1. A method of processing image data representing an original image including a plurality of image regions to create additional regions on the boundary between each pair of said plurality of image regions with the aid of a computer, said method comprising the steps of: (a) allocating a region number to each of said plurality of image regions; (b) preparing a neighbour window consisting of a plurality of pixels, said plurality of pixels including a center pixel and a plurality of adjoining pixels, each adjoining pixel allocated a predetermined priority; and (c) while successively shifting said neighbour window on said original image, allocating a new region number different from the aforementioned region numbers of said plurality of image regions to said center pixel located at the center of said neighbour window, to thereby produce a pair of additional regions each having the new region numbers on a respective side of said boundary between each pair of said plurality of image regions, said new region number being allocated to said center pixel on the conditions that: (i) said center pixel is located in one of said plurality of image regions; (ii) the region number of at least one of said adjoining pixels in said neighbour window is different from the region number previously allocated to said center pixel; and (iii) the region number of a first adjoining pixel which has the highest priority among the adjoining pixels having a different region number from the region number previously allocated to said center pixel is equal to one of the region numbers originally allocated to said plurality of image regions; while (iv) maintaining the region number previously allocated to said center pixel on the condition that the region number of said first adjoining pixel is different from the region numbers originally allocated to said plurality of image regions.