1. A method of evaluating whether an image under test is similar to a reference image, the method comprising performing a test on pairs of blocks in turn, each pair of blocks comprising a block of the image under test and a corresponding block of the reference image, the test comprising calculating a difference measure indicative of the difference between the blocks and comparing the difference measure to a threshold, wherein the test is passed if the difference measure betters the threshold, and the image under test and the reference image are deemed to be similar if the test is passed by all of the pairs of blocks.