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, 8 deletions

diff --git a/source/blender/makesdna/DNA_fluid_types.h b/source/blender/makesdna/DNA_fluid_types.h
index 835af3c6ff8..0cbef540306 100644
--- a/source/blender/makesdna/DNA_fluid_types.h
+++ b/source/blender/makesdna/DNA_fluid_types.h
@@ -480,10 +480,6 @@ enum {
   SM_HRES_FULLSAMPLE = 2,
 };
 
-typedef struct FluidDomainVertexVelocity {
-  float vel[3];
-} FluidDomainVertexVelocity;
-
 typedef struct FluidDomainSettings {
 
   /* -- Runtime-only fields (from here on). -- */
@@ -509,8 +505,6 @@ typedef struct FluidDomainSettings {
   struct GPUTexture *tex_flags;
   struct GPUTexture *tex_range_field;
   struct Object *guide_parent;
-  /** Vertex velocities of simulated fluid mesh. */
-  struct FluidDomainVertexVelocity *mesh_velocities;
   struct EffectorWeights *effector_weights;
 
   /* Domain object data. */
@@ -607,9 +601,8 @@ typedef struct FluidDomainSettings {
   int mesh_smoothen_pos;
   int mesh_smoothen_neg;
   int mesh_scale;
-  int totvert;
   short mesh_generator;
-  char _pad6[6]; /* Unused. */
+  char _pad6[2]; /* Unused. */
 
   /* Secondary particle options. */
   int particle_type;