Wasn't it Warp who wrote:
>  I was experimenting with a small macro which creates a mesh shaped like
>a lathe from a spline. (The advantage of meshes over lathes is speed. Another
>one is that with lathes you sometimes get artifacts which you don't get with
>meshes.)

An alternative would be to consider that a spline function can also be
used in an isosurface. I've put a little example of this about halfway
down this page <http://www.econym.demon.co.uk/IsoTut/splines.htm>.

As it happens, this ends up being a ridiculously slow parametric
isosurface, so I use Ingo's macro to convert it to a mesh2 for the
actual rendering, so we end up doing the same thing.

This offers a little bit more flexibility. You can easily arrange for
the cross section to be something other than a circle and the axis to be
something other than a straight line as seen on that page. You can also
add other functions to the isosurface to achieve displacements.


#macro MakeEvenSplinePoints(Points)
  #local Amnt = dimension_size(Points, 1)-1;
  #local Ind = 0;
  #while(Ind <= Amnt)
    Ind/Amnt, Points[Ind]
    #local Ind = Ind+1;
  #end
#end

#declare Points = array[18]
{ <0,.1>,<1,0>,<1.2,.05>,<1,.1>,<.2,.3>,
  <.25,.4>,<.1,.9>,<.1,2.5>,<.3,2.8>,<.3,3.1>,
  <1,3.25>,<1.25,4>,<1,5>,<.9,5>,<1.1,4.5>,
  <1.15,3.8>,<1,3.4>,<0,3.2>
}

#declare Spline = function {spline { cubic_spline
MakeEvenSplinePoints(Points) }}

camera { location -z*14 look_at 0 angle 35 orthographic translate y*2.5
}
light_source { <100,200,-300>, 1 }
light_source { <-100,20,-30>, <1,.5,0>*.5 }
plane { -z,0 pigment { checker rgb 1, rgb .5 } }



#declare Fx = function(x,y) {(Spline(u).x * sin(v))}
#declare Fy = function(x,y) {Spline(u).y}
#declare Fz = function(x,y) {(Spline(u).x * cos(v))}
#declare Umin =  0;
#declare Umax =  1;
#declare Vmin =  -pi;
#declare Vmax =  1.001*pi;
#declare Iter_U = 80;
#declare Iter_V = 20;

#include "param.inc"
Parametric()

object {Surface
  no_shadow
  pigment { rgb 1 } finish { specular .5 }
}

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