1. School of Computer Science and Technology, North University of China, Taiyuan 030051, China
2. Shanxi Key Laboratory of Machine Vision and Virtual Reality, Taiyuan 030051, China
3. Shanxi Province’s Vision Information Processing and Intelligent Robot Engineering Research Center, Taiyuan 030051, China
4. Department of Computer Science, University of Warwick, Coventry CV4 7AL, UK