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 3D 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." A common example of this latter type is the popular Magic Eye series of books.