The second derivative of all of the POV-Ray noise functions is discontinuous
at integer X, Y, or Z values. That causes the *first* derivative of the
*shading* of objects based off of the noise to be discontinuous there. This
creates a noticeable grid effect.

It's the way the noise is made to work internally. I've heard that there are
ways to fix or reduce the effect, but they haven't been implemented in
POV-Ray.

Try creating a pigment function based on the bozo pattern (which is really
just f_noise3d as a pattern, but be sure to add a color_map{[0 rgb 0][1 rgb
1]}), and then turbulating it, and using that function instead of f_noise3d.
The turbulence will at least move the grid around so it won't be quite so
noticeable.

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

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