1. Department of Computer Science, Chongqing University, Chongqing, 430044, China
2. Department of Computer Science, University of Reading, P O Box 225, Whiteknights, Reading, Berkshire, RG6 6AY, UK
3. Department of Computer Science, Shenyang Institute of Technology, Shenyang, China
4. Institute of Multimedia Network and Telecommunication Engineering, Wuhan Technological University of Surveying and Mapping, Wuhan, 430079, China