Path: utzoo!attcan!uunet!shakti!dilip
From: dilip@ncst.ernet.in (Dilip Khandekar)
Newsgroups: comp.graphics
Subject: Re: Lunar Distortions
Keywords: raytracing, distortions.
Message-ID: <1097@shakti.ncst.ernet.in>
Date: 19 Nov 90 08:25:51 GMT
References: <27332@cs.yale.edu>
Reply-To: dilip@ncst.ernet.in (Dilip Khandekar)
Organization: National Centre for Software Technology, Bombay, INDIA
Lines: 36

In article <27332@cs.yale.edu> musgrave-forest@CS.YALE.EDU (F. Ken Musgrave) writes:
>
>  Here's another little question spawned by moon-renderings:
>
>  A very wide-angle view of a scene (i.e., a landscape), with a sphere
>(i.e., a moon) in an extreme corner of the image, sports one very distorted
>sphere in the image, when rendered using the standard virtual-screen model
>for ray tracing.  (See the cover of Jan. '89 IEEE CG&A for an example.)
>
>  Seems that this is a version of the sphere-to-plane mapping problem, and
>therefore inadmissible to a non-distorting solution.
>
>  Can anyone out there prove this conjecture right or wrong, or demonstrate
>some nice workaround?
>

   Yes, this distortion is due to sphere-to-plane mapping. Actually, the 
volume defined by the rays joining every point on the sphere to the center of
projection will be a cone. Now an intersection of this cone with a plane 
will invariably result in an ellipse. So EVERY sphere, should look like an
ellipsoid in a ray-traced image which uses this model. But the distortion is
prominent only for spheres located in extreme corner of the scene.

   I also encountered the same problem and would be interested in any method
or projection-model which circumvents this problem. If the pin-hole camera
model is not a good model for the human eye-brain system then is there any
other model which is more accurate?

- dilip

===============================================================================
Dilip Khandekar
Graphics and CAD Divn.,                 email:   dilip@ncst.ernet.in
National Centre for Software Tech.,           or uunet!shakti!dilip
Juhu,   Bombay,   INDIA.                  Tel: +91-022-6201606,6201574.
===============================================================================