From: drascic@ie.utoronto.ca (SpIke)
Subject: Re: How do I generate the two views for a 3d image (LCD glasses)
Organization: University of Toronto, Department of Industrial Engineering
Date: Wed, 2 Sep 1992 04:27:45 GMT

rick@tekig1.PEN.TEK.COM (Fredrick Wilson) writes:

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

>By "stereo window" I mean something very similar to your "Display Surface."
>I think of it as a window in a wall.  Objects are viewed through this
>window. ...

>I set my window by deciding what object in the picture I want to
>have AT the window.  Usually it is the nearest object, usually
>it is at the bottom.  Sometimes I want something to come through
>the window at me so I pick something behind that object.  I then
>crop the left and right edges of both the left and right views so 
>the selected object is the same distance from the edges in both views.
>(Both views are now the same width, too.)
>The stereo window is now set.

This is a very pragmatic, practice oriented approach that can work fine.
The approach we use, however, is a little more complicated.  With a
geometric model of the viewing apparatus, and knowledge of the limits of
comfortable eye convergence and divergence, and taking into account the
desired range in the camera space in which we wish our operators to
work, we define our camera separation and convergence accordingly.
Other factors included in our computations are the lenses used, the size
of the display, and the average viewing distance.

A few years ago I designed a mount for our stereoscopic video camera
pair that permits their separation and convergence angle to be
controlled by a computer.  Using heuristics and input from the operator,
the computer will eventually be able to adjust the cameras continuously,
optimising the stereo depth resolution while keeping eye-stress to a
minimum.  

>Parallel-view photos have a little extra on the left side of the left view,
>and a little extra on the right side of the right view.  This techique takes
>those off and gives the window a very sharp, realistic appearance.

What kind of viewing technique do you use?  We've got alternating
field displays (showing the left and right views alternately, switching
rapidly at either 60 Hz or 120 Hz depending the equipment we use) and
active liquid crystal shuttering glasses.  The lenses switch from clear
to black.  The left lens is clear only when the left image is displayed,
and the right shutter only when the right image is displayed.  In this
way we use one display and channel separate images to the two eyes.

>Most of my experience is with single-lense photography.  Some with 
>computer graphics.  The concepts should apply to all forms.  If I 
>take a photo of a video, and then digitize the photo, and add 
>digital graphics to it, it's all the same.

Unless you want to have real-time displays.

>>experiments
>>conducted here at the University of Toronto and elsewhere have shown that
>>properly designed converged camera setups can produce an image that is
>>highly pleasing to the viewer, even for long periods of time.

>Yes, if the convegence angle is small (less than a few degrees), and
>the subject is appropriate.

Exactly.  (That's why I said "can".)  Most of the work we are doing is
telerobotics, with people-sized devices.  Converged cameras work great,
mostly because we have to limit the camera separation in order to avoid
straining the eyes of the operators, most of whom are not experts at
viewing SV displays and who cannot fuse images with huge disparities.

>I took converged photos for about a year,
>wondering why some worked and some didn't.  Then I took one of a 
>water faucet on the side of a house.  The straight lines in the siding
>on the house were impossible to view properly, and painful to try to view.
>That convinced my to take parallel pictures and let my eyes do all the
>converging.

Naturally, large convergence angles lead to large vertical disparities
and "convergence errors".  From the sounds of it, this photo probably
has some very large disparities, and many novices to stereoscopic
viewing would probably have trouble fusing the image.

>>>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.

>>This will work for computer graphics, but not for video.

>It works for photography too, but it's better to take the originals
>so they don't have to be rotated.  I agree that rotating your
>swiched video views on a monitor would be a royal pain.  We probably
>make something here at Tek in either the TV group or the stereoscopic
>monitor group that is capable of it, but I'm sure most people would
>rather not have to use it.

You must have some really fancy stuff at Tek.  In Star Trek: The Next
Generation, I saw them use a two-dimensional photograph and extrapolate
the solid three-dimensional original, but even their computers were
straining to do it.

Oh, you mean filming the left image, rotating the real object, and then
taking the right image!  Hmm.  Can't do that in real-time video,
though.

>>>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.

>>"Painful"?  Hmm.  All blanket statements are incorrect, including this one.

>If I had a scanner I could show you what I mean with the faucet photo.
>It HURTS to look at that sucker.  Less convergence would be less
>painful, but would also have less stereo effect.  To get the same
>amount of stereo effect back it would take seperating the cameras
>in a parallel fashion.

From one photo, you want to generalise to all converged-camera
situations?  Despite having said yourself that "some worked and some
didn't?"  Hmm.  Not very good science, methinks.

You are right about that photo, naturally.  Extreme stereoscopic depth
resolution with close up distances cannot be done using a converged
camera configuration.

	SpIke.

David Drascic
Depth of Industrial Engineering
University of Toronto
drascic@ie.utoronto.ca
Newsgroups: alt.3d