evice that presents each eye with a slightly different photograph of the same scene, which generates the illusion of depth. The photographs are taken from slightly different perspectives, one approximating the view from the left eye and the other representing the view from the right eye. The View-Master, a children's toy, is a modern type of stereoscope.Filmmakers have made use of binocular disparity to create 3-D (three-dimensional) movies. In 3-D movies, two slightly different images are projected onto the same screen. Viewers wear special glasses that use colored filters (as for most 3-D movies) or polarizing filters (as for 3-D IMAX movies). The filters separate the image so that each eye receives the image intended for it. The brain combines the two images into a single three-dimensional image. Viewers who watch the film without the glasses see a double image.Another phenomenon that makes use of binocular disparity is the autostereogram. The autostereogram is a two-dimensional image that seemingly becomes three-dimensional when the viewer relaxes or defocuses the eyes, as if focusing on a point in space behind the image. The two-dimensional image usually consists of random dots or lines, which, when viewed properly, coalesce into a previously unseen three-dimensional image. Most autostereograms are produced using computer software. The mechanism by which the autostereogram works is complex, but it employs the same principle as the stereoscope and 3-D movies. That is, each eye receives a slightly different image, which the brain fuses into a single three-dimensional image. The autostereogram was first popularized in the Magic Eye series of books in the early 1990s, although its invention traces back to 1979.Although binocular disparity is a very useful depth cue, it is only effective over a fairly short range—less than 3 m (10 ft). As our distance from objects increases, the binocular disparity decreases—that is, the ima...
