From: rick@tekig1.PEN.TEK.COM (Fredrick Wilson)
Newsgroups: alt.3d
Subject: Re: How do I generate the two views for a 3d image (LCD glasses)
Date: 28 Aug 92 22:13:33 GMT
Organization: Tektronix, Inc., Beaverton,  OR.

In article <BtpExz.4t3@ie.utoronto.ca> drascic@ie.utoronto.ca (SpIke) writes:

>When working with stereosopic photography, it may be quite easy shift the
>images horizontally relative to each other.

Setting the stereo window involves cropping, too, unless you have
shifted the film or CCD behind the lense.


>When working with stereoscopic video (SV), it is FAR from easy.  Must of the
>work I do involves the use of SV and stereoscopic graphics (SG).  And I'm
>afraid that camera convergence DOES contribute to the location of the viewer
>space (i.e. where the viewer sees the final images).  

Well, yes, because by converging the cameras you are adjusting the window.
(You are also creating convergence error.) If you change convergence in
such a way as to leave the window alone, the depth won't change.
If you change the window and leave the convergence alone, the depth
will change.  Depth is set only by the window, but changes in camera
convergence without changing camera seperation also have the effect 
of changing the window.


>But if you have to work with stereo displays for several hours
>at a time doing telerobotic work, it is important to reduce distortions to a
>minimum.

That's why you don't want convergence error.  Here's a blurb from
"ESD: The Electronic System Design Magazine" April 1988 in an article
titled "Stereoscopic 3D Imaging."  (I wish I could include the example
pictures.)

"Figure 1a shows how a cube would be viewed by two cameras with
parallel lines of sight, and Figure 1b shows two cameras with
convergent lines of sight.  When viewing these examples in stereo,
the first is easily seen and scanned by the eyes because the
surfaces that are normal to the viewing direction register
properly.  The second is difficult or impossible to see because of
the difference in perspective of the two cameras.  The y-positions
of common points do not register."

Convergence error can be compensated for to some extent by rotating
the images (and cropping again) but you can only fix part of
the picture at a time.  You can't fix the whole picture at once without
fancy image processing.

I have other references too, but those books are at home.

>Furthermore, sliding the left and right images horizontally with respect to
>each other reduces the useful stereoscopic area.

You have a choice.  Good stereo with a little of the edge cropped off,
or painful convergence error.  I used to do the latter, now I do the
former because it's more pleasing to view.


>The issues are far from being as decided as Mr. Wilson is.

Yes, I'm afraid I am decided.  The best stereo is done with parallel
views with care given to window cropping and placement.
Attempting to use convergence to avoid having to crop the window
will result in second rate stereo.

BTW, I don't mean to be confrontational.  I'm just trying to help you
understand what I think you are missing.


   |       | |       |
   | WILSON| |WILSON |     STEREO
   | R  SON| | R SON |
   | WI  ON| |W I ON |     rick@sable.pen.tek.com
   | WIC  N| |WI C N |     Tektronix
   | WILK  | |WIL K  |     Beaverton, Orygun
   | WILSON| |WILSON |   
   |       | |       |
Newsgroups: alt.3d