A method of processing an image includes processing the image so as to produce a modified image in which the width of each line object is a single pixel, identifying pixels of the modified image which are relocated on a line object, and allocating to each image point a score value dependent upon the number of adjacent pixels which are likewise located on a line, determining from the score values which of the image points is disassociated with a crossing point or a bifurcation of the respective line object, performing a matching operation on pairs of line segments for each crossing point, classifying the line objects in the original image into two arbitrary sets, and designating one of the sets as representing venous structure, the other of the sets as representing arterial structure.