1. School of Mathematics, Northwest University, Xi'an, Shaanxi, China
2. National Laboratory of Pattern Recognition, Institute of Automation, Chinese Academy of Sciences, Beijing, China
3. Department of Computing Science, University of Alberta, Edmonton, AB, Canada
4. Key Laboratory of Intelligent Perception and Image Understanding of Education Ministry of China, School of Artificial Intelligence, Xidian University, Xi'an, Shaanxi, China