Wasn't it Anthony D. Baye who wrote:

>The only way I can think of is to use the object pattern.
>f/ex.

That technique comes top of the list in the "Things That Don't Work"
section in my isosurface tutorial.
http://www.econym.demon.co.uk/isotut/dont.htm

Unfortunately, any path through the isosurface encounters a point where
the value jumps instantly from 0 to 1, so the actual max_gradient of
every point on the surface is infinite. Instead of getting a few nasty
little holes in our nice isosurface, we get a few tiny patches of
surface and the rest is hole, unless very extreme values of max_gradient
are used (and then it takes ages).

Also, the idea of adding a displacement function to the surface doesn't
work quite like what you'd intuitively expect. Adding a function to the
object pattern function gives one region of space where the function
evaluates to 1.0+displacement and in the rest of space it evaluates to
0.0+displacenent. 

-- 
Mike Williams
Gentleman of Leisure

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