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, 1 insertions, 4 deletions

diff --git a/source/blender/io/alembic/exporter/abc_writer_mesh.h b/source/blender/io/alembic/exporter/abc_writer_mesh.h
index 0e1792b9dab..fb8a01a3bbf 100644
--- a/source/blender/io/alembic/exporter/abc_writer_mesh.h
+++ b/source/blender/io/alembic/exporter/abc_writer_mesh.h
@@ -45,7 +45,6 @@ class ABCGenericMeshWriter : public ABCAbstractWriter {
    * exported object. */
   bool is_subd_;
   ModifierData *subsurf_modifier_;
-  ModifierData *liquid_sim_modifier_;
 
   CDStreamConfig m_custom_data_config;
 
@@ -70,10 +69,8 @@ class ABCGenericMeshWriter : public ABCAbstractWriter {
   void write_subd(HierarchyContext &context, Mesh *mesh);
   template<typename Schema> void write_face_sets(Object *object, Mesh *mesh, Schema &schema);
 
-  ModifierData *get_liquid_sim_modifier(Scene *scene_eval, Object *ob_eval);
-
   void write_arb_geo_params(Mesh *me);
-  void get_velocities(Mesh *mesh, std::vector<Imath::V3f> &vels);
+  bool get_velocities(Mesh *mesh, std::vector<Imath::V3f> &vels);
   void get_geo_groups(Object *object,
                       Mesh *mesh,
                       std::map<std::string, std::vector<int32_t>> &geo_groups);