From: rwoodsaj@cc.curtin.edu.au
Subject: Re: How do I generate the two views for a 3D image (LCD glasses)
Organization: Curtin University of Technology
Date: Mon, 7 Sep 1992 04:03:24 GMT

This as been a fascinating conversation and I've just been itching to to get
in my two cents worth.  With over 20 messages so far I've had trouble keeping
up...

With regards to the original question (does anyone remember it?)  

>  My question is how to generate the left and right images for a
>stereo display.  Three transforms come to mind:  1) pan the "camera" left
>and right about 10 degrees (or less) 2) dolly the camera left or right
>perpendicular to "forward" 3) dolly the camera one way and pan back the
>other simultaneously, which I think is what they eyes do relative to the
>nose.  I may be missing the actual transform entirely- I haven't given this
>too much thought yet. 

The money is behind door number two.  The parallel camera configuration
produces the least distortions.

With a stereoscopic "video" display (I am NOT talking about cross-eyed stereo
here) it is necessary to make to virtual or real cameras 'converge' at a
certain distance.  I call this the "convergence distance".  Converge may not
be a very good word to use here, but bear with me.  What it refers to is the
fact that the sight lines of the two cameras overlap at a certain distance
from the cameras.  

The distance at which the images from the two cameras converge sets the
object distance for which 3D images will appear on the surface of the
stereoscopic display.  If an object is closer to the cameras than the
convergence distance, it will appear in front of the display surface (in
viewer space).  If an object is positioned further away from the cameras than
the convergence distance, then it will appear inside the monitor (in monitor
space).

In stereoscopic display with real cameras the convergence distance can be set
by three methods:
(A) The cameras can be angled in or toed-in.  This is often called the
converged camera configuration. 
(B) the cameras are parallel and the CCD or film is (axially) shifted behind
the lens (the lenses are parallel and the CCD/film lie in the same plane) and
(C) The image is shifted at display.  In video this requires electronics to
process the image.  In film this simply requires a shift in the film behind
the projector lens.  

There is also a fourth convergence method which only relevant to projected
stereoscopic images which involves converging (toeing-in) the projectors (if
two are used).  Obviously this is not relevant to video.

In (C) there is the problem of cropping of the images and blank areas on
either side of the screen for the left and right images.  In addition, with
video it is particularly hard to perform a horizontal shift of the image (an
order of magnitude more complex than just displaying the image).  This does
not occur with options (A) and (B).

If we are talking about computer graphics, methods (B) and (C) are
essentially the same thing since they are implemented in a single equation. 
There is no need to calculate the intermediate step of the film or CCD.  And
since there is no film or CCD limiting the size of the aquired image there is
no cropping or blank edges.

In (A) (The converged camera configuration), the cameras (lenses and CCDs)
are angled inwards and there are a two distortions apparent.  The first and
most obvious is keystone distortion.  Objects at a equal depth from the
cameras will appear taller at one side of the image than the other.  This
results in vertical height differences (vertical disparity) between the two
images which in turn makes it harder for the stereoscopic image to be viewed. 
The other distortion is depth plane curvature which means that a plane of
equal depth away from the cameras will actually appear on the monitor as a
curved surface.  At the centre of the screen the plane will appear closer
than at the edges of the screen.  A diagram would help here but words will
haveto suffice.  

With (B), the parallel camera configuration, there is no keystone distortion
and no depth plane curvature.

Unfortunately real-world cameras with real-world lenses and real-world CCDs
don't allow the easy manipulation of the CCD behind the lens.  David Drascic
and I and many other 3D-video researchers use two conventional video cameras
mounted in a stereo-pair.  It would be nice to use an off the shelf item like
the Toshiba 3D-cam but it is the only one available and is not high enough
spec. for my application which requires high resolution and low light
illumination and more importantly is not available in PAL.

Most conventional video cameras have the CCD mounted behind the lens in a way
that is very inaccessible and not conducive to being moved and adjusted in an
accurate manner.  In some cameras I have the CCD is rock solid (no horizontal
CCD movement possible).  Other cameras the CCD mounting is so sloppy any
slight vibration would readjust the position of the CCD (mucking up both
vertical and horizontal alignment).  It is therefore usually necessary to
rework the CCD mount so that acurate CCD alignment is possible.  This is
vital to the parallel camera configuration, therefore the easiest way to
align video cameras is to converge them.  The CCDs are fixed in position and
the cameras are rotated around until accurate convergence distance and
alignment is attained.  In most situations the amount of distortions incurred
because of this compromise are small.

Is this coherent?

Rick and David write:
>>I've heard that there are problems with crosstalk, however.

>More so than with active glasses?  (You haven't seen it yet.  I guess
>you wouldn't know yet.)  That would surprize me because some of the
>active consumer glasses on the market are actually made with a
>*cheap* version of Tek shutter materials. (I don't recall which ones.)
>At least they were a few years ago.  (Tek didn't want to market a 
>cheap consumer product for fear that the product would be used to 
>prejudge our high-end industrial products.)

>I'm not an LC shutter expert, but I think it's true that the best LC
>shutters
>don't have as good an extinction ratio as the best plain polorizers, so the
>crosstalk would of course not be as good as the best stereo you
>have seen.  

We have both a pair of Stereographics Crystaleyes Active LCD glasses and
Tektronix SGS410 Polarising screen.  The Crystaleyes definitely have less
crosstalk, not by much but there is definitely a difference.  This probably
comes from the fact that the active glasses only use a small piece of LC cell
in front of each eye rather than a large LC cell over the entire face of the
monitor.  We prefer the polarising screen though because the passive glasses
(like sunglasses) and less obtrusive than the large Crystaleyes.

Rick writes:
>I think I just had a lightbulb turn on concerning the different
>geometries of a split pair vs. an overlaped polorized pair.  But for
>what I'm thinking about to be optimum the camera convergence needs to match
>and track the eye convergence as the eyes scan in and out of the image
>space.  Is that true?  

NO WAY!  If the convergance of the cameras changes with the changes in
convergence of the eyes then the brain would become incredibly confused.  I'm
talking MEGA-SPEW here.  I personally believe eyes should percieve a
relatively stable image (in the depth dimension anyway).  Apart from
relatively slow changes in camera parameters they should stay the same while
an operator is concentrating on the image (eg. in teleoperation), otherwise
the image would appear to more back and forth in front of you all the time.



Rick, your photo of a water faucet (tap in this part of the world) may be a
case of excessive camera separation increasing the effect of keystone
distortion in the image from the converged camera configuration.  It may also
be that the depth in the image is also too great.  This would not be fixed by
using the parallel camera configuration, but by reducing the camera
separation.  Without knowing your setup it is hard to comment.  What camera
separation, camera focal length, convergence distance , film size and display
size were you using?


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  Andrew Woods   | Centre for Marine Science and Technology
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                 | Curtin University of Technology,
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