Perspective Issue:

On 2/17/98 Kai-Mikael Jää-Aro wrote:
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From kai@nada.kth.se Tue Feb 17 12:24:26 1998
Date: Mon, 16 Feb 1998 21:02:44 +0100
Subject: Perspective rendering problems on IDesk

Greetings from chilly Stockholm,
we have lately realised that we are having a problem with the
perspective rendering on our ImmersaDesk - objects seem to follow the
viewer's head around, rather than staying in one place.  The effect
becomes more pronounced the further away the objects are.

At first we thought that this might be due to some tracking problem, but
it does not look as if the tracking error we can measure (BTW, do you
have any suggestion for calibrating the tracker?) is large enough to
cause such a big error.  We have run auto_config to see if our config
files are off, but that does not seem to affect the problem much.

Are you aware of others having similar problems and what to do about it?
(If you want I can send you an example application that clearly
demonstrates it.)
-- 
Kai-Mikael Jää-Aro	email:  kai@nada.kth.se	    "Don't win, don't lose."
CID	 		voice:  +46 8 790 62 79		-- Yoshigasaki-sensei
NADA, KTH		fax:    +46 8 10 24 77
SE-100 44  Stockholm	telex:  812 6154 1156 SICS 
SWEDEN			teletex:2401-812 6154 1156=SICS

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In response I requested the example application:
Tom Coffin wrote:
> 
> please send me the example application. I will try to
> write up a process for tracking the desk.
> 

OK, I have included a CAVELib 2.6 binary and the source code.  The
intention is to draw a line of blobs stretching into the desk box, lying
on the floor, but the line of blobs seemingly slants upwards towards the
farther end and also leans towards the sides if one moves towards the
sides.

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I took the attached binary and could not run it 
because of the feature99 built in to the licensed
copy of the cavelibrary that they have. Fortunately
they also attached the .c file which I compiled
and ran on both our ImmersaDesk (using the spacepad 
tracking system) and the Iwall (using the Flock of Birds)
-
I cannot comment on "seemingly slants upwards towards the
father end" so this very well could be a configuration
issue.
-
Re:"objects seem to follow the viewer's head around, 
rather than staying in one place.  The effect
becomes more pronounced the further away the objects are."

This does seem to happen and appears to a be correct
rendering of the viewers perspective. (see diagram below)



Though the perspective is rendered accurately, physiologically
or psychologically it appears incorrect. The example program
can be downloaded here. ftp://ftp.ncsa.uiuc.edu/VR/Apps/
The 355Kb file is called "cave". This can also be seen in
other cave apps such as "Crayoland" - as experienced viewers,
we seem to have adapted to it.

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