record kinect bag file, reconstruct disparity topic

Hi, I have followed following tutorial [0] to record Kinect data efficiently. It reconstructs the /camera/depth_registered/points topic properly but I also need the topic /camera/depth_registered/disparity. Is it possible to reconstruct this topic from the raw topics, or do I have to record it explicitly? Cheers, Oier [0] http://www.ros.org/wiki/openni_launch/Tutorials/BagRecordingPlayback