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, 3 insertions, 1 deletions

diff --git a/source/blender/io/alembic/intern/abc_reader_curves.h b/source/blender/io/alembic/intern/abc_reader_curves.h
index 075ed5ca6a1..11b23c8a8cf 100644
--- a/source/blender/io/alembic/intern/abc_reader_curves.h
+++ b/source/blender/io/alembic/intern/abc_reader_curves.h
@@ -45,7 +45,9 @@ class AbcCurveReader : public AbcObjectReader {
   void readObjectData(Main *bmain, const Alembic::Abc::ISampleSelector &sample_sel);
   struct Mesh *read_mesh(struct Mesh *existing_mesh,
                          const Alembic::Abc::ISampleSelector &sample_sel,
-                         int read_flag,
+                         const int read_flag,
+                         const char *velocity_name,
+                         const float velocity_scale,
                          const char **err_str);
 
   void read_curve_sample(Curve *cu,