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From: jbn@glacier.STANFORD.EDU (John B. Nagle)
Newsgroups: comp.graphics
Subject: Re: 3-D perceptual abilities
Keywords: TV 3-D graphic
Message-ID: <18067@glacier.STANFORD.EDU>
Date: 31 Jan 89 18:03:20 GMT
References: <6382@thorin.cs.unc.edu> <25652@sgi.SGI.COM> <985@wasatch.UUCP> <35258@bbn.COM> <1104@nic.MR.NET>
Reply-To: jbn@glacier.UUCP (John B. Nagle)
Organization: Stanford University
Lines: 49

In article <1104@nic.MR.NET> jjc@sun1.UUCP (Jon Camp) writes:
>
>1) As Benie Cosell posted, 3-D perception is much more than stereopsis.  It 
>involves parallax, focus, accomodation, obscuration, perspective, memory, a 
>great many other functions which I am not aware of and most likely some 
>that no one has ever measured.  In our everyday lives, stereopsis is not 
>even the primary means of depth perception.   Stereopsis IS, however, 
>relatively inexpensive to simulate, and is therefore the only contact 
>most people have with  "3-D display". 

     This subject has been studied in some detail by developers of flight
simulators.  See "Flight Simulation", by J.M Rolfe and K.J. Staples,
ISBN 0-521-35751-9, section 7.2, "The Psychophysics of Visual Perception".
They identify eight main non-binocular cues of distance, which I will not
give here.  I do recommend this book to anyone involved in the generation
of realistic imagery.

>3) The common wisdom is that stereopsis is most effective within the reach 
>of our hands.  

     "Flight Simulation" references T. Gold, 1972, "The Limits of Stereopsis
For Depth Perception in Dynamic Visual Situation", Society for Information
Display, International Symposium, Digest of Technical Papers, who reports
that stereopsis dominates differential size and motion parallax out to
about 17m (64m if the observer fixates his eyes on the moving object.)
This is with the observer moving at about 0.5m/sec.  Faster movement
brings the limit closer.  This is somewhat beyond the reach of the hands,
and in fact stereo vision systems have been built for in-flight refueling
simulators.

>4) As one who has experience viewing stereo and other 3-D representations, 
>I wish to report that stereopsis alone gives me a sensation of "viewing 
>fatigue", possibly because stereo so vividly presents SOME depth cues 
>while perversely witholding others.  This is a personal experience, NOT a 
>rigorous criticism of stereoptic display.

       Viewing fatigue for 3D imagery is a serious problem.  The phenomenon
is moderately well understood, and has been written up in technical papers
of the SMPTE, from the point of view of understanding how to make 3D movies.
When viewing images that are not in the same scale as real life, some rather
strict rules must be followed to avoid visual fatigue.  Unfortunately, I
don't have the paper around, but it was by someone in Hollywood who provides
3D gear to filmmakers.  One useful gadget they offer is a pocket calculator
preprogrammed with the calculations needed to set up a shot for 3D.  For
close-ups, this is non-trivial.  They also offer a special leader for 3D
films that allows the projectionist to align the system properly.  Failure 
to do this correctly will induce headaches in some of the audience.

						John Nagle