1. A method of obtaining a digital code representative of a skin-covered body part, the method including: acquiring, in a processing device, an image of the skin-covered body part, the image including a plurality of pixels, each pixel having an associated shade value in a range of shade values; identifying a plurality of subsets of pixels from the plurality of pixels, each subset of pixels including at least two pixels having a common one of a plurality of designated shade values in the range of shade values; for each of a plurality of combinations of pixels taken from the pixels in the subsets of pixels, determining a geometric measure of the pixels in said combination; encoding the geometric measures into a digital code for the skin-covered body part; and outputting the digital code from the processing device.