On Thu, 2 Sep 1999 17:06:21 +0200, Gail Shaw wrote:
>Can anyone tell me how the built in isosurface function helix1
>works. I tried
>
>isosurface {
> function { "helix1" <2,1,0.2,1,0,1,0> }
> threshold 1
> sign 1
> pigment {Red}
>}
>
>but just got a cylinder.
>
>Any help will be appreciated.

Try this:


isosurface {
 function { "helix1" <2,1,0.2,1,1,1,0> }
 threshold 0
 max_gradient 20 method 2
 bounded_by {box {-10,10}}
 sign 1         
 pigment {red 1} finish {ambient .4}
}

Most of the internal functions are designed to work with a threshold of
zero.  By default, they're bounded by a small box, so you need to add a
bounding volume if you want to see anything interesting.  I also added
the max_gradient 20 method 2 to eliminate some gaps in the result.

Finally, the docs for parameter 4 ("shape parameter") are useless, but
a little playing shows that it is the ratio of height to width for the
ellipses the "strands" are made out of.  Zero is a bad number, and will
in fact crash on some architectures.  Replacing it with a one gives 
you a circular cross-section, which is what I assume you wanted.  I'll
try to remember to fix this in the next version of the docs.

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