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A plane image or pair of 2-D images which, when appropriately viewed using both eyes, produces an image which appears to be three-dimensional. By taking a pair of photographs from slightly different angles and then allowing one eye to view each image, a stereogram is not difficult to produce.
Amazingly, it turns out that the 3-D effect can be produced by both eyes looking at a single image by defocusing the eyes at a certain distance. Such stereograms are called ``random-dot stereograms.''
References
Bar-Natan, D.  ``Random-Dot Stereograms.''  Math. J. 1, 69-71, 1991.
 
Fineman, M.  The Nature of Visual Illusion.  New York: Dover, pp. 89-93, 1996.
 
Julesz, B.  Foundations of Cyclopean Perception.  Chicago, IL: University of Chicago Press, 1971.
 
Julesz, B.  ``Stereoscopic Vision.''  Vision Res. 26, 1601-1611, 1986.
 
Terrell, M. S. and Terrell, R. E.  ``Behind the Scenes of a Random Dot Stereogram.''  Amer. Math. Monthly 101, 715-724, 1994.
 
Tyler, C.  ``Sensory Processing of Binocular Disparity.''  In Vergence Eye Movements: Basic and Clinical Aspects.
  Boston, MA: Butterworth, pp. 199-295, 1983.