ls per unit of length of the image sensor used, [0089] b is the half-length of the major axis of, the image ellipsoid ID1, expressed in number of pixels, i.e. b=D2/2*Npl, [0090] b is also the radius of the image disk ID2, expressed in number of pixels, [0091] ???O??? designates the centres of the initial image Img1 as well as the centre of the image generated Img2, [0092] i and j are the coordinat