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Autostereogram - Wikipedia

An autostereogram is a single-image stereogram (SIS), designed to create the visual illusion of a three- dimensional ( 3D ) scene from a two-dimensional image. In order to perceive 3D shapes in these autostereograms, one must overcome the normally automatic coordination between accommodation (focus) and horizontal vergence (angle of one's eyes). The illusion is one of depth perception and involves stereopsis : depth perception arising from the different perspective each eye has of a three-dimensional scene, called binocular parallax .