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Pseudo-stereoscopy

If the wrong eyes are used to look at a stereoscopic pair, the result is pseudo-stereoscopy. Often this optical illusion causes considerable confusion, but it is sometimes possible for stereo pairs to look good as either true or false stereo. 

Here are two examples where pseudo-stereoscopy of opaque objects works well. Despite the false versions giving reverse reality, the unfamiliar world shown is almost believable.

One pair is set up for parallel viewing, (U stereo) the other for cross-eye viewing (X stereo). By placing identical stereo pairs beside each other, we get the opposite stereo version in the middle:

Right  

Left

.

Right  Left    X stereo pairs

Left  

Right

.

Left  Right    U stereo pairs
 
 

In both pictures there is a promontory sticking forwards in the correct version, which turns into a "cave" projecting backwards in the pseudo-stereoscopic version.

  • Notice how it takes time for the reverse version to become acceptable to your brain.
  • The3D effect gradually improves, which shows stereoscopic perception is a complex combination of binocular vision and brain computations, which take time to achieve.
  • Once you have both versions accepted by the brain, you can flip from stereo to pseudo-stereo without any noticeable delay, showing the depth perceptions, once computed, are held in memory.
  • In this case it is short-term memory, since if you come back to these pictures after a delay, your brain has to take time for the computations all over again.

The double method for showing stereo-pairs is used a lot on this web site, to allow for people who can see U stereo but not X stereo, so you will get plenty of practice with other images which do NOT look nearly so satisfactory in pseudo-stereo. Such as the dandelion on this page.

Stereoscopic X-rays are notorious for pseudo-stereoscopy. Since X-rays  make objects look transparent, it is hard to know which is the correct version, as both seem acceptable.  (Try the background for this page and see how the shell points forwards, or away, depending on which pair you fuse.)

Anaglyphs are easily seen in pseudo-stereo - just get the glasses around the wrong way. (Usually anaglyphs are set up for left eye red, right eye green, like port and starboard running lights)

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