Sketching a grey-tone pattern out of its distance transform

We describe a procedure to create an abstraction of a grey-tone pattern by sketching its regions which have locally higher intensities. The sketch is a set of simple digital lines, qualitatively analogous to the skeleton representation computed in the case of a single-valued pattern. The grey-tone pattern is regarded as constituted by a number of regions with constant grey-value, and the skeleton is found by detecting suitable pixels on the Distance Transform of the pattern. Computation of the Distance Transform is accomplished according to the city-block distance, by ordered propagation over regions with increasing grey-values. Neighbourhood conditions are used to detect the set of the skeletal pixels, which is subsequently reduced to unit thickness. Finally, the skeleton undergoes a pruning process which removes a part or all of some of its branches.