Path: utzoo!attcan!uunet!cs.utexas.edu!asuvax!enuxha!hollasch
From: hollasch@enuxha.eas.asu.edu (Steve Hollasch)
Newsgroups: comp.graphics
Subject: Arbitrarily-Shaped Light Sources
Keywords: ray trace light graphics algorithm
Message-ID: <1438@enuxha.eas.asu.edu>
Date: 16 Sep 90 22:33:01 GMT
Organization: Arizona State University
Lines: 47


    How do raytracers make light sources out of arbitrary objects?  I
thought a while back that one approach would be to find the direction to
the object from the illuminated point, fire a random cone of rays at the
object, and assign some fraction of the object's light to the point for
each unobstructed ray.

    The main drawback of this approach, as I see it, is that it would
yield a mottled illuminated area, and the mottling would vary in a
random manner.

    About five minutes ago I had an idea for another approach:

    -  Find the 2D bounding box (from the illuminated point's view) of the
       illuminating object.

    -  From this box, get the two orthogonal basis vectors.

    -  Now subdivide this bounding box (using the basis vectors), just
       as you would the original raytrace grid.

    -  For each light ray fired, determine if the ray intersects the
       illuminating object.  If it does, increment the `silhouette'
       counter.  If the light ray intersects no other object, then
       increment the `light' counter.

    -  Once done, the light that shines on the illuminated point is
       (light_counter/silhouette_counter) * object_light.

    This technique would also lend itself to numerous optimizations.
For example, if you assume that all light objects cast a convex
silhouette, then you could use binary search techniques to locate the
edges of the silhouette.  That is, you can assume that all scan lines
will be runs of space-silhouette-space intersections, hone in on the
edges, and then multiply the resulting silhoette width by the scanline
height to get the relative area.

    Is there a better way to do this?  I haven't come across this
problem in any of the graphics texts I've read.

______________________________________________________________________________
Steve Hollasch                       Arizona State University @ Tempe, Arizona
hollasch@enuxha.eas.asu.edu  |  uunet!mimsy!oddjob!noao!asuvax!enuxha!hollasch
-- 
______________________________________________________________________________
Steve Hollasch                       Arizona State University @ Tempe, Arizona
hollasch@enuxha.eas.asu.edu  |  uunet!mimsy!oddjob!noao!asuvax!enuxha!hollasch