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From: drco@sphinx.UUCP
Newsgroups: comp.graphics
Subject: AmigaWorld Ray-Tracing Article
Message-ID: <1514@sphinx.uchicago.edu>
Date: Wed, 22-Apr-87 12:22:47 EST
Article-I.D.: sphinx.1514
Posted: Wed Apr 22 12:22:47 1987
Date-Received: Fri, 24-Apr-87 03:44:16 EST
Reply-To: drco@sphinx.UUCP (david lee griffith)
Organization: U Chicago Computation Center
Lines: 39

This is my first posting ever so please be patient.

I do not currently own an Amiga (although my next student loan may go towards
a 500 if it turns out to be as good as the propaganda says it is), but 
the postings about the AmigaWorld ray-tracing article interested me, so
I bought a copy.  The long and fruitless comparisons between the Amiga
and the Cray seem to have obscured a crucial point about the program 
printed in the article. Namely that the ray-tracing algorithm given therein
is really, really slooooow.
     As written, the program will test for each pixel and each
sphere in the universe whether the line from the observer to the
patch of the perspective plane corresponding to the pixel intersects that 
sphere.  With 50 - 100 spheres in the universe, and 320x400 pixels
(or however many there are) that is a whole bunch of comparisons. 
If each comparison requires two floating-point multiplications and
a subtraction, no wonder the program takes so long!  Some sort of pre-
processing (a simplified Z-buffering comes to mind) should be included
so that only necessary comparisons are done and !!NO!! line-sphere 
intersection tests should be done on pixels that turn out to be 
background. Of course your programs are going to take hours if you use
such brute force techniques as using ray-tracing to do hidden surface
elimination.  
     Other problems with the code are less obvious. Little things
like the fact that the color of a mirrorred sphere is irrelevent
(I want my jugglers to have golden balls to work with :-) ), and
that if a bright red light reflects off of a shiny surface the 
highlights turn out white. I won't even talk about the really 
unrealistic model of specular reflection.  
     In short, what we have so nicely listed in the article is 
some really slow and sloppily conceived code.  This would not bother
me except that an improved version of it is being made available for
commercial release.  Please tell me that not all commercial Amiga
software is of this quality.  Such a wonderful sounding machine 


-- 
...ihnp4!gargoyle!sphinx!drco				Dave Griffith
                                                        University of Chicago
                                                        Mathematics Dept.