An autostereogram is a single-image stereogram (SIS), designed to trick the human brain into perceiving a three-dimensional (3D) scene in a two-dimensional image. In order to perceive 3D shapes in these autostereograms, the brain must overcome the normally automatic coordination between focusing and convergence.
The simplest type of autostereogram consists of horizontally repeating patterns and is known as a wallpaper autostereogram. When viewed with proper convergence, the repeating patterns appear to float in the air above the background. The Magic Eye series of books features another type of autostereogram called a random dot autostereogram. In this type of autostereogram, every pixel in the image is computed from a pattern strip and a depth map. Usually, a hidden 3D scene emerges when the image is viewed with proper viewing technique.
There are two ways an autostereogram can be viewed: wall-eyed and cross-eyed. Most autostereograms are designed to be viewed in only one way, which is usually wall-eyed. Wall-eyed viewing requires that the two eyes adopt a relatively divergent angle, while cross-eyed viewing requires a relatively convergent angle.