Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!seismo!uwvax!oddjob!gargoyle!ihnp4!inuxc!iuvax!pur-ee!uiucdcs!uiucdcsm!shirley From: shirley@uiucdcsm.cs.uiuc.edu Newsgroups: comp.graphics Subject: Re: Ray tracing and caustics. Message-ID: <4400009@uiucdcsm> Date: Fri, 17-Jul-87 01:25:00 EDT Article-I.D.: uiucdcsm.4400009 Posted: Fri Jul 17 01:25:00 1987 Date-Received: Sat, 18-Jul-87 19:49:18 EDT References: <23177@sun.uucp> Lines: 34 Nf-ID: #R:sun.uucp:23177:uiucdcsm:4400009:000:1412 Nf-From: uiucdcsm.cs.uiuc.edu!shirley Jul 17 00:25:00 1987 I was also in the dark about the nuts & bolts of path tracing after reading Kajiya's paper. I was able to ask him for more detail after a talk he gave at the last SIGGRAPH conference. I asked him how a caustic appears under a clear sphere with his method. He said (approximately): Many rays are sent from the eye point through a pixel and hit the matte surface under the sphere. For each ray, a shadow ray is sent toward the light source, and since they are blocked by the sphere there is no direct lighting contribution. A reflection ray is also generated for each primary ray and some will hit the clear sphere and be refracted (twice) and hit the light source. This causes a large brightness contribution for those rays, which averaged in with the "dead" rays gives you the correct brightness for the caustic. This leads to two obvious questions: how are the reflection rays chosen, and how bright is the "large contribution"? Also you should note that if a reflection ray from a matte surface hits a light source DIRECTLY, no extra contribution should be added, because the direct lighting term was already calculated directly. I have modified my ray tracer to "path trace" and the results are nice, but (not suprisingly) noise is a real problem unless MANY rays are used, especially when caustics are involved. Peter Shirley U of Illinois at UC