Wasn't it Oleguer Vilella who wrote:
>Hello Mike,
>
>Can you explain again how did you do this translation?
>====================================
>isosurface {
>   function{DNA(f_r(x,z,y) ,f_th(x,z,y),z)}
>====================================
>What's f_r and what's f_th?
>

If you're not familiar with polar coordinates, I don't think I can teach
the concept in a newsgroup posting.

If you are familiar with polar coordinates, then f_r(), f_th() and
f_ph() are standard functions (in functions.inc) which give you the
polar coordinates R, Theta and Phi. On a globe, they would be altitude,
longitude and latitude.

By transforming a function from Cartesian (ordinary xyz) coordinates to
polar coordinates we can bend the space that contains the function so
that the grid lines of the space are wrapped round into circles.

There are spherical polar coordinates (R Theta Phi) and cylindrical
polar coordinates (R Theta z) and it's the second of these that I used
for this scene.

-- 
Mike Williams
Gentleman of Leisure

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