1. A method of determining variations of a test image with respect to a reference image, the method comprising: a) storing the reference image in a memory, wherein the reference image comprises a plurality of pixels, wherein each of the pixels comprises at least one value; b) storing the test image in a memory, wherein the test image comprises a plurality of pixels, wherein each of the pixels comprises at least one value; c) specifying a region in the test image and a corresponding region in the reference image; d) specifying a plurality of sub-regions in the test image region and a plurality of corresponding sub-regions in the reference image region; e) calculating an average of the values of the pixels in each of the sub-regions in the test image region and the reference image region; f) calculating differences between average values for respective corresponding sub-regions in the test image region and the reference image region; and g) generating a test result, indicative of the differences of the test image with respect to the reference image in the specified region, based on the calculated differences.