Re: one-eyed stereo

[P3D John O. Merritt <JOMERRITT@xxxxxxxxxxx>]

Binocular disparity means, by definition, disparity between the
two (bi) eyes (ocular), that is, disparity or differences between
the L eye retinal image and the R eye retinal image.  

A single viewpoint cannot have any binocular disparity, by
definition, and a single eye cannot have any binocular disparity
because there is no other eye to have disparity with.  

A single eye can see the binocular disparity between two images,
say, in viewing a cross-polarized projected stereo pair without
wearing the polarized glasses... one eye can see the double image
formed when the L and R images are superimposed.  That one eye
can see that some objects are more "doubled" on the screen, while
other objects have little disparity, and then the observer can 
consciously infer which objects are closer to the screen plane and
which are farther.  But this does not enter the "stereoposis" system
and produce that sense of "air space" between objects that comes
most strongly from normal binocular viewing.  

So, while one eye can see the disparities between a stereo pair
of images, the brain cannot make "stereopsis" out of those disparities
unless they are fed into the L and R eyes in the way humans evolved
to do.  Rapid alternation between the L and R mages of a stereo
pair can be perceived as motion parallax, and seen with one eye,
but again does not produce that unique and clear sense of "seeing
airspace" between objects, as is obtained with normal binocular
viewing.

It is possible, however, to get really good stereo by using two
eyes to view a single photograph.  The single photo can be taken
from one viewpoint, with no adapaters, just an ordinary photo.
The secret is that the scene must have several copies of some
identical objects...such as columns on the Supreme Court, or
identical airplanes parked side by side on an airfield.  Then,
by simply doing a little cross-eye viewing, you look at one
column with the R eye, and a different column with the L eye,
and you have the "same" column seen from slightly different
angles, and "true stereo"---  same with the airplanes, the
effect depending on the planes being apparently identical.

That's a good bar bet: that you can get true stereo from a single
photograph, taken with an ordinary camera.  

You can even get hyper stereo, as with a telestereoscope, by 
cross-viewing the columns in a real building.


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