1. A method of identifying an object in an image, the method comprising: determining at least one feature map for each of a plurality of cells in the image; determining a self-dissimilarity between a first feature map associated with a first one of said cells and a second feature map associated with a second cell, wherein the self-dissimilarity is determined by determining a sum over thresholds of a difference in area between the first feature map and the second feature map; forming an appearance signature for the object based on the determined self-dissimilarity; determining a distance between the appearance signature of the object in the image and appearance signatures of each of a plurality of further objects; and identifying the object in the image based on the determined distances.