STARLab, Vrije Universiteit Brussel (VUB), Bldg G/10, Pleinlaan 2, Brussels, Belgium;School of Computer Science and Information Technology, RMIT University, Bld 10.10, 376-392 Swanston Street, Melbourne, VIC, Australia;Facultad de Informática, Universidad Politécnica de Madrid, Campus de Montegancedo S/N, Boadilla del Monte, Madrid, VIC, Spain