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, 6 insertions, 0 deletions

diff --git a/source/blender/io/alembic/intern/abc_customdata.h b/source/blender/io/alembic/intern/abc_customdata.h
index e9736555ead..4fba6a2f0f7 100644
--- a/source/blender/io/alembic/intern/abc_customdata.h
+++ b/source/blender/io/alembic/intern/abc_customdata.h
@@ -122,6 +122,12 @@ void read_custom_data(const std::string &iobject_full_name,
                       const CDStreamConfig &config,
                       const Alembic::Abc::ISampleSelector &iss);
 
+void read_velocity(const Alembic::Abc::ICompoundProperty &prop,
+                   const Alembic::Abc::PropertyHeader *prop_header,
+                   const Alembic::Abc::ISampleSelector &selector,
+                   const CDStreamConfig &config,
+                   const char *velocity_name,
+                   const float velocity_scale);
 typedef enum {
   ABC_UV_SCOPE_NONE,
   ABC_UV_SCOPE_LOOP,