From: dchapman@ncsa.uiuc.edu (Dee A. Chapman)
Subject: TECH: Distortion in VR view
Date: Tue, 19 Jan 93 15:23:53 CST
Organization: Nat'l Ctr for Supercomp App (NCSA) @ University of Illinois


Hi,

I've noticed some "distortion" in viewing 3D graphics objects which
were rendered using an SGI SkyWriter (GL Library).  For example, if I 
create two divergent rectangular boxes, and view them on the flat screen
of the computer console, they will appear to converge or be parallel:

				back
	-----		-----		-----		-----
	\    \	       /    /		|   |		|   |		
	 \    \	      /    /		|   |		|   |
	  \    \     /    /   ======>   |   |		|   |
	   \	\   /    /		|   |		|   |
	    -----   -----      front	-----           -----
	("real" life view)	        (graphic representation)

I believe this effect is due to the use of the "perspective" GL
library call.  (void perspective(Angle fovy,aspect, znear, zfar)) I
realize that the objects should appear to converge as they get farther
away from the viewer, however the effect I'm referring to is that
them seem to converge excessively. I can lessen this "converging" effect 
by varying the fovy parameter.  However, this can introduce other inaccuracies. 
>From a simulation point of view, I'd like to make this graphics representation 
as close to "real" life representation as possible.

In addition, I've noticed when I view the 3D graphics objects using
a Flight Helmet head mounted display, the "converging" effect is
lessened.  I'm thinking that is probably due to the lensing in the
helmet.  Any other ideas?

I'd really appreciate either some references to papers or books that deal 
with these issues, or your own personal experiences with this.  
Email would be preferable.  Thanks a lot for your time.