by Gene Levine
There is a lot of information on the Internet concerning all aspects of autostereogram history, science, and creation. There are even programs and/or programming code available to make make your own SIS or SIRDS. Since it is my wish to promote autostereograms as an art, rather than a science, I'll only try and give a quick overview of what we're dealing with.
A stereogram is a flat two dimensional image viewed in such a fashion as to produce a three-dimensional effect. Traditionally done with opto-mechanical devices: when such an effect is accomplished with the eyes alone it is called an autostereogram, or more often simply a stereogram.
From birth, we learn to view the world using our normal stereo-vision. With this, each eye focuses on an object, seeing the same object on the same plane, but the small distance between each eye feeds the brain enough different information to interpret a sense of 3D depth. This phenomena is called stereopsis.
Stereopsis takes place within the brain, and that is key to the stereogram 3D effect. Unlike normal convergent vision, with stereograms we either diverge or cross our eyes to feed the brain subtle differences in 2D images. The stereo offset is innate to the 2D image, not an actual 3D environment. The brain does not care, and the subtle differences within a 2D image's pattern create the 3D effect off the 2D surface.
There are several kinds of stereograms, but only two ways to view them for the 3D effect. (Figures 2, & 3)
PARALLEL VISION: Each eye locks on the image separately but parallel, as opposed to converging as with normal vision. This is the most commonly used way for viewing stereograms.
CROSS-EYED VISION: Each eye views the image separately, but right eye views left side, and left eye views right.
There is much confusion between Parallel and cross-eyed viewing. The effect is essentially the opposite. Eventually, your eyes will tell you which works with what. To the best of my knowledge, all hidden-image stereograms require parallel viewing. All stereograms on this site are made for parallel viewing. Cross-eyed viewing is frequently used for stereo-pairs. Cross-eyed viewing is more likely to cause eyestrain.
An interesting esthetic with stereograms is no matter how profound the effect of depth appears, all is in focus at one time. After all, it is really a two-dimensional representation. This is what often gives stereograms their ethereal window and meditative appeal.