In article <WASm$FAUWHICFwAT@econym.demon.co.uk>,
 Mike Williams <nos### [at] econymdemoncouk> wrote:

>>Wasn't it kurtz le pirate who wrote:
>>>hello,
>>>
>>>i want to model cable with 2 parts : cylindrical and part of torus.
>>>
>>>for cylindrical, i have no problem. code i use is :
>>>
>>>#declare p1 = 8;    // number of helixes 
>>>#declare p2 = 0.25; // number of turns per unit length 
>>>#declare p3 = 1.75; // minor radius
>>>#declare p4 = 1.50; // major radius
>>>#declare p5 = 1.00; // shape parameter
>>>#declare p6 = 1;    // cross section type -> circle
>>>#declare p7 = 0;    // cross section rotation angle
>>>
>>>#declare containerSize = 3.20;
>>>#declare maxGradiantValue = 10;
>>>
>>>#declare cableFunction = function { f_helix1(x, y, z, p1, p2, p3, p4, 
>>>p5, p6, p7) }
>>>
>>>#declare cableCyl = isosurface {
>>>  function { cableFunction(x,y,z) }
>>>  contained_by { 
>>>    box { <+containerSize, 0.00, +containerSize> <-containerSize, 
>>>-cableLen, -containerSize>   }
>>>    }
>>>  max_gradient maxGradiantValue
>>>  }
>>>
>>>
>>>for "torus part", i do this code :
>>>
>>>#declare cableTorus = isosurface {
>>>  function { cableFunction(f_r(x,y,z)-MajorRadius, y, f_th(x,y,z)) }
>>>  contained_by { 
>>>    sphere { 0,*MajorRadius1.2 }
>>>    }
>>>  max_gradient maxGradiantValue
>>>  rotate 90*x 
>>>  }
>>>with MajorRadius = 80.00
>>>
>>>
>>>... but all the thing a get is a plain torus !!!
>>>
>>>anyone can help me ?
>>>thanks
>>
>>
>>The easy problem: If you're trying to follow my example code from 
>><http://www.econym.demon.co.uk/isotut/substitute.htm#polar>
>>you'll need to flip the axes like I do. 
>>
>>Try declaring a second cableFunction with flipped axes like this:
>>
>>#declare cableFunction2 = function { f_helix1(x, z, y, p1, p2, p3,
>>p4,p5, p6, p7) }
>>
>>#declare cableTorus = isosurface {
>>  function { cableFunction2(f_r(x,y,z)-MajorRadius, y, f_th(x,y,z)) }
>>  contained_by { 
>>    sphere { 0,MajorRadius*1.2 }
>>    }
>>  max_gradient 1.3
>>  rotate 90*x 
>>  }
>>
>>The problem you'll then notice is that p2 only relates to the number of
>>turns per unit length before the substitution, so you only get something
>>like pi*p2 turns around the whole loop, and that's nowhere near enough.
>>
>>You might think that would be easy to solve by increasing the value of
>>p2, but unfortunately you hit the problem that f_helix1 only behaves
>>predictably when the major radius is larger than the minor radius. If
>>you increase p2, things go horribly wrong.
>>
>>So, I think you've got to use f_helix2, like this:
>>
>>#declare cableFunction2 = function { f_helix2(x, z, y, 0,
>>p2*MajorRadius, p3, p4,p5, p6, p7) }
>>
>>#declare cableTorus = isosurface {
>>  function { cableFunction2(f_r(x,y,z)-MajorRadius, y, f_th(x,y,z)) }
>>  contained_by { 
>>    sphere { 0,MajorRadius*1.2 }
>>    }
>>  max_gradient 1.3
>>  rotate 90*x 
>>  }
>>
>>However, f_helix2 doesn't support multi-stranding, so if you want an
>>eight strand cable you'll need eight copies of that isosurface. That's
>>not quite as bad as it seems because the low value of max_gradient means
>>that it renders pretty quickly.
>>
>>#declare Angle = 360/p2/MajorRadius/8;
>>
>>#declare N=0;
>>#while (N<8)
>>  object {cableTorus pigment {rgb 1} rotate z*Angle*N}
>>  #declare N=N+1;
>>#end
>>
>>This then leaves you with the final problem that this cable doesn't look
>>quite the same as the cylindrical one. That's because it's still using
>>f_helix1. So you'll need to do the same thing with that if you want them
>>to match.

thanks Mike, I will try to put all that into practice

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