1. Universita’ degli Studi di Milano, Department of Computer Science, Via Comelico 39/41, I-20135 Milano, Italy
2. University of Trento, Department of Mathematics, Via Calepina 14, I-38122 Trento, Italy
3. Fondazione Bruno Kessler, Center for Information and Communication Technology, Technologies of Vision, Via Santa Croce 77, I-38122 Trento, Italy