AUTOSTEREOGRAMS: work in a rather complex way. A repeating pattern (often of random dots) is perturbed by reference to a depth map. The depth map uses a grey scale image to represent distance from the viewer. White is closest and black furthest from the observer. Using computer programs, the pixels of the pattern are shifted left or right depending on the depth map. The brain can decipher the resulting pattern and reconstructs a 3-dimensional image from it. Observing the effect can take some practice and some people never experience the 3D effect.
