1. Object detection apparatus for detecting objects in a test image, said apparatus comprising: a comparator to compare blocks of a test window of said image with reference data indicative of the presence of an object to generate index figures indicative of a degree of similarity between a region and said reference data; storage to store probability data corresponding to possible values of said index figure and said block position; logic operable, in respect of a current block, to access a probability value from said storage in dependence on that block's position within said test window and said index figure generated in respect of that block; and a combiner to combine said probability values corresponding to blocks in a test window to generate a result indicative of the probability of that test window containing an object; in which said probability values in said storage are ordered by block position then by index figure.