IRISA/INRIA, Campus de Beaulieu, 35042 Rennes Cedex, France. Patrick.Durand@irisa.fr|IRISA/INRIA, Campus de Beaulieu, 35042 Rennes Cedex, France. Patrick.Durand@irisa.fr|IRISA/INRIA, Campus de Beaulieu, 35042 Rennes Cedex, France. Patrick.Durand@irisa.fr|IRISA/INRIA, Campus de Beaulieu, 35042 Rennes Cedex, France. Patrick.Durand@irisa.fr