1. School of Artificial Intelligence, Optics and Electronics (iOPEN), Northwestern Polytechnical University, Xi’an, China
2. Department of Computer Science, Colorado School of Mines, Golden, CO, USA
3. Department of Electrical and Computer Engineering, University of Pittsburgh, Pittsburgh, PA, USA
4. National Engineering Laboratory for Visual Information Processing and Applications and the Institute of Artificial Intelligence and Robotics, Xi’an Jiaotong University, Xi’an, China