Wasn't it Hartmut Wagener who wrote:
>Hi,
>
>i have written a converter to convert from the stl-format to povray. The 
>first version of this converter is now some years old, the last one is 
>ready now written in java.
>
>But i have some problems with rendering the scenes. In binary.images you 
>can see two images, the one showing the result of the rendering, the 
>other showing the calculated norm-vectors that really look as they 
>should be for a smooth scene.
>
>When gouraud-shading a 3d-scene there might be a error called 
>mach-banding, but with phon-shading by interpolating the norm-vectors in 
>stead of the shading-values this mach-banding should dissapear.
>
>But you can see that some triangles are not calculated smooth. I can 
>post the stl-file (shortened) and the java-prog for examination, perhaps 
>it is my fault and i am not able to see it ...
>
>The problems appear in all of my stl2pov-versions, not only in the new 
>java-version ...

Although the displayed calculated normal vectors look as if they're
pointing in roughly the correct directions, I suspect that they might
not be exactly correct.

In particular, I suspect that some of the vertices have slightly
different normals in each of the smooth triangles they are used in. This
sort of thing can happen if you're calculating the normals one triangle
at a time. When I do this sort of thing, I now calculate the normals for
each of the vertices first and store them in an array.

-- 
Mike Williams
Gentleman of Leisure

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