"David Robinson" <tdf### [at] ezrscom> wrote:
> Can POV nativly build a helix that wraps around itself, to form a closed
> loop? Imagine a helix following the path of a curve.
>
> All I need to specify is circle diameter, helix thickness and number of
> twists (as well as the usual colour/texture etc).
>
> I expect this way to be a lot quicker, if it is supported...

There is an isosurface called the Helical Torus that does
what you're looking for. Some info on the parameters
are available at this (very excellent!) resource provided
by Mike Williams:

http://www.econym.demon.co.uk/isotut/nfunc2.htm#ht

Here are a couple of helical tori you can use as starting
points. I used the following parameter values for thin rope
trim around a window:

 f_helical_torus(x,y,z, 100, 60, 2,
         1.0, 1,
         0.5,
         0.25, 1, 1.0, 0)

These parameters were for a thick rope around the base of
a pillar:

 f_helical_torus(x,y,z, 6.0, 8, 2,
         0.5, 1,
         0.5,
         0.25, 1, 1.0, 0)

Isosurfaces are not usually considered a newusers feature
as they can be confusing, but once you get used to them
they are much more powerful than normal csg objects.

Also, if you need a mesh, I believe there are macros on
Mike's website that convert isosurfaces to meshes, but
I've not used them.

For non-circular curves, you can draw a normal
isosurface rope (f_helix1) along the path of a spline.

-Mark Slone

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