Stereograms

A stereogram is a pair of two-dimensional panels depicting the view of a scene or an object from the vantage points of the right and left eyes. Observing the panels superimposed in a stereoscope results in the experience of three-dimensionality by virtue of the fact that object depth is encoded as right/left position difference in the panels. Because in a stereoscope one views a representation rather than a real scene, the word illusion is sometimes associated with a stereogram.Originally, stereogram referred to a pair of stereo images which could be viewed using a stereoscope. Other types of stereograms include anaglyphs and autostereograms. The stereogram was discovered by Charles Wheatstone in 1838. He found an explanation of binocular vision which led him to construct a stereoscope based on a combination of prisms and mirrors to allow a person to see 3D images from two 2D pictures. Oliver Wendell Holmes, Sr. invented an improved form of stereoscope in 1861, which had no mirrors and was inexpensive to produce. These stereoscopes were immensely popular for decades. Stereograms were re-popularized by the creation of autostereograms on computers, wherein a 3D image is hidden in a single 2D image, until the viewer focuses the eyes correctly. The Magic Eye series is a popular example of this. Magic Eye books refer to autostereograms as stereograms, leading most people to believe that the word stereogram is synonymous with autostereogram. Salvador Dali created some impressive stereograms in his exploration in a variety of optical illusions.

Types of Stereograms:
Stereoscopic imaging relies on the use of a stereoscope to present a slightly different image to each eye. The stereo pair can be viewed with the naked eye, if the images are placed side by side. The stereo pair is then viewed using the same viewing technique used to see autostereograms.
Anaglyph images, also recognized as "red/green" or "magenta/cyan" images, combine two stereo images from slightly different viewpoints into a single image. These images may then be viewed with "anaglyph glasses", which use color filters to moderate the light reaching each eye to create the illusion of a three dimensional image.
Random dot stereograms employ either two stereoscopic images or one anaglyph. The input image (or images) contain random dots with no discernible shapes. When the proper viewing device is used, a hidden 3D scene emerges from these random dots.
Autostereograms produce an illusion of depth using only a single image. The image is usually generated by computer by repeating a narrow pattern from left to right. By decoupling eye convergence from focusing operations, a viewer is able to trick the brain into seeing a 3D scene.
SIRDS (Single Image Random Dot Stereogram) is a form of autostereogram where each repeated pattern is altered slightly, creating a hidden image which is not discernible unless the right viewing technique is used.
Wiggle-gram is an animated computer image which gives 3D percept without using glasses using only a single image. It usually contains a few frames.