ASCII stereogram

ASCII stereograms are a form of ASCII art based on stereograms to produce the optical illusion of a three-dimensional image by crossing the eyes appropriately using a single image or a pair of images next to each other.
To obtain the 3D effect (in Figure 1 for instance), it is necessary for the viewer to diverge their eyes such that two adjacent letters in the same row come together (). To help in focusing, try to make the two capital Os at the top look like three. Ensure that the image of the central dot is stable and in focus. Once this has been done, look down at the rest of the image and the 3D effect should become apparent. If the Os at the bottom of Figure 1 look like three, then the effect is reversed. It is also possible to obtain opposite 3D effects by crossing the eyes rather than diverging them .
Figure 2 demonstrates the effect even more dramatically. Once the 3D image effect has been achieved (), moving the viewer's head away from the screen increases the stereo effect even more. Moving horizontally and vertically a little also produces interesting effects.
Figure 3 shows a Single Image Random Text Stereogram (SIRTS) based on the same idea as a Single Image Random Dot Stereogram (SIRDS). The word "Hi" in relief can be seen when the image clicks into place. ()
Some people have included stereograms in their "signature" at the end of electronic mail messages and news articles. Figure 4 is such an example.
Moving animated versions of ASCII stereograms are possible too.
Text emphasis.
The stereo effect can be used to highlight individual words in a text, as a sort of "secret message". The effect can be disguised when the paragraph is block justified.
Sources.
Figures 1, 2, 3 and 4 are due to David B. Thomas, Jonathan Bowen, Charles Durst and Marty Hewes respectively. These four stereograms appeared on the publicly accessible "alt.3d" USENET newsgroup. Figure 5 was invented on the spot by a Wikipedian.
Originally adapted from an article on "ASCII Stereograms" by the author of that article (and with his permission).