1. Guangxi Key Laboratory of Multimedia Communications and Network Technology, School of Computer, Electronics and Information, Guangxi University, Nanning 530004, China
2. Department of Computer Science, National University of Singapore, Singapore 119077, Singapore
3. Department of Computer Science and Engineering, Hong Kong University of Science and Technology, Hong Kong 999077, China
4. School of Computer Science and Technology, Shandong University, Qingdao 266237, China