Steve Baker (steve++at++mred.bgm.link.com)
Wed, 6 Nov 96 14:55:32 -0500
Nathaniel Bletter <nat++at++od.sri.com> said:
> As Dennis Pierce said, you can use a multi-pass rendering scheme to get rid of
> distortion. This has been used in the past and is documented on Paul Haeberli's
> texture mapping tricks page in Graphica Obscura
> (http://www.sgi.com/grafica/texmap/index.html). You basically rerender the
> final square image onto a surface curved opposite from the distortion you are
> trying to correct. The second rendering pass doesn't take too long since you
> can turn the z-buffer off as there is only one surface.
This is interesting stuff - but you should realise that for display on
a FLAT screen (such as a CRT), the image that Performer generates for
a single channel IS NOT DISTORTED IN ANY WAY .... provided you didn't
lie to Performer about where the eye is with respect to the screen.
In order to get this information to Performer, you can either use
the simple setup with a horizontal and vertical field-of-view - or
create your own fancy offset frustum. You need the latter if the
viewers eye is not positioned on an imaginary line which is
perpendicular to the center of the channel.
It follows that any attempt to get rid of this aparrent distortion
is doomed to failure of one sort or another - unless the end result
of that is to put the simulated eyepoint back where it should be.
The fancy distortion correction ideas out there (typically
two-pass tricks on SGI boxes...performed in hardware on some
other big-box IG's) are usually there to cope with curved
screens of one kind or another (toroidal, spherical, whatever).
There is often a temptation in computer graphics to try to see more
of the virtual world through this small screen-sized window than
you should be able to see. People do this by kludging the field of
view - it's a useful trick - but distortion is a penalty that
you simply can't avoid. It's just how geometry works.
Steve Baker 817-619-1361 (Vox-Lab)
Hughes Training Inc. 817-619-8776 (Vox-Office/Vox-Mail)
2200 Arlington Downs Road 817-619-4028 (Fax)
Arlington, Texas. TX 76005-6171 Steve++at++MrEd.bgm.link.com (eMail)
http://www.hti.com (external) http://MrEd.bgm.link.com/staff/steve (intranet)
http://web2.airmail.net/sjbaker1 (external)
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