f && mesh_data.ceil_positions != NULL &&
-      mesh_data.ceil_positions->size() == positions->size()) {
+  if (config.use_vertex_interpolation && config.weight != 0.0f &&
+      mesh_data.ceil_positions != NULL && mesh_data.ceil_positions->size() == positions->size()) {
     read_mverts_interp(mverts, positions, mesh_data.ceil_positions, config.weight);
     return;
   }
@@ -457,7 +457,7 @@ static void read_mesh_sample(const std::string &iobject_full_name,
   }
 }
 
-CDStreamConfig get_config(Mesh *mesh)
+CDStreamConfig get_config(Mesh *mesh, const bool use_vertex_interpolation)
 {
   CDStreamConfig config;
 
@@ -471,6 +471,7 @@ CDStreamConfig get_config(Mesh *mesh)
   config.totpoly = mesh->totpoly;
   config.loopdata = &mesh->ldata;
   config.add_customdata_cb = add_customdata_cb;
+  config.use_vertex_interpolation = use_vertex_interpolation;
 
   return config;
 }
@@ -646,7 +647,9 @@ Mesh *AbcMeshReader::read_mesh(Mesh *existing_mesh,
     }
   }
 
-  CDStreamConfig config = get_config(new_mesh ? new_mesh : existing_mesh);
+  Mesh *mesh_to_export = new_mesh ? new_mesh : existing_mesh;
+  const bool use_vertex_interpolation = read_flag & MOD_MESHSEQ_INTERPOLATE_VERTICES;
+  CDStreamConfig config = get_config(mesh_to_export, use_vertex_interpolation);
   config.time = sample_sel.getRequestedTime();
   config.modifier_error_message = err_str;
 
@@ -936,11 +939,13 @@ Mesh *AbcSubDReader::read_mesh(Mesh *existing_mesh,
   }
 
   /* Only read point data when streaming meshes, unless we need to create new ones. */
-  CDStreamConfig config = get_config(new_mesh ? new_mesh : existing_mesh);
+  Mesh *mesh_to_export = new_mesh ? new_mesh : existing_mesh;
+  const bool use_vertex_interpolation = read_flag & MOD_MESHSEQ_INTERPOLATE_VERTICES;
+  CDStreamConfig config = get_config(mesh_to_export, use_vertex_interpolation);
   config.time = sample_sel.getRequestedTime();
   read_subd_sample(m_iobject.getFullName(), &settings, m_schema, sample_sel, config);
 
-  return config.mesh;
+  return mesh_to_export;
 }
 
 }  // namespace blender::io::alembic