Paul Bourke posted this in c.g.r.r today:
http://www.swin.edu.au/astronomy/pbourke/modelling/spring/

I fed the parametric function into MegaPov, but I'm not completely happy 
with it. On the page Paul says: "It is a parametric equation in terms of 
parameters u and v. Both range from 0 to 2 pi, increasing u will give 
multiple windings."
When I use <0,2*pi>,<0,2*pi> for the uv boundaries, the scene stays black. 
Can somebody explain me this and what the uv boudary exactly is?

//===scene
#version unofficial MegaPov 0.6;
global_settings {assumed_gamma 1.0}
light_source {<500,500,-500> rgb 1}
camera {location <0,0,-5> look_at <0,0,0>}

#declare r1 = 0.5;
#declare r2 = 0.5;
#declare periodlength=1.5;

parametric {
   function
   r2 * (sin(v) + (periodlength * u / pi)),
   (1 - (r1 * cos(v))) * cos(u),
   (1 - (r1 * cos(v))) * sin(u)
   
   <-2*pi,-2*pi>,<2*pi,2*pi> //<0,2*pi>,<0,2*pi>
   <-2,-1.5,-1.5>,<2,1.5,1.5>
   accuracy 0.01
   precompute 20,[x,y,z]
   pigment {rgb 1}
}
//===end scene


Ingo

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Pov-Ray    : http://members.home.nl/seed7/

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