Wasn't it Bill Hails who wrote:

>http://thyme.homelinux.net/IsoCylinder.html

One thing that immediately jumps out as being a bit dodgy is the fact
that

        (CyFn(x, y, z).gray - 0.5) * Intensity

can be negative for some points on some pigment patterns. At such points
the isosurface will be slightly larger than the specified Radius and can
cause a thin sliver of surface fall outside the contained_by box.

I can't quite see how that would cause exactly the effect that you
showed in the image you posted, but you  could try doing something like
this to force the box to be big enough to contain the bits that stand
proud of the surface

        #declare Box_Width = Radius + sqrt(Intensity*0.5);
        contained_by {
            box {
                <-Box_Width, 0, -Box_Width>,
                <Box_Width, Height, Box_Width>
            }
        }

Or, more simply, ensure that your surface fits inside the box by not
subtracting 0.5 from the pigment

        (CyFn(x, y, z).gray) * Intensity

-- 
Mike Williams
Gentleman of Leisure

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