A stereogram is any image that, through one of several techniques, is able to convey the experience of depth perception to the viewer by means of stereopsis for binocular vision. Originally, stereogram referred to a pair of stereo images which could be viewed using a stereoscope. Nowadays, there are many other methods to display stereoscopic images.
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. Between 1849 and 1850, David Brewster, a Scottish scientist, improved the Wheatstone stereoscope by using lenses instead of mirrors, thus reducing the size of the device. 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. Salvador Dalí created some impressive stereograms in his exploration in a variety of optical illusions. In the early 1900s, devices like the View-Master were developed, gaining moderate popularity.
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.
Stereoscopic imaging relies on the principle of stereoscopy 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 a parallel view or cross-eyed view method. The stereo pair can also be viewed with devices such as the Stereoscope or the View-Master. More advanced methods include Head-mounted displays, Polarized 3D images, Active shutter 3D images and other 3D display methods. The following are some examples of techniques that are mainly used for still images.
- 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. An autostereogram can be observed by allowing the eyes to focus behind the image (diverge), or, less often, in front of it (converge). These two methods are also known as wall-eyed and cross-eyed, respectively. The slight differences in vertical repetitions of figures or random dots create the illusion of depth in the 2D image, just as the slight difference in perspective between one's eyes creates the perception of depth on 3D objects and scenes.
- According to Magic Eye, a maker of autostereograms, "most people prefer the diverging method". However, with normal stereograms, this imposes a limit on the size of the image, since there is a limit to how much the eyes diverge; images created for the cross-eyed method can be larger. If a stereogram is viewed with the wrong method, the depth information is seen 'reversed'; points intended to be in the background appear in the foreground and vice versa.
- 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.
- A Bubblegram or Laser Crystal is a 3D image composed of points suspended in a medium, typically a plastic block.
Practical uses 
While stereograms have typically been used for amusement, including stereographic cards, 3D films, printings using anaglyph and pictures, posters and books of autostereograms, there are also serious uses of the technologies.
In the 19th Century, it was realized that stereoscopic images provided an opportunity for people to experience places and things far away, and many tour sets were produced, and books were published allowing people to learn about geography, science, history, and other subjects. Such uses continued till the mid 20th Century, with the Keystone View Company producing cards into the 1960s.
Space exploration 
The two cameras that make up each rover's Pancam are situated 1.5m above the ground surface, and are separated by 30 cm, with 1 degree of toe-in. This allows the image pairs to be made into scientifically useful stereoscopic images, which can be viewed as stereograms, anaglyphs, or processed into 3D computer images.
The ability to create realistic 3D images from a pair of cameras at roughly human-height gives researchers increased insight as to the nature of the landscapes being viewed. In environments without hazy atmospheres or familiar landmarks, humans rely on stereoscopic clues to judge distance. Single camera viewpoints are therefore more difficult to interpret. Multiple camera stereoscopic systems like the Pancam address this problem with unmanned space exploration.
Clinical uses 
Mathematical, scientific and engineering uses 
As in the stereopair image of the lake, stereopair photographs are sometimes used to help visualise aerial photographs. Cartographers may also generate stereopairs using computer programs in order to visualise topography in three dimensions. In biology and chemistry, complex molecular structures are often rendered in stereopairs. The same technique can also be applied to any mathematical (or scientific, or engineering) parameter that is a function of two variables, although in these cases it is more common for a three-dimensional effect to be created using a 'distorted' mesh or shading (as if from a distant light source).
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A random dot autostereogram encodes a 3D scene which can be 'seen' with proper viewing technique (). Click on thumbnail to see full-size image
Stereogram of an Asiatic hybrid lilium. To view the image cross your eyes until four images appear (), then allow the images to converge to a set of three, focusing at the center of the image.
Curious rock with a jutting portion at "Home Plate" via the Mars Spirit Rover. (Animated GIF image for stereoscopic perception).
Statue by John Gibson in Rome
- Pinker, S. (1997). The Mind's Eye. In How the Mind Works (pp. 211–233). ISBN 0-393-31848-6
- CenturyTel Portal accessed 21 March 2009
- Horibuchi, S. (1994). Salvador Dalí: the stereo pair artist. In Horibuchi, S. (Ed.), Stereogram (pp.9, pp.42). San Francisco: Cadence Books. ISBN 0-929279-85-9
- Magic Eye Inc. (2004). Magic Eye: Beyond 3D. Kansas City: Andrews McMeel Publishing. ISBN 0-7407-4527-1
- University of Virginia The Stereoscope In America, accessed 21 March 2009.
- "Pancam technical brief" (PDF). Cornell University. Retrieved 2006-06-30.
- Bartiss, OD MD, Michael (2005-01-25). "Convergence Insufficiency". WebMD. Retrieved 2006-06-30.
- David F. Watson (1992). Contouring. A Guide to the Analysis and Display of Spatial Data (with programs on diskette). In: Daniel F. Merriam (Ed.); Computer Methods in the Geosciences; Pergamon / Elsevier Science, Amsterdam; 321 pp. ISBN 0-08-040286-0
Further reading 
- Scott B. Steinman, Barbara A. Steinman and Ralph Philip Garzia. (2000). Foundations of Binocular Vision: A Clinical perspective. McGraw-Hill Medical. ISBN 0-8385-2670-5
- BBC's h2g2 entry on 3D Stereograms
- How to create wiggle stereograms
- stereograma - open source stereogram maker
- Make your own stereogram