have components only along x for the two volume elements 7 and 8 which belong to the segmented volume on either side of the change in level with respect to the plane 5 it can be shown that the standard computations based on the shape of the segmented volume i e as it appears herein with its set back leads to the determination of two slightly inclined normals 12 and 13 respectively it can be assumed as a first approach that the normals 12 and 13 are substantially parallel to each other from the volume element 6 to the volume element 7 with the normals approaching each other we are before a visible surface with a concave shape by contrast from the volume element 8 to the volume element 9 with the normals 11 and 13 being divergent we are before a convex surface if these volume elements 7 to 9 are subjected mathematically to an illumination coming from a direction 14 for example with an orientation 1 1 1 these respectively concave and convex surfaces will have a luminosity level different from each other from this difference in luminosity which winds all along the edge 15 of the set back in the protuberance 4 with respect to the plane 5.