University of the Aegean, Department of Cultural Technology and Communication, Mytilene, GR-81100,Greece;University of Bologna, Department of Informatics, Electronics and Systems, V.le Risorgimento, 2 - 40136 Bologna, Italy;Hong Kong Polytechnic University, Department of Computing, Hung Hom, Kowloon, Hong Kong;INRIA Paris - Rocquencourt, Domaine de Voluceau, B.P.105, 78153 Le Chesnay, France