Cycles: support rendering PointCloud motion blur from attribute

This adds support to render PointCloud motion blur from a standard
"velocity" attribute.

This implementation is similar to that of the Mesh geometry, and
perhaps some code could be deduplicated through a more generic API.
`mesh_need_motion_attribute` was renamed `object_need_motion_attribute`
as it does not really require a mesh and moved to `util.h` so that
it can be shared.

This fixes T94622.

Reviewed By: brecht

Maniphest Tasks: T94622

Differential Revision: https://developer.blender.org/D13719

intern/cycles/blender/mesh.cpp

@@ -1086,40 +1086,6 @@ static void create_subd_mesh(Scene *scene,
 
 /* Sync */
 
-/* Check whether some of "built-in" motion-related attributes are needed to be exported (includes
- * things like velocity from cache modifier, fluid simulation).
- *
- * NOTE: This code is run prior to object motion blur initialization. so can not access properties
- * set by `sync_object_motion_init()`. */
-static bool mesh_need_motion_attribute(BObjectInfo &b_ob_info, Scene *scene)
-{
-  const Scene::MotionType need_motion = scene->need_motion();
-  if (need_motion == Scene::MOTION_NONE) {
-    /* Simple case: neither motion pass nor motion blur is needed, no need in the motion related
-     * attributes. */
-    return false;
-  }
-
-  if (need_motion == Scene::MOTION_BLUR) {
-    /* A bit tricky and implicit case:
-     * - Motion blur is enabled in the scene, which implies specific number of time steps for
-     *   objects.
-     * - If the object has motion blur disabled on it, it will have 0 time steps.
-     * - Motion attribute expects non-zero time steps.
-     *
-     * Avoid adding motion attributes if the motion blur will enforce 0 motion steps. */
-    PointerRNA cobject = RNA_pointer_get(&b_ob_info.real_object.ptr, "cycles");
-    const bool use_motion = get_boolean(cobject, "use_motion_blur");
-    if (!use_motion) {
-      return false;
-    }
-  }
-
-  /* Motion pass which implies 3 motion steps, or motion blur which is not disabled on object
-   * level. */
-  return true;
-}
-
 void BlenderSync::sync_mesh(BL::Depsgraph b_depsgraph, BObjectInfo &b_ob_info, Mesh *mesh)
 {
   /* make a copy of the shaders as the caller in the main thread still need them for syncing the
@@ -1144,7 +1110,7 @@ void BlenderSync::sync_mesh(BL::Depsgraph b_depsgraph, BObjectInfo &b_ob_info, M
 
     if (b_mesh) {
       /* Motion blur attribute is relative to seconds, we need it relative to frames. */
-      const bool need_motion = mesh_need_motion_attribute(b_ob_info, scene);
+      const bool need_motion = object_need_motion_attribute(b_ob_info, scene);
       const float motion_scale = (need_motion) ?
                                      scene->motion_shutter_time() /
                                          (b_scene.render().fps() / b_scene.render().fps_base()) :

intern/cycles/blender/pointcloud.cpp

@@ -37,12 +37,52 @@ static void fill_generic_attribute(BL::PointCloud &b_pointcloud,
   }
 }
 
-static void copy_attributes(PointCloud *pointcloud, BL::PointCloud b_pointcloud)
+static void attr_create_motion(PointCloud *pointcloud,
+                               BL::Attribute &b_attribute,
+                               const float motion_scale)
+{
+  if (!(b_attribute.domain() == BL::Attribute::domain_POINT) &&
+      (b_attribute.data_type() == BL::Attribute::data_type_FLOAT_VECTOR)) {
+    return;
+  }
+
+  BL::FloatVectorAttribute b_vector_attribute(b_attribute);
+  const int num_points = pointcloud->get_points().size();
+
+  /* Find or add attribute */
+  float3 *P = &pointcloud->get_points()[0];
+  Attribute *attr_mP = pointcloud->attributes.find(ATTR_STD_MOTION_VERTEX_POSITION);
+
+  if (!attr_mP) {
+    attr_mP = pointcloud->attributes.add(ATTR_STD_MOTION_VERTEX_POSITION);
+  }
+
+  /* Only export previous and next frame, we don't have any in between data. */
+  float motion_times[2] = {-1.0f, 1.0f};
+  for (int step = 0; step < 2; step++) {
+    const float relative_time = motion_times[step] * 0.5f * motion_scale;
+    float3 *mP = attr_mP->data_float3() + step * num_points;
+
+    for (int i = 0; i < num_points; i++) {
+      mP[i] = P[i] + get_float3(b_vector_attribute.data[i].vector()) * relative_time;
+    }
+  }
+}
+
+static void copy_attributes(PointCloud *pointcloud,
+                            BL::PointCloud b_pointcloud,
+                            const bool need_motion,
+                            const float motion_scale)
 {
   AttributeSet &attributes = pointcloud->attributes;
+  static const ustring u_velocity("velocity");
   for (BL::Attribute &b_attribute : b_pointcloud.attributes) {
     const ustring name{b_attribute.name().c_str()};
 
+    if (need_motion && name == u_velocity) {
+      attr_create_motion(pointcloud, b_attribute, motion_scale);
+    }
+
     if (attributes.find(name)) {
       continue;
     }
@@ -111,7 +151,11 @@ static void copy_attributes(PointCloud *pointcloud, BL::PointCloud b_pointcloud)
   }
 }
 
-static void export_pointcloud(Scene *scene, PointCloud *pointcloud, BL::PointCloud b_pointcloud)
+static void export_pointcloud(Scene *scene,
+                              PointCloud *pointcloud,
+                              BL::PointCloud b_pointcloud,
+                              const bool need_motion,
+                              const float motion_scale)
 {
   /* TODO: optimize so we can straight memcpy arrays from Blender? */
 
@@ -141,7 +185,7 @@ static void export_pointcloud(Scene *scene, PointCloud *pointcloud, BL::PointClo
   }
 
   /* Export attributes */
-  copy_attributes(pointcloud, b_pointcloud);
+  copy_attributes(pointcloud, b_pointcloud, need_motion, motion_scale);
 }
 
 static void export_pointcloud_motion(PointCloud *pointcloud,
@@ -193,7 +237,7 @@ static void export_pointcloud_motion(PointCloud *pointcloud,
   }
 
   /* Export attributes */
-  copy_attributes(pointcloud, b_pointcloud);
+  copy_attributes(pointcloud, b_pointcloud, false, 0.0f);
 }
 
 void BlenderSync::sync_pointcloud(PointCloud *pointcloud, BObjectInfo &b_ob_info)
@@ -207,7 +251,13 @@ void BlenderSync::sync_pointcloud(PointCloud *pointcloud, BObjectInfo &b_ob_info
 
   /* TODO: add option to filter out points in the view layer. */
   BL::PointCloud b_pointcloud(b_ob_info.object_data);
-  export_pointcloud(scene, &new_pointcloud, b_pointcloud);
+  /* Motion blur attribute is relative to seconds, we need it relative to frames. */
+  const bool need_motion = object_need_motion_attribute(b_ob_info, scene);
+  const float motion_scale = (need_motion) ?
+                                 scene->motion_shutter_time() /
+                                     (b_scene.render().fps() / b_scene.render().fps_base()) :
+                                 0.0f;
+  export_pointcloud(scene, &new_pointcloud, b_pointcloud, need_motion, motion_scale);
 
   /* update original sockets */
   for (const SocketType &socket : new_pointcloud.type->inputs) {

intern/cycles/blender/util.h

@@ -18,6 +18,7 @@
 #define __BLENDER_UTIL_H__
 
 #include "scene/mesh.h"
+#include "scene/scene.h"
 
 #include "util/algorithm.h"
 #include "util/array.h"
@@ -670,6 +671,40 @@ static inline uint object_ray_visibility(BL::Object &b_ob)
   return flag;
 }
 
+/* Check whether some of "built-in" motion-related attributes are needed to be exported (includes
+ * things like velocity from cache modifier, fluid simulation).
+ *
+ * NOTE: This code is run prior to object motion blur initialization. so can not access properties
+ * set by `sync_object_motion_init()`. */
+static bool object_need_motion_attribute(BObjectInfo &b_ob_info, Scene *scene)
+{
+  const Scene::MotionType need_motion = scene->need_motion();
+  if (need_motion == Scene::MOTION_NONE) {
+    /* Simple case: neither motion pass nor motion blur is needed, no need in the motion related
+     * attributes. */
+    return false;
+  }
+
+  if (need_motion == Scene::MOTION_BLUR) {
+    /* A bit tricky and implicit case:
+     * - Motion blur is enabled in the scene, which implies specific number of time steps for
+     *   objects.
+     * - If the object has motion blur disabled on it, it will have 0 time steps.
+     * - Motion attribute expects non-zero time steps.
+     *
+     * Avoid adding motion attributes if the motion blur will enforce 0 motion steps. */
+    PointerRNA cobject = RNA_pointer_get(&b_ob_info.real_object.ptr, "cycles");
+    const bool use_motion = get_boolean(cobject, "use_motion_blur");
+    if (!use_motion) {
+      return false;
+    }
+  }
+
+  /* Motion pass which implies 3 motion steps, or motion blur which is not disabled on object
+   * level. */
+  return true;
+}
+
 class EdgeMap {
  public:
   EdgeMap()