1. Laboratory for Computation and Visualization inMathematics and Mechanics, Institute of Mathematics, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
2. Department of Computer Science, University of Oxford, Oxford, U.K.
3. Department of Engineering Science, Natural Interaction Lab, Institute of Biomedical Engineering, University of Oxford, Oxford, U.K.
4. Department of Computer Science and Technology, University of Cambridge, Cambridge, U.K.