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

4 files changed, 1 insertions, 37 deletions

diff --git a/source/blender/makesdna/DNA_fluid_defaults.h b/source/blender/makesdna/DNA_fluid_defaults.h
index 95f5b8b66b0..4135c4d40a8 100644
--- a/source/blender/makesdna/DNA_fluid_defaults.h
+++ b/source/blender/makesdna/DNA_fluid_defaults.h
@@ -50,7 +50,6 @@
     .tex_flags = NULL, \
     .tex_range_field = NULL, \
     .guide_parent = NULL, \
-    .mesh_velocities = NULL, \
     .effector_weights = NULL, /* #BKE_effector_add_weights. */ \
     .p0 = {0.0f, 0.0f, 0.0f}, \
     .p1 = {0.0f, 0.0f, 0.0f}, \
@@ -122,7 +121,6 @@
     .mesh_smoothen_pos = 1, \
     .mesh_smoothen_neg = 1, \
     .mesh_scale = 2, \
-    .totvert = 0, \
     .mesh_generator = FLUID_DOMAIN_MESH_IMPROVED, \
     .particle_type = 0, \
     .particle_scale = 1, \
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;
diff --git a/source/blender/makesdna/DNA_modifier_defaults.h b/source/blender/makesdna/DNA_modifier_defaults.h
index 1b3dbd148df..5b2694f420b 100644
--- a/source/blender/makesdna/DNA_modifier_defaults.h
+++ b/source/blender/makesdna/DNA_modifier_defaults.h
@@ -419,10 +419,6 @@
     .velocity_scale = 1.0f, \
     .reader = NULL, \
     .reader_object_path = "", \
-    .vertex_velocities = NULL, \
-    .num_vertices = 0, \
-    .velocity_delta = 0.0f, \
-    .last_lookup_time = 0.0f, \
   }
 
 #define _DNA_DEFAULT_MirrorModifierData \
diff --git a/source/blender/makesdna/DNA_modifier_types.h b/source/blender/makesdna/DNA_modifier_types.h
index 8520786a030..31daa778b03 100644
--- a/source/blender/makesdna/DNA_modifier_types.h
+++ b/source/blender/makesdna/DNA_modifier_types.h
@@ -2144,10 +2144,6 @@ enum {
   MOD_NORMALEDIT_MIX_MUL = 3,
 };
 
-typedef struct MeshCacheVertexVelocity {
-  float vel[3];
-} MeshCacheVertexVelocity;
-
 typedef struct MeshSeqCacheModifierData {
   ModifierData modifier;
 
@@ -2163,25 +2159,6 @@ typedef struct MeshSeqCacheModifierData {
   /* Runtime. */
   struct CacheReader *reader;
   char reader_object_path[1024];
-
-  /* Vertex velocities read from the cache. The velocities are not automatically read during
-   * modifier execution, and therefore have to manually be read when needed. This is only used
-   * through the RNA for now. */
-  struct MeshCacheVertexVelocity *vertex_velocities;
-
-  /* The number of vertices of the Alembic mesh, set when the modifier is executed. */
-  int num_vertices;
-
-  /* Time (in frames or seconds) between two velocity samples. Automatically computed to
-   * scale the velocity vectors at render time for generating proper motion blur data. */
-  float velocity_delta;
-
-  /* Caches the scene time (in seconds) used to lookup data in the Alembic archive when the
-   * modifier was last executed. Used to access Alembic samples through the RNA. */
-  float last_lookup_time;
-
-  int _pad1;
-  void *_pad2;
 } MeshSeqCacheModifierData;
 
 /* MeshSeqCacheModifierData.read_flag */