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

2 files changed, 149 insertions, 0 deletions

diff --git a/source/blender/io/alembic/ABC_alembic.h b/source/blender/io/alembic/ABC_alembic.h
index ddf75aa3258..67f8aeb0a67 100644
--- a/source/blender/io/alembic/ABC_alembic.h
+++ b/source/blender/io/alembic/ABC_alembic.h
@@ -128,6 +128,16 @@ struct CacheReader *CacheReader_open_alembic_object(struct AbcArchiveHandle *han
                                                     struct Object *object,
                                                     const char *object_path);
 
+bool ABC_has_vec3_array_property_named(struct CacheReader *reader, const char *name);
+
+/* r_vertex_velocities should point to a preallocated array of num_vertices floats */
+int ABC_read_velocity_cache(struct CacheReader *reader,
+                            const char *velocity_name,
+                            float time,
+                            float fps,
+                            int num_vertices,
+                            float *r_vertex_velocities);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/source/blender/io/alembic/intern/alembic_capi.cc b/source/blender/io/alembic/intern/alembic_capi.cc
index 7cde2d4fe73..89eb9ed41a1 100644
--- a/source/blender/io/alembic/intern/alembic_capi.cc
+++ b/source/blender/io/alembic/intern/alembic_capi.cc
@@ -22,6 +22,7 @@
 
 #include <Alembic/AbcMaterial/IMaterial.h>
 
+#include "abc_axis_conversion.h"
 #include "abc_reader_archive.h"
 #include "abc_reader_camera.h"
 #include "abc_reader_curves.h"
@@ -70,7 +71,10 @@
 #include "WM_api.h"
 #include "WM_types.h"
 
+using Alembic::Abc::IV3fArrayProperty;
 using Alembic::Abc::ObjectHeader;
+using Alembic::Abc::PropertyHeader;
+using Alembic::Abc::V3fArraySamplePtr;
 using Alembic::AbcGeom::ICamera;
 using Alembic::AbcGeom::ICurves;
 using Alembic::AbcGeom::IFaceSet;
@@ -79,9 +83,11 @@ using Alembic::AbcGeom::INuPatch;
 using Alembic::AbcGeom::IObject;
 using Alembic::AbcGeom::IPoints;
 using Alembic::AbcGeom::IPolyMesh;
+using Alembic::AbcGeom::IPolyMeshSchema;
 using Alembic::AbcGeom::ISampleSelector;
 using Alembic::AbcGeom::ISubD;
 using Alembic::AbcGeom::IXform;
+using Alembic::AbcGeom::kWrapExisting;
 using Alembic::AbcGeom::MetaData;
 using Alembic::AbcMaterial::IMaterial;
 
@@ -859,3 +865,136 @@ CacheReader *CacheReader_open_alembic_object(AbcArchiveHandle *handle,
 
   return reinterpret_cast<CacheReader *>(abc_reader);
 }
+
+/* ************************************************************************** */
+
+static const PropertyHeader *get_property_header(const IPolyMeshSchema &schema, const char *name)
+{
+  const PropertyHeader *prop_header = schema.getPropertyHeader(name);
+
+  if (prop_header) {
+    return prop_header;
+  }
+
+  ICompoundProperty prop = schema.getArbGeomParams();
+
+  if (!has_property(prop, name)) {
+    return nullptr;
+  }
+
+  return prop.getPropertyHeader(name);
+}
+
+bool ABC_has_vec3_array_property_named(struct CacheReader *reader, const char *name)
+{
+  AbcObjectReader *abc_reader = reinterpret_cast<AbcObjectReader *>(reader);
+
+  if (!abc_reader) {
+    return false;
+  }
+
+  IObject iobject = abc_reader->iobject();
+
+  if (!iobject.valid()) {
+    return false;
+  }
+
+  const ObjectHeader &header = iobject.getHeader();
+
+  if (!IPolyMesh::matches(header)) {
+    return false;
+  }
+
+  IPolyMesh mesh(iobject, kWrapExisting);
+  IPolyMeshSchema schema = mesh.getSchema();
+
+  const PropertyHeader *prop_header = get_property_header(schema, name);
+
+  if (!prop_header) {
+    return false;
+  }
+
+  return IV3fArrayProperty::matches(prop_header->getMetaData());
+}
+
+static V3fArraySamplePtr get_velocity_prop(const IPolyMeshSchema &schema,
+                                           const ISampleSelector &iss,
+                                           const std::string &name)
+{
+  const PropertyHeader *prop_header = schema.getPropertyHeader(name);
+
+  if (prop_header) {
+    const IV3fArrayProperty &velocity_prop = IV3fArrayProperty(schema, name, 0);
+    return velocity_prop.getValue(iss);
+  }
+
+  ICompoundProperty prop = schema.getArbGeomParams();
+
+  if (!has_property(prop, name)) {
+    return V3fArraySamplePtr();
+  }
+
+  const IV3fArrayProperty &velocity_prop = IV3fArrayProperty(prop, name, 0);
+
+  if (velocity_prop) {
+    return velocity_prop.getValue(iss);
+  }
+
+  return V3fArraySamplePtr();
+}
+
+int ABC_read_velocity_cache(CacheReader *reader,
+                            const char *velocity_name,
+                            const float time,
+                            float velocity_scale,
+                            int num_vertices,
+                            float *r_vertex_velocities)
+{
+  AbcObjectReader *abc_reader = reinterpret_cast<AbcObjectReader *>(reader);
+
+  if (!abc_reader) {
+    return -1;
+  }
+
+  IObject iobject = abc_reader->iobject();
+
+  if (!iobject.valid()) {
+    return -1;
+  }
+
+  const ObjectHeader &header = iobject.getHeader();
+
+  if (!IPolyMesh::matches(header)) {
+    return -1;
+  }
+
+  IPolyMesh mesh(iobject, kWrapExisting);
+  IPolyMeshSchema schema = mesh.getSchema();
+  ISampleSelector sample_sel(time);
+  const IPolyMeshSchema::Sample sample = schema.getValue(sample_sel);
+
+  V3fArraySamplePtr velocities = get_velocity_prop(schema, sample_sel, velocity_name);
+
+  if (!velocities) {
+    return -1;
+  }
+
+  float vel[3];
+
+  int num_velocity_vectors = static_cast<int>(velocities->size());
+
+  if (num_velocity_vectors != num_vertices) {
+    return -1;
+  }
+
+  for (size_t i = 0; i < velocities->size(); ++i) {
+    const Imath::V3f &vel_in = (*velocities)[i];
+    copy_zup_from_yup(vel, vel_in.getValue());
+
+    mul_v3_fl(vel, velocity_scale);
+
+    copy_v3_v3(r_vertex_velocities + i * 3, vel);
+  }
+
+  return num_vertices;
+}