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From: gt4417a@prism.gatech.EDU (SKELTON,JOEL PHILLIP)
Newsgroups: comp.graphics
Subject: phunky phong triangles in rayshade 4.0
Keywords: rayshade raytrace Bob n' Ray
Message-ID: <31202@hydra.gatech.EDU>
Date: 12 Jun 91 14:31:37 GMT
Organization: Georgia Institute of Technology
Lines: 39

Has anyone else noticed problems with shading discontinuities
on objects composed of phong triangles in rayshade 4.0? As an
example if I create a sphere out of phong triangles (with all
of the normals defined correctly, i checked on that) and render
it with a light somwhewhere close to the eye point then I get
a picture of a smoothly shaded sphere. If I put the light away from
the eye and to the side of the sphere then I see a line that traces
the edge of some of the triangles where the shading suddenly
goes to almost black. The effect is something like this:


                     .....
                   .    \  .
smoothly shaded -.      /    . ------- almost black
                 .      \    .
                 .      /    .
                   .	\  .
                     .....


This is not just Mach-banding, I know what that looks like. 

If anyone else has seen this effect or has a guess as to 
why it is happening I'd really appreciate hearing from
you.

btw. , I just used the sphere as a simplified example
so I don't need any suggestions to "just use the sphere
primitive".


jps
bongo fury

-- 
SKELTON,JOEL PHILLIP
Georgia Institute of Technology, Atlanta Georgia, 30332
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