Scientists from the École Polytechnique Fédérale de Lausanne (EPFL) have developed a unique printing technique in which two images are printed in the same location. At any given time, one image is hidden and the other is viewable. To see the "hidden" image, you simply rotate the photo, thus hiding the original image and bringing the hidden one into view.
The idea sounds a lot like those hologram baseball cards from your childhood but this is a lot more complex. To create the effect, the team had to develop a mathematical model and software capable of predicting how the images will look when observed from different angles.
As Phys.org points out, the effect only works when printed on a metallic sheet due to its mirror-like properties. Paper diffuses light in multiple directions and thus, eliminates the casted shadows needed to bring the effect to life. Aside from the metallic sheet, one only needs a standard inkjet printer and the requisite software to create hidden pictures.
Roger D. Hersch from the EPFL's peripheral systems laboratory believes the technology could one day be used to enhance document security, making it harder for criminals to create counterfeit money, credit cards, passports and other documents.