1 Computer Vision Center, Universitat Autonoma de Barcelona, Spain; Computer Science Department, Universitat Autonoma de Barcelona, Spain. Electronic address: dberga@cvc.uab.es.
2 Centro de Investigacion en Tecnoloxias da Informacion, Universidade Santiago de Compostela, Spain.
3 Computer Vision Center, Universitat Autonoma de Barcelona, Spain; Computer Science Department, Universitat Autonoma de Barcelona, Spain.