1. Method of image processing intended to temporally modulate a sequence of source images with a modulation image comprising at least one pattern said sequence of source images being modulated with the modulation image in such a way that said at least one pattern is invisible to the human eye when the images of the sequence are displayed and that artefacts appear when said images are copied by picture-taking during their display, wherein the shape and/or the position of said pattern of the modulation image is determined by analysis of the source image that said modulation image modulates.