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, 5 insertions, 0 deletions

diff --git a/source/blender/blenkernel/BKE_attribute.h b/source/blender/blenkernel/BKE_attribute.h
index 5fda30224c6..7476474258b 100644
--- a/source/blender/blenkernel/BKE_attribute.h
+++ b/source/blender/blenkernel/BKE_attribute.h
@@ -66,6 +66,11 @@ bool BKE_id_attribute_remove(struct ID *id,
                              struct CustomDataLayer *layer,
                              struct ReportList *reports);
 
+struct CustomDataLayer *BKE_id_attribute_find(const struct ID *id,
+                                              const char *name,
+                                              const int type,
+                                              const AttributeDomain domain);
+
 AttributeDomain BKE_id_attribute_domain(struct ID *id, struct CustomDataLayer *layer);
 int BKE_id_attribute_data_length(struct ID *id, struct CustomDataLayer *layer);
 bool BKE_id_attribute_required(struct ID *id, struct CustomDataLayer *layer);