P3D Retinal rivalry & other visual tricks

[George Themelis <gthemelis@xxxxxxxxx>]

--- Rehotshots@xxxxxxx wrote:

> Has anyone out there ever tried to make a stereo photo or 
> slide out of 2 totally different scenes, but related?  
> Kind of like double exposure in conventional photography?  

There is a variety of effects that fit in this category.  First,
you can make double exposures in 3d just like you do in 2d, only
in 3d they are more complicated to shoot and more effective to
view because of the added realism of the medium.  Some examples
that come in mind are Tomy Thomas' "bodiless cowboy" (double
exposure where the cowboy is missing his legs) or a series of
"ghost" pictures where, as a results of double exposure, a
figure ("ghost") appears transparent.

In this case the right and left view have identical information,
i.e., they are not different, expect for the small differences
that create the sense of depth.  These differences (stereoscopic
deviations) are very well-defined, i.e. they are in the
horizontal direction and usually have very small magnitude.  If
there are other differences between left and right views in a
stereo pair we use the term "retinal rivalry" to describe them. 
It is like the two eyes fighting with each other to present the
information to the brain.  The result might be seeing only the
right view or only the left view or a mixture of the two.

There are many examples of retinal rivalry.  The simplest and
unintentional is when a scene with "sparkling" reflections is
photographed and the left view contains reflections not seen in
the right view and visa versa.  Like reflections in water, snow,
jewelry, etc.  This creates an interesting visual effect and it
is usually desirable.

A more dramatic example is the case where, say, the left eye
sees a white object in a black background and the right eye sees
the same object but black in a white background.  The end
results can be something "gray" or alternating right and left
views.

The eye/brain complex is very sensitive to even small changes
between the two views.  One trick stereoscopists like to do is
to freeview the "find the 7 differences" cartoons.  The
differences pop out right away!  A good way to impress friends
and family.

Those who take sequential stereo pictures are very well aware of
unintentional differences between R and L views because of
movements between the exposures, like the leaves in a tree,
people or cars moving, etc.  These usually "spoil" the stereo
image.  But there are cases in which, as a result of very
careful planning or just sheer luck, these movements actually
contribute to the stereo effect!  One example is the movement of
clouds.  You can take a "hyperstereo" of clouds from a fixed
stop, using their movement.  If you move the camera during the
two exposures then you record a stereo of the ground and if the
clouds have moved in the right direction then they will be in
3d, located in the background.

Quite a few people have experimented with retinal rivalry in
stereo photography. Perhaps someone can list succesful
experiments?

George

=====
George Themelis (DrT-3d@xxxxxxxxxxxxxxxx)
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