Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!uxc!uxc.cso.uiuc.edu!m.cs.uiuc.edu!shirley
From: shirley@m.cs.uiuc.edu
Newsgroups: comp.graphics
Subject: Re: MTV-Raytracer on ATARI ST
Message-ID: <4400031@m.cs.uiuc.edu>
Date: 8 Mar 89 16:22:00 GMT
References: <1171@laura.UUCP>
Lines: 21
Nf-ID: #R:laura.UUCP:1171:m.cs.uiuc.edu:4400031:000:726
Nf-From: m.cs.uiuc.edu!shirley    Mar  8 10:22:00 1989




The problem may be pretty simple.  When a ray is reflected (in the
form "origin + t * direction", the next intersection is chosen to
be the one where t is positive and otherwise the smallest.  There
will always be an intersection at t = 0 (the reflecting surface).
Most ray tracers have some code like

EPSILON = <some small positive number>
for all intersections
	find minimum t > EPSILON

This avoids the problem that occurs when that t = 0 turns into a
t = +small.  If single precision is used the EPSILON must be
bigger.  I think the often-referenced SIGGRAPH ray-tracing
tutorial talks about how to handle this problem in several ways,
but the hack above is the easiest to implement.

Peter Shirley
shirley@cs.uiuc.edu