Department of Computer Science, National Tsing-Hua University, Hsinchu, Taiwan, ROC. hongjie@iis.sinica.edu.tw|Department of Computer Science, National Tsing-Hua University, Hsinchu, Taiwan, ROC. hongjie@iis.sinica.edu.tw|Department of Computer Science, National Tsing-Hua University, Hsinchu, Taiwan, ROC. hongjie@iis.sinica.edu.tw|Department of Computer Science, National Tsing-Hua University, Hsinchu, Taiwan, ROC. hongjie@iis.sinica.edu.tw