AUTOSTEREOGRAMS or Single Image Stereograms use a repeating pattern (often of random dots) 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 away. A computer program shifts the pixels of the background to the left or right depending on the depth map. The brain can decipher the resulting pattern and reconstructs a 3-dimensional image. Getting the effect can take practice and some people never succeed. To get the effect, stare at the image (preferably at a corner) with your eyes focused on infinity and wait for the image to pop out.