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

1 files changed, 24 insertions, 1 deletions

diff --git a/source/blender/blenkernel/intern/attribute.c b/source/blender/blenkernel/intern/attribute.c
index e9444cf88a6..ee8ef5e97f7 100644
--- a/source/blender/blenkernel/intern/attribute.c
+++ b/source/blender/blenkernel/intern/attribute.c
@@ -51,7 +51,7 @@ typedef struct DomainInfo {
   int length;
 } DomainInfo;
 
-static void get_domains(ID *id, DomainInfo info[ATTR_DOMAIN_NUM])
+static void get_domains(const ID *id, DomainInfo info[ATTR_DOMAIN_NUM])
 {
   memset(info, 0, sizeof(DomainInfo) * ATTR_DOMAIN_NUM);
 
@@ -223,6 +223,29 @@ bool BKE_id_attribute_remove(ID *id, CustomDataLayer *layer, ReportList *reports
   return true;
 }
 
+CustomDataLayer *BKE_id_attribute_find(const ID *id,
+                                       const char *name,
+                                       const int type,
+                                       const AttributeDomain domain)
+{
+  DomainInfo info[ATTR_DOMAIN_NUM];
+  get_domains(id, info);
+
+  CustomData *customdata = info[domain].customdata;
+  if (customdata == NULL) {
+    return NULL;
+  }
+
+  for (int i = 0; i < customdata->totlayer; i++) {
+    CustomDataLayer *layer = &customdata->layers[i];
+    if (layer->type == type && STREQ(layer->name, name)) {
+      return layer;
+    }
+  }
+
+  return NULL;
+}
+
 int BKE_id_attributes_length(ID *id, const CustomDataMask mask)
 {
   DomainInfo info[ATTR_DOMAIN_NUM];