1. A method of superimposing a first image onto a second image, wherein each image is defined by a plurality of pixels, the method comprising: changing target pixels of the second image corresponding in position to pixels of the first image to be superimposed on the second image, wherein changing target pixels of the second image comprises changing the values of some of these target pixels of the second image to respective new values, each of which is based on the original value of at least one pixel of the second image.