@lcrs has a post on it: http://lewisinthelandofmachines.tumblr.com/post/159532447318/houdini-2d-motion-vectors-from-mantra-2d
@lcrs has a post on it: http://lewisinthelandofmachines.tumblr.com/post/159532447318/houdini-2d-motion-vectors-from-mantra-2d