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From: markv@pixar.com (Mark VandeWettering)
Newsgroups: sci.virtual-worlds
Subject: Re: Real-time raytrace and some related "ramblings"
Message-ID: <1991May28.203108.6738@pixar.com>
Date: 28 May 91 20:31:08 GMT
References: <1991May13.193721.5473@milton.u.washington.edu> <1991May14.
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Organization: Pixar -- Point Richmond, California
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In article <1991May14.202045.1020@milton.u.washington.edu> lance@motcsd.csd.
mot.com (lance.norskog) writes:

>Ray-tracing is inherently 2D, while radiosity is inherently 3d.
>Ray-tracing works from the eye to a background, radiosity works from
>a light source until the light peters out.  

Raytracing is _not_ inherently 2D.  It is an approximation to solving
the "rendering equation" or global illumination problems.  Raytracing makes
the assumption that the only "interesting" light transport occurs in directions
that end up at your eye position (or recursively, the point of origin for a 
ray).  While one may argue (correctly) that this does not simulate all 
possible light paths, it does make a reasonable attempt at reproducing some
subset of possible lighting situations.

Radiosity also makes a similar assumption.  The idea behind (most) radiosity 
implementations is that there is no view dependent illumination in the scene.
In other words, all scene elements are diffuse reflectors.  This too, is 
a reasonable assumption, which yields a different subspace of possible
illumination situations.  

>You can do a radiosity
>pass once, and save all the surface pixels in a 3D sparse data structure.
>(Voxels are one technique.)  Then, just move around the 3-space and
>continuously walk the data structure, displaying successive images
>from your pre-computed database.  You can't move the objects, and
>you yourself are transparent (no visible effect on the shading)
>but it's computationally much less intensive.  

And produces a different set of effects.  Interesting, but also different.

>Also, radiosity gives a more realistic, software look.  Check some
>computer graphics books.

"Realism" is subjective.  Hybrid algorithms that combine raytracing and 
radiosity seem to be the most realistic, because they are able to capture
reflection and refraction from both diffuse and specular surfaces.

>Takes a lot more RAM, though.  You have to store enough detail for
>each surface that it looks OK close up.  Also, you have to move
>through the database quickly using a 3D line-drawing algorithm.
>It is pre-sorted, though.

Storage can be higher.  The main problem is that in order to avoid aliasing,
you patches should all be on the order of a pixel in size.  The reason that 
most radiosity pictures look okay is because we expect diffuse illumination
to change only slowly over the picture.  Still, look carefully at places where
tables and chairs meet the ground, and you will not see the sharper shadows
that one would expect.  This is largely due to insufficient grid resolution.

Mark VandeWettering