[1] Univ Paris Sud, INRIA, LRI, F-91465 Orsay, France
[2] Georgia Inst Technol, Sch Interact Comp, Atlanta, GA 30332 USA
[3] Georgia Inst Technol, GVU Ctr, Atlanta, GA 30332 USA
[4] Chalmers Univ Technol, Dept Comp Sci & Engn, S-41296 Gothenburg, Sweden