I wrote this code to work with the geodesic macro. it takes a position 
vector and rotateds an object (here a torus) so it's y axis is along 
that vector. the position vector is on a sphere centered at the origin, 
so it is exactly the same as a surface normal. Uses trig to get z and y 
euler angles.

#macro do_corner(position)

#local a = position.x;
#local b = position.y;
#local c = position.z;

// union
// {
  torus
  {
   .08,.035
   pigment {color rgb <1,1,0>}
// }
// cylinder
// {
//  <0,-.2,0>,<0,.2,0>,.04
//  pigment {color rgb <1,0,0>}
// }
   #local thetaz = asin(b);
   #local thetay = asin(c);

   #if (a<0)
   #local thetaz = pi-thetaz;
   #end
   #if (a>0)
   #local thetay = -thetay;
   #end

   rotate 90*z
   rotate z*thetaz*(180/pi)
   rotate y*thetay*(180/pi)

   translate position
  }

#end


Doug Eichenberg wrote:
> Let's say I have an object "A" at the origin, and a known surface normal
> <x,y,z>.  I want to find the rotation vector <Rx,Ry,Rz> of the surface
> normal <x,y,z>, such that when I rotate "A":
> 
> object{ A rotate<Rx,Ry,Rz> }
> 
> it lies along the surface normal.
> 
> I tried doing this as follows:
> 
> object{ A rotate<atan2d(Norm.y,Norm.z), atan2d(Norm.x,Norm.z),
> atan2d(Norm.x,Norm.y)>
> 
> but I seem to be getting strange results.
> 
> Anyone have code they used for doing this?  Or know how in general?
> Thanks for the help.
> 
> --
> Doug Eichenberg
> www.getinfo.net/douge
> dou### [at] nlsnet
> 
>

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