code and an i code and a set of c codes delineating the course of the vein progressing downwards these various codes advantageously appear in the output code stream in the order in which their respective conditions arise in the left to right line by line comparison process thus for fig1 b we output an h code and an i code for region 1 an h code and an i code for region 2 and h 0 code and an i code for region 3 and then for each successive image line three c codes for the right edges of regions 1 2 and 3 respectively a vein that bifurcates forms a new region such as region 3 in fig1 c in this case the left leg of the bifurcation is taken as a continuation of the parent vein and the right leg is treated as a new vein thus for the first image line in which region 3 occurs output first a c code to indicate a connection to the parent vein in the previous image line it can be seen that this vein continues to demarcate the right edge of region 1 following this an h code and an i code for region 3 are output this establishes the new vein defining the right edge of region 3 this is followed by a c code for the right edge of region 2 for each successive line three c codes are output for the right.