Wasn't it Max Ulbrich who wrote:
>Hi,
>
>I try to get a sum over an expression that contains data of an array. I 
>try to use a macro for this purpose and later call this in the function 
>part of the isosurface. In the part where I try to extract the x, y and 
>z coordinates from the macro arguments something goes wrong. Although I 
>do not get an error message, the x coordinate seems always to be 1.
>It seems as if x gets evaluated before the call of the macro. Actually I 
>would prefer to get this done just when the call happens. My guess is 
>that macro isn't the right solution. Who can help me?

I notice that this time you're not actually trying to index the arrays
at run time. You use atmx[0], which can be evaluated at parse time. So
in this case you can just write a loop inside your function definition
and it will all work.

It didn't produce anything very interesting when I ran it, but perhaps
that's because I didn't have very interesting values of vg, v50 or
atmx[0].

#declare F = function {
   #local i=0;
   #while (i<=arrcnt)
       +(1-1/(1+exp(-(sqrt(pow((x-atmx[0])/vg,2)
       +pow((y-atmy[0])/vg,2)
       +pow((z-atmz[0])/vg,2))-v50/vg))))
     #declare i=i+1;
   #end
}

isosurface {
   function{ 1 - F(x,y,z) }
   contained_by {
     box { <26,-27,22>,<37,-34,27> }
   }
   threshold 0.5
   max_gradient 5
   pigment {rgb x}
   translate <-35,32,-24>
   scale 0.4
}


Or, alternatively, instead of having a loop you can just multiply the
contained expression by arrcnt, since the expression evaluetes to the
same value for all values of "i".

-- 
Mike Williams
Gentleman of Leisure

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