I'm not a programmer type and I might be wrong, but there seems to be a
problem of ambiguity. This is how I envision it: the normals of a smooth
triangle are calculated via interpolation of its 3 corner normals. When one
normal is rotated more than 180 degrees in relation to the others, how
should POV know which way to do the interpolated rotation? Bad ASCII art:

 |   /
--- --- ---
         \

 |   \
--- --- --- ---
         /   \

The end result is the same, but the intermediate normals are different. As
long as the angle difference is less than 180 degrees there is no ambiguity.

Margus

Gordon wrote:
> 
> It does seem strange to me as well. In the particle system I am working on,
> I'm using smooth triangles as the particles. I calculate normals that point
> at 90 degrees to the average normal of the triangle. These normals are
> parallel to the surface and have had no error mesages like this.
> 
> Any programmer types who get into the POV code care to explain in detail?
> 
> Thanks
> Gordon
> 
> david sharp wrote in message <36F### [at] interportnet>...
> >
> >fourth dimension?
> >a ball doesn't have to twist into a fourth dimension to have
> >normals pointing in *every* direction.
> >
> >Since Nathan Kopp posted that at least that one triangle was OK with
> >POV-Ray 3.1a, I find this still mysterious.

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