1. Object detection apparatus configured to detect 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 a position of the current block within said test window and said index figure generated in respect of the current 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 the test window including an object, wherein said probability values in said storage are ordered in the storage by block position, then by index figure.