> I've not looked at the source, but I vaguely seem to remember something
> about POV using different (faster) code for triangles that it considers
> to be flat. I guess what might be happening is that POV spots that all
> three normals are the same and therefore treats both triangles as being
> flat.

That would make sense.

> I don't know if I'd call this a bug, because a face with three equal
> normals that are not perpendicular to the face is physically
> unrealistic.

True, but then, any surface with normals that aren't perpendicular to it at
all is unrealistic - including the average smooth triangle. I know this
example is slightly odd, but I hope the developers would agree that this
isn't the best way for POV-Ray to behave in this situation. The actual code
in which I discovered this had normals which were pretty close to the true
normals of the triangles. I had two adjacent triangles which were close to
parallel but not quite, and I wanted them to appear parallel by assigning
normals like this.

It's also unlikely but possible that this situation could come up in a
normal smooth triangle mesh outputted by a modeller.

If your assessment is correct, then the problem could probably be avoided by
evaluating the dot product of the supplied normals and the true normal, and
skipping the optimization if the dot product is less than 1-epsilon.

Anyway, thanks for the workaround, it will probably help me solve my
problem.

 - Slime
 [ http://www.slimeland.com/ ]

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