1 Hamlyn Centre, Imperial College London, London SW7 2AZ, UK.
2 Laboratory for Computational Sensing and Robotics, Whiting School of Engineering, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218-2608, USA.
3 Hamlyn Centre, Imperial College London, London SW7 2AZ, UK. g.z.yang@imperial.ac.uk.