Modifiers: export motion blur velocity through attribute

Previously fluid simulation and Alembic modifiers had a dedicated function
to query the velocity for motion blur. Now use a more generic system where
those modifiers output a velocity attribute.

Advantages:
* Geometry and particle nodes can output velocity through the same mechanism,
  or read the attribute coming from earlier modifiers.
* The velocity can be preserved through modifiers like subdivision surface or
  auto smooth.
* USD and Alembic previously only output velocity from fluid simulation, now
  they work with velocity from other sources too.
* Simplifies the code for renderers like Cycles and exporters like
  Alembic and USD.

This breaks compatibility:
* External renderers and exporters accessing these velocities through the
  Python API now need to use the attribute instead.
* Existing modifier node setups that create an attribute named "velocity"
  will render differently with motion blur.

Differential Revision: https://developer.blender.org/D12305

1 files changed, 0 insertions, 2 deletions

diff --git a/source/blender/blenloader/intern/versioning_290.c b/source/blender/blenloader/intern/versioning_290.c
index cb7a8ad592a..0e89c15ca1e 100644
--- a/source/blender/blenloader/intern/versioning_290.c
+++ b/source/blender/blenloader/intern/versioning_290.c
@@ -1092,8 +1092,6 @@ void blo_do_versions_290(FileData *fd, Library *UNUSED(lib), Main *bmain)
           if (md->type == eModifierType_MeshSequenceCache) {
             MeshSeqCacheModifierData *mcmd = (MeshSeqCacheModifierData *)md;
             mcmd->velocity_scale = 1.0f;
-            mcmd->vertex_velocities = NULL;
-            mcmd->num_vertices = 0;
           }
         }
       }