Abstract: A centralised game server in a bank (50) of game servers runs a game program for use by a user at a remote terminal (52, 56, 58). In the game server, the game program sends a first set of graphics instructions to a first graphics processing unit (76) which is intercepted by an instruction interception module (74). The first set of instructions, including vertex data, transformation data and texture data are passed to the first graphics processing unit (76) whilst a specially manipulated version of the instructions is generated and passed to a second graphics processing unit (78). The first graphics processing unit (76) renders the image data as the game intended whilst the second graphics processing unit (78) is used to render specially adapted graphics data from which to extract compression assistance data used for compression, e.g. motion vectors.