Department of Computer Science and Computer Vision Center, Universitat Autònoma de Barcelona, 08193 Bellaterra (Barcelona), Catalonia, Spain;Department of Architecture, Design and Media Technology, Aalborg University, Niels Jernes Vej 14, 9220 Aalborg East, Denmark;National Laboratory of Pattern Recognition (NLPR), Institute of Automation, Chinese Academy of Sciences, 95 Zhongguancun East Road, Haidian District, Beijing 100190, PR China