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From: nwatson@enuxha.eas.asu.edu (Nathan F. Watson)
Newsgroups: comp.graphics
Subject: Re: phunky phong triangles in rayshade 4.0
Keywords: rayshade raytrace Bob n' Ray
Message-ID: <3574@enuxha.eas.asu.edu>
Date: 12 Jun 91 19:20:02 GMT
References: <31202@hydra.gatech.EDU>
Organization: Arizona State University, Tempe, AZ
Lines: 59

In article <31202@hydra.gatech.EDU>, gt4417a@prism.gatech.EDU (SKELTON,JOEL PHILLIP) writes:
> Has anyone else noticed problems with shading discontinuities
> on objects composed of phong triangles in rayshade 4.0?

> ... 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
>                  .      \    .
>                  .      /    .
>                    .	\  .
>                      .....
> 
> jps
> bongo fury

My guess is that triangles to the left of the boundary have their TRUE
outward normals pointing toward the light source, and so include the
mentioned light source in their shading calculations.  The triangles to the
right have their TRUE outward normals pointing away from the light source
and so eliminate the mentioned light source in shading calculations.
The left triangles along the boundary will be smoothly shaded using the
light source, whereas right triangles will be "black".  Because the
phongly-calculated normals along the boundary are not necessarily
normal to the vector to the light source (and may, indeed, point away
from the light source), the transition will not be smooth.

A proposed solution (that I'm not sure will work):

Handle lighting calculations w.r.t. the mentioned light source as follows:
   (a) For triangles whose vertex normals all point toward the light
       source:  Use phongly-calculated normals as usual.
   (b) For triangles whose vertex normals all point away from the light
       source:  Reject the triangle as it is not lit by the light source.
   (c) For triangles with vertex normals point both at and away from the
       light source:  Calculate the phong-normal at each point and determine
       whether the light source should be used.

I suspect that triangles in (a) will usually have their outward normals
pointing toward the light source, those in (b) will usually have their
outward normals pointing away from the light source.

Those in (c) will have outward normals pointing toward and away from
the light source.  The mentioned check should be applied in either case
to provide a smooth transition.

Sorry for the wordy description.


-- 
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Nathan F. Watson                             Arizona State University
nwatson@enuxha.eas.asu.edu                Computer Science Department
"Remember:  No matter where you go, there you are." - Mr. B. Banzai