Key Laboratory of Intelligent Perception and Image Understanding of Ministry of Education, Xidian University, Xian, 710071, China. neouma@mail.xidian.edu.cn.|Key Laboratory of Intelligent Perception and Image Understanding of Ministry of Education, Xidian University, Xian, 710071, China.|Key Laboratory of Intelligent Perception and Image Understanding of Ministry of Education, Xidian University, Xian, 710071, China.|Department of Computer Science, University of Surrey, Guildford, GU2 7XH, UK.