1. Department of Electrical Engineering, Universitat Politècnica de Catalunya (UPC), C. Colom 1, 08222 Terrassa, Spain
2. Computer Vision Center (CVC), Universitat Autonoma de Barcelona (UAB), 08193 Bellaterra, Spain
3. Department of Electrical Engineering, Eindhoven University of Technology, 5611 AZ Eindhoven, The Netherlands
4. James Watt School of Engineering, University of Glasgow, Glasgow G12 8QQ, UK