MAS - Ecole Centrale Paris, Grande Voie des Vignes, Chatenay-Malabry, France;I.N.R.I.A, 2004 route des lucioles,, Sophia-Antipolis, France;Department of Mathematics, UCLA, Sophia-Antipolis;Image and Pattern Analysis Group, Heidelberg Collaboratory for Image Processing, University of Heidelberg, Sophia-Antipolis, Germany