autostereogram

An autostereogram is a single-image stereogram designed to produce the perception of a three-dimensional scene when viewed with relaxed eyes. It encodes depth information into a two-dimensional picture, allowing the brain to fuse two slightly different views from the same image without the use of special glasses.

Most common are random-dot autostereograms, in which a depth map is translated into horizontal shifts of a repeating texture within a single image. When a viewer looks with unfocused or diverged (or crossed) eye positions, the overlaid patterns align at corresponding locations, and disparities between the two views create the impression of depth.

Other autostereograms use patterned textures or programs that map depth values to texture displacements. The technique relies on the brain’s ability to match corresponding points between the left and right eye images and interpret binocular disparity as depth.

Viewing often requires a free-viewing or "Magic Eye" method: the eyes must be allowed to diverge or converge until the hidden image snaps into view. Some people cannot perceive depth from autostereograms, and prolonged attempts can cause eye strain or headaches. The concept traces back to Béla Julesz’s random-dot stereograms (1959), with popularity rising in the 1990s through books and magazines.