Two photos in one print

French researchers have succeeded in printing a photo which changes upon rotation. Researchers from the École Polytechnique Fédérale de Lausanne discovered this when they were experimenting with metal printing. They make use of so-called halftone printing, a technique where tiny dots of cyan, magenta and yellow appear to form an image depending on the viewing angle. They found that when this technique is used on metal surfaces, small colour changes took place. This made them develop the technique and they have succeeded in manipulating colours in such a way that it seems as if there are two photos in one frame.