Methods and apparatus, including computer program products, for designating a memory region for receiving image frames from an image frame server, the image frames are from a first sequence of image frames. Each image frame in the first sequence of image frames is associated with a time position on a first timeline. A needed image frame is selected, the needed image frame associated with a second time position on a second timeline. The second time position is mapped on the second timeline to a first time position on the first timeline. The image frame server is requested to provide an image frame from the first sequence of image frames at the first time position on the first timeline.