1. A method of identifying a characteristic of an image comprising the steps of: a) obtaining from the image a plurality of digitised sample strings, each sample string representative of one or more linear image samples; b) generating from the sample strings a variation code indicative of variation between the sample strings by applying a vector operator to the sample strings to generate a variation code, and c) repeating steps (a) and (b) to generate a plurality of variation codes, said variation codes together defining an overall variation code; and d) identifying a characteristic of the image in dependence upon all or part of the overall variation code.