To compare the similarity of two images, pixel by pixel, you can find the relative L2 error, which just involves subtracting one image from the other, summing the squared value of it, and then getting the square root of it. So if the person had not moved at all, subtracting the current face with the previous face should give a very low number at each pixel, but if they had just moved slightly in any direction, subtracting the pixels would give a large number and so the L2 error will be high. As the result is summed over all pixels, the value will depend on the image resolution. So to get the mean error we should divide this value by the total number of pixels in the image. Let's put this in a handy function, getSimilarity(), as follows: