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From: skinner@saturn.ucsc.edu (Robert Skinner)
Newsgroups: comp.graphics
Subject: Re: Ray tracing and caustics.
Message-ID: <543@saturn.ucsc.edu>
Date: Mon, 13-Jul-87 15:45:04 EDT
Article-I.D.: saturn.543
Posted: Mon Jul 13 15:45:04 1987
Date-Received: Fri, 17-Jul-87 04:37:31 EDT
References: <219@sugar.UUCP>
Organization: U.C. Santa Cruz, CIS/CE.
Lines: 29
Keywords: ray-tracing caustics algorithm reality
Summary: see Jim Kajiya's "Rendering Equation", Siggraph '86

In article <219@sugar.UUCP>, peter@sugar.UUCP (Peter DaSilva) writes:
> I was looking at a beautiful ray-traced image not so long ago and wondering
> why it looked less than real. There was something wrong with a transparent
> object in it. I had to refer to a *real* transparent object to figure out
> what it was: it had a nice diffuse shadow, but there was no caustic in and
> around the shadow from the refraction of light rays from the light source.


See Jim Kajiya's 'Rendering Equation' paper in the Siggraph '86 proceedings.
It has an image with caustics, and caustics were mentioned repeatedly in the 
Advanced Image Synthesis tutorial that year (I'm told).  

Kajiya's paper describes the equations for solving all rendering problems, 
which includes radiosity, caustics, color bleeding, etc.  He never 
explicitly says in the paper how these effects were accomplished, 
but it seems like they trace multiple rays in random directions from 
any ray/surface intersection.  If these rays intersect an illuminated surface,
they contribute to the illumination of the first surface, just like a
light source, hence you get color bleeding (see the underside of the 
upright structure in the above mentioned image).  
If the ray happens to hit a light after refraction through
a glass ball, the highlight will be reflected and will appear as a caustic.

Seems plausible, but does anyone REALLY know how Kajiya does it?
How are these extra rays chosen?  How many are used?  
etc, etc.

Robert Skinner
skinner@saturn.ucsc.edu