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stereo pairs and 3D

Yukiharu Hadeishi YHADEISH at biomed.med.yale.edu
Tue Dec 1 20:51:31 EST 1992

In some recent post, Frank Richardson (FRICHARD at biomed.med.yale.edu) wrote:

> For those who know how to adjust the focal points of their vision
> so as to view 3D images (as published in the popular literature)
> What is the mechanism behind this interesting phenomenon?  What
> particular visual snafu makes possible the false sense of 3D when
> you are just looking at 2D images?

The only real trick to seeing stereopairs as 3D images is fooling your visual
system into thinking that it's seeing a single image and not a pair of images. 
The two images in these stereopairs are usually two computer-generated pictures
of the same 3D object.  By allowing each of your eyes to focus on just one of
the images in the stereopair, then, it will seem as though what you're looking
at is the whole 3D object itself, sitting twice as far from your face as the
page (although, of course, correspondingly larger than the images on the
page...).  Hmmn, well, either twice as far from your face, or half as far from
your face, if you're crossing your eyes to see the image, I suppose... although
that's probably the harder way for most people to get the 3D effect.

Of course, the stereograms don't work for everybody--- probably because they
have gotten so accustomed to only focusing at a given plane of focus when their
eyes are "crossed" at a given angle.  (Try focusing on your fingertip, as you
move your fingertip from arms-length to the tip of your nose...)  One easy
exercise that anybody can do to dissociate these two things is to hold your
fingertips in front of you such that they are a centimeter or two apart.  (This
works best if you're in a well-lit place, preferably with a white background
behind your fingertips.)  Now, look beyond your fingertips--- and take note of
the double-nailed ghost finger floating between them!  (You all remember doing
this when you were seven, right?  ;-)  )  Focus on that ghost finger--- if you
can bring it into sharp focus, without changing the angles of your eyeballs,
you've taken a step in the right direction...  :-)

Of course, if you can't see the "ghost finger", you might be one of those
numerous minority who aren't really seeing with both eyes.  In that case, wink
at your fingers a few times--- it should become clear which of your eyes is the
"dominant" one.  By winking with the dominant eye, you might be able to
generate that ghost finger... good luck!  :-)

  -- yh.

P.S.  I hope I've got this mostly right; please feel free to correct any
glaring errors.  Send all *truly* harsh comments directly to me, at the address
below.  We don't want to clutter up this newsgroup with flames...  ;-)

Yukiharu "Yuki" Hadeishi   -   Lurker at Large
Internet:  yhadeish at biomed.med.yale.edu
The Interdepartmental Neuroscience Program will have
absolutely nothing to do with any of my opinions...

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