Department of Computer Science and Information Engineering, University of Trento, via Sommarive 5, I-38123 (Povo) Trento, Italy. passerini@disi.unitn.it.|School of Computer Science and Technology, Xidian University, Xian, PR China. jyzhang@mail.xidian.edu.cn.|School of Computer Science and Technology, Xidian University, Xian, PR China. jyzhang@mail.xidian.edu.cn.