1. University of Rennes I, CNRS UMR 6074, IRISA, F-35042 Rennes, France
2. VisAGeS U746 Unit/Project, INRIA, IRISA, F-35042 Rennes, France
3. VisAGeS U746 Unit/Project, IRISA, INSERM, F-35042 Rennes, France
4. UR341 Mathematiques et Informatique Appliquées, INRA, F-78352 Jouy en Josas Cedex, France
5. VISTA Project-Team, IRISA, INRIA, F-35042 Rennes, France