Published by the IEEE Computer Society
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• The author is with the School of Computer Science, The University of Manchester, Manchester M13 9PL, UK.
Manuscript received 18 Mar. 2008; accepted 15 July 2008.
Recommended for acceptance by Z. Ghahramani.
For information on obtaining reprints of this article, please send e-mail to: firstname.lastname@example.org, and reference IEEECS Log Number TPAMI-2008-03-0155.
Digital Object Identifier no. 10.1109/TPAMI.2008.186.