P3D Telling right from left

["Dr. George A. Themelis" <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

We mentioned the relative shifts of foreground w/respect to 
background found in EVERY stereo pair.  It is worth mentioning here 
that the direction of these shifts (stereoscopic deviations) can 
identify the left and right images.  

There are several ways to do this by looking at the negatives or 
the uncut roll.  For example, most stereo cameras use "ID marks",
i.e. simple notches in the camera apertures, to identify the
right from left images.  Charles Piper has published a list of
such marks (see his book "The Technical Page").  These, not only
identify the left from right, but also the camera that took the
pair!  For example, the Realist uses a round notch at the bottom
of the right camera aperture, which appears reversed at the top
of the right image.

Another way is knowing the sequence of the images on the film.
The first image is the right one.

But what if someone throws on you an unmounted pair of prints? 
How can you then tell the right from left with no ID marks or
not knowing the sequence in the film?

Here is how to do it:  Have you noticed when driving that the moon 
always stays with you, while nearby objects zoom in and out moving 
from right to left?  Same way, if you look at your stereo pair first 
left and then right, you should see the nearby objects "zooming in and 
out" or moving from left to right with respect to the background.  
Try it!  (Personally, I am lazy bum and I freeview instead of using
my brain to think!)

Interesting comment:  To use this method you must be able to tell
what is in front and what is in the back.  Distiguish foreground
from background.  If you cannot do that then you have a perfect
candidate for pseudo-stereo! 

George Themelis


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