Hello Mike,

Can you explain again how did you do this translation?
====================================
isosurface {
   function{DNA(f_r(x,z,y) ,f_th(x,z,y),z)}
====================================
What's f_r and what's f_th?

Thanks,
Oleguer


news:jD8### [at] econymdemoncouk...
>
> Here's an alternative way of getting a circular helix that's a bit
> easier to manage than f_helical_torus. What you do is first create an
> ordinary f_helix1, then translate it off to one side and transform it
> into cylindrical polar co-ordinates. This gives you access to the
> f_helix1 control parameters which are better understood than the
> f_helical_torus parameters.
>
> I did the translation (x => x-R3) as a separate step from the
> co-ordinate change (x => f_r(x,z,y) , y => f_th(x,z,y)) to keep things
> simple.
>
> The use of (x,z,y) rather than (x,y,z) is due to the fact that f_th is
> defined as atan2(x,z), whereas I wanted atan2(x,y).
>
> #version 3.6;
> global_settings {assumed_gamma 1.0}
> #include "functions.inc"
>
> camera {location  <0,0, -20> look_at 0 angle 20}
> light_source { <-100,200,-300> colour rgb 1 }
> background { rgb 0 }
>
> #declare R1=0.1;     // helix minor radius
> #declare R2=0.3;     // helix major radius
> #declare R3=2.0;     // torus major radius
> #declare Strands=2;  // DNA = double strand
>
> #declare Turns=8.5;  // number of helix turns in the loop
>                      // Turns*Strands should be an integer
>                      // to avoid a discontinuity
>
> #declare DNA=function { f_helix1 (x-R3, y, z, Strands,
>                         Turns, R1, R2, 1, 1, 0) }
>
> isosurface {
>    function{DNA(f_r(x,z,y) ,f_th(x,z,y),z)}
>         max_gradient 1.3
>         contained_by {sphere {0, 3} }
>         pigment {rgb 1}
> }
>
>
>
> For real DNA, the two strands aren't 180 degrees out of phase like this.
> I'm not sure what the exact phase difference is, but here's a version
> with the strands 120 degrees out of phase. If you know what the phase
> difference should be, you can plug that number in where I've written
> "120".
>
> I've also changed the "shape parameter" from 1.0 to 0.6 in this version
> to compensate for the fact that the change of co-ordinates had caused
> the cross section to become elliptical instead of circular. This will
> probably need to be adjusted whenever you change things like R3 and
> Turns.
>
> #version 3.6;
> global_settings {assumed_gamma 1.0}
> #include "functions.inc"
>
> camera {location  <0,0, -20> look_at 0 angle 20}
> light_source { <-100,200,-300> colour rgb 1 }
> background { rgb 0 }
>
> #declare R1=0.15;    // helix minor radius
> #declare R2=0.3;     // helix major radius
> #declare R3=2.0;     // torus major radius
> #declare Strands=1;  // DNA = double strand
>
> #declare Turns=8;    // number of helix turns in the loop
>                      // Turns*Strands should be an integer
>                      // to avoid a discontinuity
>
> #declare DNA=function { f_helix1 (x-R3, y, z, Strands,
>                         Turns, R1, R2, 0.6, 1, 0) }
>
> #declare Strand=
> isosurface {
>    function{DNA(f_r(x,z,y) ,f_th(x,z,y),z)}
>         max_gradient 1.3
>         contained_by {sphere {0, 3} }
>         pigment {rgb 1}
> }
>
> object{Strand}
> object{Strand rotate z*120/Turns}
>
> -- 
> Mike Williams
> Gentleman of Leisure

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