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

8 files changed, 109 insertions, 143 deletions

diff --git a/source/blender/io/alembic/intern/abc_customdata.h b/source/blender/io/alembic/intern/abc_customdata.h
index e9736555ead..4fba6a2f0f7 100644
--- a/source/blender/io/alembic/intern/abc_customdata.h
+++ b/source/blender/io/alembic/intern/abc_customdata.h
@@ -122,6 +122,12 @@ void read_custom_data(const std::string &iobject_full_name,
                       const CDStreamConfig &config,
                       const Alembic::Abc::ISampleSelector &iss);
 
+void read_velocity(const Alembic::Abc::ICompoundProperty &prop,
+                   const Alembic::Abc::PropertyHeader *prop_header,
+                   const Alembic::Abc::ISampleSelector &selector,
+                   const CDStreamConfig &config,
+                   const char *velocity_name,
+                   const float velocity_scale);
 typedef enum {
   ABC_UV_SCOPE_NONE,
   ABC_UV_SCOPE_LOOP,
diff --git a/source/blender/io/alembic/intern/abc_reader_curves.cc b/source/blender/io/alembic/intern/abc_reader_curves.cc
index 27ee35d1b39..688ba23333a 100644
--- a/source/blender/io/alembic/intern/abc_reader_curves.cc
+++ b/source/blender/io/alembic/intern/abc_reader_curves.cc
@@ -283,6 +283,8 @@ void AbcCurveReader::read_curve_sample(Curve *cu,
 Mesh *AbcCurveReader::read_mesh(Mesh *existing_mesh,
                                 const ISampleSelector &sample_sel,
                                 int /*read_flag*/,
+                                const char * /*velocity_name*/,
+                                const float /*velocity_scale*/,
                                 const char **err_str)
 {
   ICurvesSchema::Sample sample;
diff --git a/source/blender/io/alembic/intern/abc_reader_curves.h b/source/blender/io/alembic/intern/abc_reader_curves.h
index 075ed5ca6a1..11b23c8a8cf 100644
--- a/source/blender/io/alembic/intern/abc_reader_curves.h
+++ b/source/blender/io/alembic/intern/abc_reader_curves.h
@@ -45,7 +45,9 @@ class AbcCurveReader : public AbcObjectReader {
   void readObjectData(Main *bmain, const Alembic::Abc::ISampleSelector &sample_sel);
   struct Mesh *read_mesh(struct Mesh *existing_mesh,
                          const Alembic::Abc::ISampleSelector &sample_sel,
-                         int read_flag,
+                         const int read_flag,
+                         const char *velocity_name,
+                         const float velocity_scale,
                          const char **err_str);
 
   void read_curve_sample(Curve *cu,
diff --git a/source/blender/io/alembic/intern/abc_reader_mesh.cc b/source/blender/io/alembic/intern/abc_reader_mesh.cc
index 77edd4908bd..dbccc8f2851 100644
--- a/source/blender/io/alembic/intern/abc_reader_mesh.cc
+++ b/source/blender/io/alembic/intern/abc_reader_mesh.cc
@@ -27,6 +27,7 @@
 
 #include "MEM_guardedalloc.h"
 
+#include "DNA_customdata_types.h"
 #include "DNA_material_types.h"
 #include "DNA_mesh_types.h"
 #include "DNA_meshdata_types.h"
@@ -36,6 +37,7 @@
 #include "BLI_listbase.h"
 #include "BLI_math_geom.h"
 
+#include "BKE_attribute.h"
 #include "BKE_main.h"
 #include "BKE_material.h"
 #include "BKE_mesh.h"
@@ -43,8 +45,10 @@
 #include "BKE_object.h"
 
 using Alembic::Abc::Int32ArraySamplePtr;
+using Alembic::Abc::IV3fArrayProperty;
 using Alembic::Abc::P3fArraySamplePtr;
 using Alembic::Abc::PropertyHeader;
+using Alembic::Abc::V3fArraySamplePtr;
 
 using Alembic::AbcGeom::IC3fGeomParam;
 using Alembic::AbcGeom::IC4fGeomParam;
@@ -420,6 +424,52 @@ static void get_weight_and_index(CDStreamConfig &config,
   config.ceil_index = i1;
 }
 
+static V3fArraySamplePtr get_velocity_prop(const ICompoundProperty &schema,
+                                           const ISampleSelector &selector,
+                                           const std::string &name)
+{
+  for (size_t i = 0; i < schema.getNumProperties(); i++) {
+    const PropertyHeader &header = schema.getPropertyHeader(i);
+
+    if (header.isCompound()) {
+      const ICompoundProperty &prop = ICompoundProperty(schema, header.getName());
+
+      if (has_property(prop, name)) {
+        const IV3fArrayProperty &velocity_prop = IV3fArrayProperty(prop, name, 0);
+        if (velocity_prop) {
+          return velocity_prop.getValue(selector);
+        }
+      }
+    }
+    else if (header.isArray()) {
+      if (header.getName() == name) {
+        const IV3fArrayProperty &velocity_prop = IV3fArrayProperty(schema, name, 0);
+        return velocity_prop.getValue(selector);
+      }
+    }
+  }
+
+  return V3fArraySamplePtr();
+}
+
+static void read_velocity(const V3fArraySamplePtr &velocities,
+                          const CDStreamConfig &config,
+                          const float velocity_scale)
+{
+  CustomDataLayer *velocity_layer = BKE_id_attribute_new(
+      &config.mesh->id, "velocity", CD_PROP_FLOAT3, ATTR_DOMAIN_POINT, NULL);
+  float(*velocity)[3] = (float(*)[3])velocity_layer->data;
+
+  const int num_velocity_vectors = static_cast<int>(velocities->size());
+  BLI_assert(num_velocity_vectors == config.mesh->totvert);
+
+  for (int i = 0; i < num_velocity_vectors; i++) {
+    const Imath::V3f &vel_in = (*velocities)[i];
+    copy_zup_from_yup(velocity[i], vel_in.getValue());
+    mul_v3_fl(velocity[i], velocity_scale);
+  }
+}
+
 static void read_mesh_sample(const std::string &iobject_full_name,
                              ImportSettings *settings,
                              const IPolyMeshSchema &schema,
@@ -458,6 +508,13 @@ static void read_mesh_sample(const std::string &iobject_full_name,
   if ((settings->read_flag & (MOD_MESHSEQ_READ_UV | MOD_MESHSEQ_READ_COLOR)) != 0) {
     read_custom_data(iobject_full_name, schema.getArbGeomParams(), config, selector);
   }
+
+  if (!settings->velocity_name.empty() && settings->velocity_scale != 0.0f) {
+    V3fArraySamplePtr velocities = get_velocity_prop(schema, selector, settings->velocity_name);
+    if (velocities) {
+      read_velocity(velocities, config, settings->velocity_scale);
+    }
+  }
 }
 
 CDStreamConfig get_config(Mesh *mesh, const bool use_vertex_interpolation)
@@ -563,7 +620,7 @@ void AbcMeshReader::readObjectData(Main *bmain, const Alembic::Abc::ISampleSelec
   m_object = BKE_object_add_only_object(bmain, OB_MESH, m_object_name.c_str());
   m_object->data = mesh;
 
-  Mesh *read_mesh = this->read_mesh(mesh, sample_sel, MOD_MESHSEQ_READ_ALL, nullptr);
+  Mesh *read_mesh = this->read_mesh(mesh, sample_sel, MOD_MESHSEQ_READ_ALL, "", 0.0f, nullptr);
   if (read_mesh != mesh) {
     /* XXX FIXME: after 2.80; mesh->flag isn't copied by #BKE_mesh_nomain_to_mesh(). */
     /* read_mesh can be freed by BKE_mesh_nomain_to_mesh(), so get the flag before that happens. */
@@ -630,7 +687,9 @@ bool AbcMeshReader::topology_changed(Mesh *existing_mesh, const ISampleSelector
 
 Mesh *AbcMeshReader::read_mesh(Mesh *existing_mesh,
                                const ISampleSelector &sample_sel,
-                               int read_flag,
+                               const int read_flag,
+                               const char *velocity_name,
+                               const float velocity_scale,
                                const char **err_str)
 {
   IPolyMeshSchema::Sample sample;
@@ -673,6 +732,8 @@ Mesh *AbcMeshReader::read_mesh(Mesh *existing_mesh,
   /* Only read point data when streaming meshes, unless we need to create new ones. */
   ImportSettings settings;
   settings.read_flag |= read_flag;
+  settings.velocity_name = velocity_name;
+  settings.velocity_scale = velocity_scale;
 
   if (topology_changed(existing_mesh, sample_sel)) {
     new_mesh = BKE_mesh_new_nomain_from_template(
@@ -829,6 +890,13 @@ static void read_subd_sample(const std::string &iobject_full_name,
   if ((settings->read_flag & (MOD_MESHSEQ_READ_UV | MOD_MESHSEQ_READ_COLOR)) != 0) {
     read_custom_data(iobject_full_name, schema.getArbGeomParams(), config, selector);
   }
+
+  if (!settings->velocity_name.empty() && settings->velocity_scale != 0.0f) {
+    V3fArraySamplePtr velocities = get_velocity_prop(schema, selector, settings->velocity_name);
+    if (velocities) {
+      read_velocity(velocities, config, settings->velocity_scale);
+    }
+  }
 }
 
 /* ************************************************************************** */
@@ -876,7 +944,7 @@ void AbcSubDReader::readObjectData(Main *bmain, const Alembic::Abc::ISampleSelec
   m_object = BKE_object_add_only_object(bmain, OB_MESH, m_object_name.c_str());
   m_object->data = mesh;
 
-  Mesh *read_mesh = this->read_mesh(mesh, sample_sel, MOD_MESHSEQ_READ_ALL, nullptr);
+  Mesh *read_mesh = this->read_mesh(mesh, sample_sel, MOD_MESHSEQ_READ_ALL, "", 0.0f, nullptr);
   if (read_mesh != mesh) {
     BKE_mesh_nomain_to_mesh(read_mesh, mesh, m_object, &CD_MASK_EVERYTHING, true);
   }
@@ -935,7 +1003,9 @@ void AbcSubDReader::readObjectData(Main *bmain, const Alembic::Abc::ISampleSelec
 
 Mesh *AbcSubDReader::read_mesh(Mesh *existing_mesh,
                                const ISampleSelector &sample_sel,
-                               int read_flag,
+                               const int read_flag,
+                               const char *velocity_name,
+                               const float velocity_scale,
                                const char **err_str)
 {
   ISubDSchema::Sample sample;
@@ -962,6 +1032,8 @@ Mesh *AbcSubDReader::read_mesh(Mesh *existing_mesh,
 
   ImportSettings settings;
   settings.read_flag |= read_flag;
+  settings.velocity_name = velocity_name;
+  settings.velocity_scale = velocity_scale;
 
   if (existing_mesh->totvert != positions->size()) {
     new_mesh = BKE_mesh_new_nomain_from_template(
diff --git a/source/blender/io/alembic/intern/abc_reader_mesh.h b/source/blender/io/alembic/intern/abc_reader_mesh.h
index 3329b723b77..d9f89cc8085 100644
--- a/source/blender/io/alembic/intern/abc_reader_mesh.h
+++ b/source/blender/io/alembic/intern/abc_reader_mesh.h
@@ -42,7 +42,9 @@ class AbcMeshReader : public AbcObjectReader {
 
   struct Mesh *read_mesh(struct Mesh *existing_mesh,
                          const Alembic::Abc::ISampleSelector &sample_sel,
-                         int read_flag,
+                         const int read_flag,
+                         const char *velocity_name,
+                         const float velocity_scale,
                          const char **err_str) override;
   bool topology_changed(Mesh *existing_mesh,
                         const Alembic::Abc::ISampleSelector &sample_sel) override;
@@ -73,7 +75,9 @@ class AbcSubDReader : public AbcObjectReader {
   void readObjectData(Main *bmain, const Alembic::Abc::ISampleSelector &sample_sel);
   struct Mesh *read_mesh(struct Mesh *existing_mesh,
                          const Alembic::Abc::ISampleSelector &sample_sel,
-                         int read_flag,
+                         const int read_flag,
+                         const char *velocity_name,
+                         const float velocity_scale,
                          const char **err_str);
 };
 
diff --git a/source/blender/io/alembic/intern/abc_reader_object.cc b/source/blender/io/alembic/intern/abc_reader_object.cc
index 9a5ffd04bd1..a6d46c4b42a 100644
--- a/source/blender/io/alembic/intern/abc_reader_object.cc
+++ b/source/blender/io/alembic/intern/abc_reader_object.cc
@@ -167,6 +167,8 @@ Imath::M44d get_matrix(const IXformSchema &schema, const float time)
 struct Mesh *AbcObjectReader::read_mesh(struct Mesh *existing_mesh,
                                         const Alembic::Abc::ISampleSelector &UNUSED(sample_sel),
                                         int UNUSED(read_flag),
+                                        const char *UNUSED(velocity_name),
+                                        const float UNUSED(velocity_scale),
                                         const char **UNUSED(err_str))
 {
   return existing_mesh;
diff --git a/source/blender/io/alembic/intern/abc_reader_object.h b/source/blender/io/alembic/intern/abc_reader_object.h
index 89590b26b61..6e97f841a88 100644
--- a/source/blender/io/alembic/intern/abc_reader_object.h
+++ b/source/blender/io/alembic/intern/abc_reader_object.h
@@ -50,6 +50,10 @@ struct ImportSettings {
   /* From MeshSeqCacheModifierData.read_flag */
   int read_flag;
 
+  /* From CacheFile and MeshSeqCacheModifierData */
+  std::string velocity_name;
+  float velocity_scale;
+
   bool validate_meshes;
   bool always_add_cache_reader;
 
@@ -65,6 +69,8 @@ struct ImportSettings {
         sequence_len(1),
         sequence_offset(0),
         read_flag(0),
+        velocity_name(""),
+        velocity_scale(1.0f),
         validate_meshes(false),
         always_add_cache_reader(false),
         cache_file(NULL)
@@ -143,7 +149,9 @@ class AbcObjectReader {
 
   virtual struct Mesh *read_mesh(struct Mesh *mesh,
                                  const Alembic::Abc::ISampleSelector &sample_sel,
-                                 int read_flag,
+                                 const int read_flag,
+                                 const char *velocity_name,
+                                 const float velocity_scale,
                                  const char **err_str);
   virtual bool topology_changed(Mesh *existing_mesh,
                                 const Alembic::Abc::ISampleSelector &sample_sel);
diff --git a/source/blender/io/alembic/intern/alembic_capi.cc b/source/blender/io/alembic/intern/alembic_capi.cc
index deb945b767c..63565b902f3 100644
--- a/source/blender/io/alembic/intern/alembic_capi.cc
+++ b/source/blender/io/alembic/intern/alembic_capi.cc
@@ -786,7 +786,9 @@ Mesh *ABC_read_mesh(CacheReader *reader,
                     Mesh *existing_mesh,
                     const float time,
                     const char **err_str,
-                    int read_flag)
+                    const int read_flag,
+                    const char *velocity_name,
+                    const float velocity_scale)
 {
   AbcObjectReader *abc_reader = get_abc_reader(reader, ob, err_str);
   if (abc_reader == nullptr) {
@@ -794,7 +796,8 @@ Mesh *ABC_read_mesh(CacheReader *reader,
   }
 
   ISampleSelector sample_sel = sample_selector_for_time(time);
-  return abc_reader->read_mesh(existing_mesh, sample_sel, read_flag, err_str);
+  return abc_reader->read_mesh(
+      existing_mesh, sample_sel, read_flag, velocity_name, velocity_scale, err_str);
 }
 
 bool ABC_mesh_topology_changed(
@@ -860,136 +863,3 @@ CacheReader *CacheReader_open_alembic_object(CacheArchiveHandle *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;
-}