1. School of Electronic Engineering, Xidian University, Xi'an 710071, P. R. China
2. State Key Lab of Integrated Service Networks, Xidian University, Xi'an 710071, P. R. China
3. Biometrics Research Centre, Department of Computing, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
4. School of Computer Science and Information Systems, Birkbeck College, University of London, London WC1E 7HX, UK