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, 2 insertions, 0 deletions

diff --git a/source/blender/io/alembic/intern/abc_reader_object.cc b/source/blender/io/alembic/intern/abc_reader_object.cc
index 9a5ffd04bd1..a6d46c4b42a 100644
--- a/source/blender/io/alembic/intern/abc_reader_object.cc
+++ b/source/blender/io/alembic/intern/abc_reader_object.cc
@@ -167,6 +167,8 @@ Imath::M44d get_matrix(const IXformSchema &schema, const float time)
 struct Mesh *AbcObjectReader::read_mesh(struct Mesh *existing_mesh,
                                         const Alembic::Abc::ISampleSelector &UNUSED(sample_sel),
                                         int UNUSED(read_flag),
+                                        const char *UNUSED(velocity_name),
+                                        const float UNUSED(velocity_scale),
                                         const char **UNUSED(err_str))
 {
   return existing_mesh;