Norm Krumpe wrote:

> For spirals, you might check out the nice little OpenGL freeware program
> called "Helixir" at http://www.evolve.co.uk/helixir/

The homepage looks nice, but the program is Win-only unless
they release the source and somebody hacks it to work with
Mesa.

From the original question I guess that aratcliffe
meant helixes, not spirals (Helixir does both).

Here is a trick I tried: Take a torus, cut it into sectors,
and shear each sector (using "Matrix", doing y += c*x)
a little, and shift adjacent sectors up so that they match.

The math is easy (the formulas fit on a single page).
The problem with this method is that shearing distorts the
cross section of the torus into ellipses, and adjacent 
ellipses don't match exactly, which becomes visible unless
the sectors are very short and the material is rough.

Ralf

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