Department of Computer Science and Information Engineering, National Taiwan University Taipei 106, Taiwan. jsyeh@cmlab.csie.ntu.edu.tw|Department of Computer Science and Information Engineering, National Taiwan University Taipei 106, Taiwan. jsyeh@cmlab.csie.ntu.edu.tw|Department of Computer Science and Information Engineering, National Taiwan University Taipei 106, Taiwan. jsyeh@cmlab.csie.ntu.edu.tw|Department of Computer Science and Information Engineering, National Taiwan University Taipei 106, Taiwan. jsyeh@cmlab.csie.ntu.edu.tw