random-dot stereogram
a type of stereogram consisting of two images, each composed of black and white dots (or squares). Many of the dots are identical in both images, but a subset of dots in one image is offset horizontally. The images appear to consist of random dots, but when the separate images are simultaneously presented to the separate eyes, they are fused by the visual system, and the horizontal disparity of the subset of dots is interpreted as stereoscopic depth. [first devised in 1959 by Béla Julesz]