rosbag play speed

edit

Hi All I am using rosbag play --clock myrecod.bag to play file and subscribe these three topics: "/camera/rgb/image_color", "/camera/depth/image", "/camera/depth/camera_info" and I want to process each frame of these topics by fovis (http://code.google.com/p/fovis/).my recording file is 20 fps and I have time-consuming calculation on each frame of data. There for I only able to subscribe 4 fps of my recordings.

How can I slow down the play back speed of my recordings file in order to get all frames?

Thanks in Advance Ramin