Wasn't it Simon who wrote:
>That is very clear! Thanks Mike - If you don't mind some further 
>questions...
>
>Say we wanted to do something like an infinitely long crinkle-cut chip 
>(excuse the awful description)
>
>Dealing with 1 dimension at a time...
>
>y<1+sin(x*2*pi)
>y>-1+sin(x*2*pi + pi/2)
>
>how can I combine the 2? - to be more precise, how do I define the "solid" 
>bit as the area between the two?
>
>If you'd rather point me at a tutorial and leave me to it, I understand but 
>if you're willing to walk me through this, I'd greatly appreciate it
>
>Thanks in advance!

You can use this trick to do two dimensions at once

http://www.econym.demon.co.uk/isotut/substitute.htm#thick

Like this:

     function { abs(sin(2*pi*x)-y)-0.25 }

To do four directions, you can use max() to perform the intersection of
the simpler functions.

#declare  F = function(x,y) {0.1*sin(2*pi*x)+y}

isosurface {
  function { max( F(x,y-1), F(x-0.5,-y-1), F(x,z-1), F(x-0.5,-z-1)) }
        max_gradient 1.2
        contained_by{box{<-5,-2,-2><5,2,2>}}
        pigment {rgb <0.9,0.8,0.7>}
}

-- 
Mike Williams
Gentleman of Leisure

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