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A Stereogram is an optical illusion of depth created from a flat, two-dimensional image or images (wikipedia.org).
Our website is dedicated mostly to Single Image Stereograms, also known as Autostereograms. We'll be calling them just Stereograms.
A Single Image Stereogram is a two dimensional image with a hidden illusion of depth, which could be observed (seen, revealed) by viewing them in a special way.
A stereogram usually consists of repetitive patterns or objects. When you diverge (look through) or cross (look in front of) your eyes, the patterns or objects overlap, and that develops the hidden depth.
Classification by viewing method:
1. Parallel Stereograms
2. Crosseyed Stereograms
A Parallel Stereogram requires you to diverge your eyes or to look through the image, in order to see hidden depth. The hidden image will appear behind the picture you are looking through.
Crosseyed Stereogram requires you to converge your eyes or to look in front of the image, in order to see hidden depth. The hidden image will appear in front of the picture you are looking through.
Parallel stereograms are more common then cross-eyed ones, and they both have pros and cons. Some people say that cross-eyed stereograms are difficult to view and they cause more eyestrain.
On the other hand parallel stereograms have a limitation on pattern width (parallax) which makes it difficult to design large parallel posters.
If a cross-eyed stereogram is looked at with the parallel method and visa versa, a parallel stereogram is looked at with the cross-eyed method, they both will have inversed depth, the hidden image will be inside-out.
Classification by type.
There is no official terminology for all stereogram types, so that there is no official classification by type either. This is an approximate classification.
Hidden Image Stereograms
stereograms with hidden objects which are not obvious until the stereogram is revealed.
Random Dot Stereogram (RDS, SIRDS)
hidden depth is encoded by random dots.
hidden depth is encoded by a pattern, which could be any picture or drawing.
Mapped Texture Stereogram (MTS)
hidden depth is encoded by a texture, which is also used for texturing the hidden objects.
hidden depth is encoded by contours of hidden objects.
Stained Glass Stereogram
painted Contour Stereogram.
Object Array Stereograms
repetitive objects become three dimensional or appear on different levels of depth.
2D Floaters (Wallpaper)
flat objects appear on different levels of depth.
flat objects become three dimensional and could be on different levels of depth as well.
repetitive and seamless pattern becomes three dimensional.
set of small objects (clouds) form a hidden object.
Text Stereogram (ASCII)
a text document which works like a stereogram.
Hidden Text Stereogram
hidden image is formed from a text pattern.
Text Array Stereogram
repetitive words appear on different levels of depth.
Pop-up Text Stereogram
some letters pop up and form text or sentence you can read.
All the types could be combined in one stereogram to achieve various effects such as transparency, layers of depth, 3D ornaments, etc.
Most common are Patterned Stereograms, 2D/3D Floaters and their combinations.