Wasn't it Andrel C. Linnenbank who wrote:
>I am not entirely sure if this has been reported before.
>I used  povray v3.5b8 on a 750MHz PIII, windows 98
>
>When using sphere_sweep to generate a hollow tube there are 
>some problems in rendering the inside of the tube.
>
>Strange bands of discontinuous normals seem to appear in areas where
>the spheres make a bend.

I confirm this. The effect seems to happen with any inverse
sphere_sweep, or any difference {something, sphere_sweep}. The normals
in the inverse sphere_sweep are not the inverse of the normals of the
sphere_sweep.

>Second point is that I used a parameterization for the sizes
>of the spheres that depended on the clock to generate an animation.
>It was a rather complex formula. I now use a different method where 
>I generate an different include file for every frame. The reason 
>is that this seems to be much faster. Apparently the formula 
>is computed for every pixel in stead of ones per frame. I
>think some caching may improve the speed of rendering. 

I can't reproduce that with simple formulae that use the clock.
You're not using functions by any chance are you?

-- 
Mike Williams
Gentleman of Leisure

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