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Revision as of 02:02, 5 September 2004 by Omegatron (talk | contribs) (→External links)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)An autostereogram is a single-picture stereogram, designed to trick human eyes (and brains) into seeing a three-dimensional scene in a two-dimensional image. The Magic Eye series of books featured a type of autostereogram called a random dot stereogram.
How they work
The human brain accomplishes stereo vision by a complex set of mechanisms which attempt to relate the two slightly different two-dimensional images seen by the two eyes. The brain tries to assemble a three-dimensional impression by matching each point (or set of points) in one eye's view with the equivalent point (or set of points) in the other eye's view. It therefore assesses the points' positions in the otherwise inscrutable z-axis (depth).
When the brain is presented with a series of very similar points, such as in a repeating pattern like you might see on wallpaper, it has difficulty matching the two eye's views accurately. By looking at a horizontally repeating pattern, but focusing in front of the pattern, it is possible to trick the brain into matching one element of the pattern, as seen by the left eye, with another (similar looking) element, beside the first, as seen by the right eye. This gives the illusion of a plane bearing the same pattern but located behind the real wall. The distance at which this plane lies behind the wall depends only on the spacing between identical elements.
Autostereograms use this dependence of depth on spacing to create three-dimensional images. If, over some area of the picture, the pattern is repeated at smaller distances that area will appear closer than the background plane. If the distance of repeats is longer over some area, then that area will appear more distant (like a hole in the plane). Subtle changes in spacing can create the illusion of smooth gradients in distance rather than the simpler-to-achieve jumps in depth. This fine-tuned gradient requires a pattern more complex than a standard repeating-pattern wallpaper, so typically a pattern consisting of repeated random dots its used. This forms what is called a random dot stereogram
How to see them
Much advice exists about 'seeing' an autostereogram (that is, seeing the intended three-dimensional image). The first step is to understand what the intended result is. The aim is to unfocus the eyes to that the image of one iteration of a pattern from one eye overlaps the image of a different iteration of the pattern from the other eye.
Many books recommend the viewer to hold one finger between his eyes and to move it slowly towards the picture, maintaining his focus on the finger at all times, until he is correctly focused on the spot between him and the picture that will allow him to view the illusion.
Other texts recommend staring at 'infinity' - that is, focusing on something distant - and then moving the picture towards or away from the eyes until a comfortable focus is obtained. This method is less practical for viewing autostereograms on a computer screen.
Whichever method works (if either) may depend on whether the viewer is naturally cross-eyed or wall-eyed. It is estimated that some 2% of normally sighted people cannot see the illusion in autostereograms.