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From: wecker@cookie.dec.com (DAVE  TANSTAAFL  WECKER)
Newsgroups: net.micro.amiga
Subject: Ray tracing algorithm
Message-ID: <6135@decwrl.DEC.COM>
Date: Tue, 28-Oct-86 15:16:24 EST
Article-I.D.: decwrl.6135
Posted: Tue Oct 28 15:16:24 1986
Date-Received: Wed, 29-Oct-86 08:48:40 EST
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Organization: Digital Equipment Corporation
Lines: 46

Several people have asked me to post a SHORT description of exactly what a
ray tracing algorithm is.. so I'll try.

Ray tracing is a technique for generating very "life like" images
through one of the most compute intensive algorithms around. The basic steps
are as follows:

  For each pixel on the screen:
    Set the pixel to black.
    Draw a vector from the position of your eye to the pixel. (Yes you
      can do steroscopic images by defining 2 eye positions)
(a) Continue the vector until you hit something.
    Depending on the object you hit:
      Pass the vector through (with degradition along any RGB
        component, e.g. transmission of certain colors) applying any
	refraction coefficients for the object.
      Bounce the vector off of the object (with appropriate degradations and
	scatterings).
      If the object was a light source stop and back the vector out 
	(back to the original pixel with computed RGB degradations) and
	add it to the pixel value.
      If the vector goes off the edges of the "world" throw it away.
    At this point you should have a bundle of vectors (bounced, scattered,
      bent). Take each new vector and recursively call (a) until you hit
      a maximum recursion depth.
    The cumulative value at the pixel is the correct RGB value to display
      (built up from all RAYS that hit that pixel on its way to your eye).

  That's pretty much it, there are however ALOT of got-ya's:

    - Usually, there is really no such thing as "black" so you you need to
      define an ambient lighting factor for all objects.

    - The algorithm above will give you the dreaded "jaggies" whenever there
      is a straight diagonal line in the picture. The images I sent out use
      a technique called "distributed ray tracing". This simply means
      that some number of rays are sent from the eye through the pixel 
      (usually between 2 and 10) instead of 1. Each ray is slightly off
      center, so the net effect is to "blur" sharp edges avoiding the 
      jaggies".

    - How many times do you let a ray bounce inside of a glass sphere before
      you give up on it?

    - Too many things to mention... you're all just going to have to wait
      until I get the AMIGA version completed.