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| This stereogram
has six repetitions of the pattern and is designed to give a radically
different image at the second degree of eye cross (or spread) than it does
at the first. The second "stereo-level" gives the "figure four" pattern
at the right. The first gives a series of five vertical bars -- somewhat
scrambled by the figure four, which is awaiting you to cross (or spread)
to the next level. The bars then vanish and the integrated image
is all that's left.
This stereogram was was tediously typed on a typewriter. It predates home computers by many years. |
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The purpose of
this stereogram was to illustrate a kind of "fourth dimension" in a visual
object. The first three dimensions are the three of the stereo image.
The fourth is the amount of eye crossing (spreading) used to observe the
images.
It was also made to illustrate that an object perceived stereoscopically does not have to be limited in width by the width of the repeating stripe. Other stereograms were constructed at the time that illustrate many other possibilities: curved surfaces, moving surfaces, color dissonance... |
How was it made?
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| This stereogram starts with
a vertical panel of randomly typed symbols. Here, an equal-width
font was used so that the pattern has the uniformity seen in Julesz's patterns
of black and white squares.
The next step is to repeat the panel to the right of the first, but a sterescopic image is included by shifting a segment of the pattern a position or two to the right or left. That would leave some blank space behind, so we fill in that blank with new random symbols. For the third panel--which goes in the next position to the right--we repeat the second panel (not the first) and again introduce more of the desired stereoscopic image by shifting. The "uncovered" blank gets filled in with symbols that must not interfere with the other "fill-in" symbols. We can avoid the interference by thinking of the pattern as having several distinctly defined levels and consistently using the same symbols for the same level. This process is then continued to place additional panels further to the right. We can continue as far as we wish. This particular stereogram
constructs the vertical, stereoscopic bars alternating above and below
the plane of the background so that they cancel each other at the second
eye crossing or spreading.
Curved surfaces? In the above pattern, each typed symbol lies in a little horizontal plane. Distorting the symbol when the next panel is created will make the stereoscopic image lie not on that horizontal plane, but on a surface with stereoscopic curvature. Modern digital processing
lets us easily create smoothly curved surfaces in constructed stereograms.
The background image on this page was made by distorting
so that pixel motion was limited to the left-right direction only.
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