MadSci Network: Medicine

Re: Why can some people see 3-D pictures and others can't?

Date: Thu Jan 29 10:26:30 1998
Posted By: Dr. Ofer Markman, Post Doc, Physiology, Hebrew U. School of medicine
Area of science: Medicine
ID: 884920375.Me

There are few ways to "see 3D pictures", all based on different
technologies and "tricks"
The simplest and most common ones are the stereo-pairs - two pictures of
a three dimenssional object taken each in a 6 degree difference one from
another one to the left of the other, or the stereo trio in which the
two possible left-right possiblilities exist.  If placed properly these
are placed so that the center of pictures is within 6.0 - 6.5 cm from
each other, which is the eye distance between the eyes of "an evarage
person". This can be watched with special devices (such as stereo
glasses, streo chambers, stereo mirrors), or by the trained eyes, by
certain techniques - eye crossing or direct viewing. The devices and
techniques come to help us overcome the focal coupling of our both eyes,
the phenomenon in which on eye follow the focal point of the other.  In
this case we want the two eyes to focus on two pictures independently,
and trick our mind to "think it is the same picture" thus make it into
one 3D picture. One can train his eyes to focus on two places, but it is
not realy easy, some have stronger coupling then others, som with
natural decoupling ("crossed eyes") worked hard to overcome what in real
life is a dissadvantage and have hard time to de-learn it, others have
hard time for the simple reason their eye to eye disances are a bit off
6.5-7.0 a common case (consider the fact that men - women differences of
0.5 cm are the normal) I have yet to find the two eyed person who could
not at least see 3D in one of the devices, unless one of his eyes is
very weak. 
Other techniques use devices to make one picture be seen differently by
each eye, either by putting on glasses with different color filters, or
by using polarized light with polarized filters, In all such cases two
different pictures are printed or  projected one on top of the other and
the filter separates the right-eye-picture from the left eye.  Here the
task is usually easier on the viewer's part, but all kind of color
blindness (if color filters are used) or other vision dissabilities can
cause problems.   
 The third way to "make you see 3D" is the use of color illusions that
make "the normal person" feels as if it is 3D.  That can use color
tricks (color blindness can cause problems here) or psychological
illusions which may not work on all people.  "The normal" person is
usually not existant, most such techniques work for most people leaving
a few out.  Many of these tricks need a lack of focus, or "relaxing your
eyes" terms which are very hard to explain and you know you do it right
only when you see 3D. Most can be trained.
Maybe a good start is a rather old Scientific American article "Art,
Illusion, and the Visual System," Scientific American, January 1988, by
M.S. Livingstone,  or to search encyclopedias for Perception.  
The Grulier's Multimedia Encyclopedia review on the issue by Julian
Hochberg is rather good. The one on color perception is very good too.
More (very) advanced articles on the metter can be found in Physics
today Dec 1992 Vol 45 (12) from color perception (P 24) to its use in
digital color procesing etc. (In other articles)

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