> When defining the function for an isosurface I would lke to be able
> to use #while loops, for example, to compute a series to some specified
> precision.

Unfortunately, you can't really have loops (although there is a summation
function which can help in some cases) or variables, but some things can be
faked if necessary, especially with recursion.

Here are some handy tricks:

The functional equivalent of precomputing a variable for multiple later uses
is this:

#declare func1 = function(x,y,z, precomputed){...}
#declare func2 = function(x,y,z) {func1(x,y,z, computeSomething(x,y,z))}

An if/else type of thing can be done with select():

select(
    x-3, // "if x is less than 3", or more precisely "if x minus 3 is less
than zero"
    x, // then
    -x // else
)

And this can be used with recursion, which can be used to replace a while
loop. For instance, Warp once wrote this code to create a mandel fractal
(from "Re: Difference Small objects and Glass Cube" on Tuesday, September
14, 2004 11:28 AM in povray.general):

#declare MandIter =
  function(n, Re, Im, Zr, Zi)
  { select(n>50 | Zr*Zr+Zi*Zi > 4, 0,
           MandIter(n+1, Re, Im, Zr*Zr-Zi*Zi+Re, 2*Zr*Zi+Im), n/50)
  }

#declare Mandel = function(Re, Im) { MandIter(0, Re, Im, Re, Im) }

> Direct email replies welcome.

I think it's best to reply via the newsgroup so the community benefits from
the knowledge passed on.

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

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