1. A non-transitory computer readable medium carrying computer program code to, when running, implement a method for processing an image to generate a signature which is characteristic of a pattern within said image, the method comprising: receiving an image; overlaying a window at multiple locations on said image to define a plurality of sub-images within said image, with each sub-image each having a plurality of pixels having a luminance level; determining a luminance value for each said sub-image, wherein said luminance value is derived from said luminance levels of said plurality of pixels; combining said luminance values for each of said sub-images to form said signature; wherein said combining is such that said signature is independent of the location of each sub-image.