1 files changed, 35 insertions, 0 deletions
diff --git a/intern/cycles/blender/util.h b/intern/cycles/blender/util.h
index 6c396d39614..ae9cdfd4f58 100644
--- a/intern/cycles/blender/util.h
+++ b/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()