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, 4 deletions

diff --git a/source/blender/makesdna/DNA_modifier_defaults.h b/source/blender/makesdna/DNA_modifier_defaults.h
index 1b3dbd148df..5b2694f420b 100644
--- a/source/blender/makesdna/DNA_modifier_defaults.h
+++ b/source/blender/makesdna/DNA_modifier_defaults.h
@@ -419,10 +419,6 @@
     .velocity_scale = 1.0f, \
     .reader = NULL, \
     .reader_object_path = "", \
-    .vertex_velocities = NULL, \
-    .num_vertices = 0, \
-    .velocity_delta = 0.0f, \
-    .last_lookup_time = 0.0f, \
   }
 
 #define _DNA_DEFAULT_MirrorModifierData \