Cycles: add support for volume motion blur

This adds support for rendering motion blur for volumes, using their
velocity field. This works for fluid simulations and imported VDB
volumes. For the latter, the name of the velocity field can be set per
volume object, with automatic detection of velocity fields that are
split into 3 scalar grids.

A new parameter is also added to scale velocity for more artistic control.

Like for Alembic and USD caches, a parameter to set the unit of time in
which the velocity vectors are expressed is also added. For Blender gas
simulations, the velocity unit should always be in seconds, so this is
only exposed for volume objects which may come from external OpenVDB
files.

These parameters are available under the `Render` panels for the fluid
domain and the volume object data properties respectively.

Credits: kernel advection code from Tangent Animation's Blackbird based
on earlier work by Geraldine Chua

Differential Revision: https://developer.blender.org/D14629

4 files changed, 24 insertions, 2 deletions

diff --git a/source/blender/makesdna/DNA_fluid_defaults.h b/source/blender/makesdna/DNA_fluid_defaults.h
index fd48585792f..90a91c6c995 100644
--- a/source/blender/makesdna/DNA_fluid_defaults.h
+++ b/source/blender/makesdna/DNA_fluid_defaults.h
@@ -187,6 +187,7 @@
     .cache_comp = SM_CACHE_LIGHT, \
     .cache_high_comp = SM_CACHE_LIGHT, \
     .cache_file_format = 0, \
+    .velocity_scale = 1.0f, \
   }
 
 /** \} */
diff --git a/source/blender/makesdna/DNA_fluid_types.h b/source/blender/makesdna/DNA_fluid_types.h
index 11780d99af8..5a1636879bb 100644
--- a/source/blender/makesdna/DNA_fluid_types.h
+++ b/source/blender/makesdna/DNA_fluid_types.h
@@ -670,7 +670,10 @@ typedef struct FluidDomainSettings {
   char interp_method;
   char gridlines_color_field; /* Simulation field used to color map onto gridlines. */
   char gridlines_cell_filter;
-  char _pad10[7]; /* Unused. */
+  char _pad10[3]; /* Unused. */
+
+  /* Velocity factor for motion blur rendering. */
+  float velocity_scale;
 
   /* OpenVDB cache options. */
   int openvdb_compression;
diff --git a/source/blender/makesdna/DNA_volume_defaults.h b/source/blender/makesdna/DNA_volume_defaults.h
index 2025d664d40..ee98f0ea4fd 100644
--- a/source/blender/makesdna/DNA_volume_defaults.h
+++ b/source/blender/makesdna/DNA_volume_defaults.h
@@ -36,7 +36,8 @@
     .frame_duration = 0, \
     .display = _DNA_DEFAULT_VolumeDisplay, \
     .render = _DNA_DEFAULT_VolumeRender, \
-  }
+    .velocity_scale = 1.0f, \
+}
 
 /** \} */
 
diff --git a/source/blender/makesdna/DNA_volume_types.h b/source/blender/makesdna/DNA_volume_types.h
index f2f53bc910f..a2e558aa790 100644
--- a/source/blender/makesdna/DNA_volume_types.h
+++ b/source/blender/makesdna/DNA_volume_types.h
@@ -24,6 +24,11 @@ typedef struct Volume_Runtime {
 
   /** Default simplify level for volume grids loaded from files. */
   int default_simplify_level;
+
+  /* Names for scalar grids which would need to be merged to recompose the velocity grid. */
+  char velocity_x_grid[64];
+  char velocity_y_grid[64];
+  char velocity_z_grid[64];
 } Volume_Runtime;
 
 typedef struct VolumeDisplay {
@@ -75,6 +80,18 @@ typedef struct Volume {
   VolumeRender render;
   VolumeDisplay display;
 
+  /* Velocity field name. */
+  char velocity_grid[64];
+
+  char _pad3[3];
+
+  /* Unit of time the velocity vectors are expressed in.
+   * This uses the same enumeration values as #CacheFile.velocity_unit. */
+  char velocity_unit;
+
+  /* Factor for velocity vector for artistic control. */
+  float velocity_scale;
+
   /* Draw Cache */
   void *batch_cache;