Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!seismo!rutgers!ucla-cs!zen!ucbvax!jade!saturn!skinner From: skinner@saturn.ucsc.edu (Robert Skinner) Newsgroups: comp.graphics Subject: Re: Ray tracing and caustics. Message-ID: <552@saturn.ucsc.edu> Date: Mon, 20-Jul-87 13:13:56 EDT Article-I.D.: saturn.552 Posted: Mon Jul 20 13:13:56 1987 Date-Received: Fri, 24-Jul-87 02:36:13 EDT References: <23177@sun.uucp> <4400009@uiucdcsm> Organization: U.C. Santa Cruz, CIS/CE. Lines: 53 Summary: clarification wanted... In article <4400009@uiucdcsm>, shirley@uiucdcsm.cs.uiuc.edu writes: > (Kajiya) 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. > I still need some clarification.. Suppose the image is set up like this: (Reflected Ray) ^ . - . |S| <-- Light - ^ | Eye Where the 'S' is a glass sphere. If a ray is traced from the Eye to the left of the Sphere where the shadow and caustic should appear, the reflection ray goes up in the direction indicated, NO? This is should not intersect the Sphere or the Light, so how can this create a caustic? Is the term 'reflected ray' a misnomer? Are we talking about a different sort of ray than the usual 'reflection ray'? I have yet to hear an explanation more satisfying than the following: Standard shadow tracer rays are used to generate the regular shadow. In addition, a number of random rays are traced away from the surface in random directions, hoping to hit some (indirect) light source, such as a brightly illuminated surface nearby. The rays that are refracted twice through the sphere and hit the light source contribute to the caustics. Once again, How are these 'reflected rays' chosen? This seems to be Mr Shirley's question, but the overloading of the term reflected is very confusing here. Robert Skinner skinner@saturn.ucsc.edu (nothing cute here until I get my USENET act back together)