Cycles: add support for rendering deformation motion blur from Alembic caches.

This patch adds the ability to render motion blur from Alembic caches.
The motion blur data is derived from a velocity attribute whose name has
to be defined by the user through the MeshSequenceCache modifier, with a
default value of ".velocities", which is the standard name in Alembic
for the velocity property, although other software may ignore it and
write velocity with their own naming convention (e.g. "v" in Houdini).

Furthermore, a property was added to define how the velocity vectors
are interpreted with regard to time : frame or second. "Frame"
means that the velocity is already scaled by the time step and we do not
need to modify it for it to look proper. "Second" means that the unit
the velocity was measured in is in seconds and so has to be scaled by
some time step computed here as being the time between two frames (1 /
FPS, which would be typical for a simulation). This appears to be
common, and is the default behavior.

Another property was added to control the scale of the velocity to
further modify the look of the motion blur.

Reviewed By: brecht, sybren

Differential Revision: https://developer.blender.org/D2388

1 files changed, 27 insertions, 0 deletions

diff --git a/source/blender/makesrna/intern/rna_cachefile.c b/source/blender/makesrna/intern/rna_cachefile.c
index f9275ef1993..00cd95c4492 100644
--- a/source/blender/makesrna/intern/rna_cachefile.c
+++ b/source/blender/makesrna/intern/rna_cachefile.c
@@ -174,6 +174,33 @@ static void rna_def_cachefile(BlenderRNA *brna)
   RNA_def_property_ui_text(
       prop, "Object Paths", "Paths of the objects inside the Alembic archive");
 
+  /* ----------------- Alembic Velocity Attribute ----------------- */
+
+  prop = RNA_def_property(srna, "velocity_name", PROP_STRING, PROP_NONE);
+  RNA_def_property_ui_text(prop,
+                           "Velocity Attribute",
+                           "Name of the Alembic attribute used for generating motion blur data");
+  RNA_def_struct_name_property(srna, prop);
+  RNA_def_property_update(prop, 0, "rna_CacheFile_update");
+  RNA_def_property_clear_flag(prop, PROP_ANIMATABLE);
+
+  static const EnumPropertyItem velocity_unit_items[] = {
+      {CACHEFILE_VELOCITY_UNIT_SECOND, "SECOND", 0, "Second", ""},
+      {CACHEFILE_VELOCITY_UNIT_FRAME, "FRAME", 0, "Frame", ""},
+      {0, NULL, 0, NULL, NULL},
+  };
+
+  prop = RNA_def_property(srna, "velocity_unit", PROP_ENUM, PROP_NONE);
+  RNA_def_property_enum_sdna(prop, NULL, "velocity_unit");
+  RNA_def_property_enum_items(prop, velocity_unit_items);
+  RNA_def_property_ui_text(
+      prop,
+      "Velocity Unit",
+      "Define how the velocity vectors are interpreted with regard to time, 'frame' means "
+      "the delta time is 1 frame, 'second' means the delta time is 1 / FPS");
+  RNA_def_property_update(prop, 0, "rna_CacheFile_update");
+  RNA_def_property_clear_flag(prop, PROP_ANIMATABLE);
+
   RNA_define_lib_overridable(false);
 
   rna_def_cachefile_object_paths(brna, prop);