optical illusions

[P3D Peter Abrahams <telscope@xxxxxxxxxx>]

> examples of views that in 2D would be misleading, but seen in 3D
>would be revealed in true form. -- those "impossible
>objects" that you see in books on optical illusions.
>These are solid objects such as rectangular bars that
>are bent into a triangle, but the sides rotate on the
>way around so there is a confusing mismatch at some
>point.  Etc.  These are sometimes shown in actual photos,
>where someone fakes it by bending the sides in such
>a way that from one angle, it looks like a solid object,
>but there is a break somewhere where a side passes in front
>of another 
I believe these objects are photographed from a critical angle, at which
the break is covered by one end of the object.  The illusion depends on the
monocular view, since with two (stereo) photos,the break would be revealed.
Most three dimensional, sculpted objects that become illusory when
photographed likewise depend on a mono view.  Exceptions would be of
great interest.
However, the subject of optical illusions created for viewing with a stereo
photo viewer is worth investigating.  My guess is that they would need to be
hand drawn or computer generated, as opposed to photographs of solid 
objects.  Escher's endless stairways come to mind, as an example of an
impossible object that would be enhanced in 3d, but not unique to 3d.
There must be some illusions that only 'emerge' in stereo, such as an object
placed within another object through binocular fusion.
Prehaps an idea for one could be found in the Keystone-type cards used to 
test for stereo vision.  
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telscope@xxxxxxxxxx (Peter Abrahams)          
the history of the telescope, 
     the prism binocular, and the microscope


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