Department of Computing Science, Katholicke Universiteit Leuven, Celestijnenlaan 200 A, B-3001 Leuven (Heverlee) Belgium;Department of Computing Science, Katholicke Universiteit Leuven, Celestijnenlaan 200 A, B-3001 Leuven (Heverlee) Belgium;AudioVisual Centre, University College London, Windeyer Building, Cleveland Street, London WIP 6DB, England;AudioVisual Centre, University College London, Windeyer Building, Cleveland Street, London WIP 6DB, England;Department of Computer Science, University College London, Gower Street, London WC1E 6BT, England;Department of Computer Science, University College London, Gower Street, London WC1E 6BT, England;Imperial College of Science, Technology and Medicine, Royal School of Mines, Centre for Computing Services, Prince Consort Road, London SW7 2BP, England