1. Department of Computer Science, Brown University, Providence, Rhode Island 02912, USA;
2. School of Interactive Computing, Georgia Institute of Technology, Atlanta, Georgia 30332, USA;
3. Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, New York 14853, USA;
4. Department of Computer Science and Electrical Engineering, University of Maryland, Baltimore County, Baltimore, Maryland 21250, USA;