1. a Department of Computer Science and Engineering, POSTECH, San 31, Hyoja-Dong, Nam-Gu, Pohang 790-784, South Korea
2. b Center for Intelligent Robotics, KIST, 39-1, Hawolgok-Dong, Seoungbuk-Gu, Seoul 136-791, South Korea
3. c Department of Computer Science and Engineering, POSTECH, San 31, Hyoja-Dong, Nam-Gu, Pohang 790-784, South Korea;, Email: dkim@postech.ac.kr