[1] Department of Computer Science, University of Kentucky, Lexington, KY 40506-0046, United States
[2] Computer Science Division-EECS, University of California, Berkeley, CA 94720-1776, United States
[3] Department of Computer Science, Dept. of Optom. and Vision Science, University of California, Berkeley, CA, United States
[4] Tsinghua University, Beijing, China