1. A method of analyzing an image made up of a plurality of pixels, each pixel having a brightness intensity, the method comprising: identifying a subset of the pixels; for each identified pixel: defining a local patch of pixels surrounding the identified pixel; and grouping the pixels in the local patch into a number of bins; dividing the local patch into a plurality of sub-patches, for each sub-patch: determining how many pixels in each sub-patch fall into each of the bins to create an intermediate vector; and concatenating the intermediate vectors to form a feature vector and describe the local patch.