Wasn't it normdoering who wrote:
>Has anyone ever tried to use the trace function in a macro to generate a
>mesh2 file from an isosurface?

Ingo Janssen has created param.inc that can make a mesh2 from a
parametric isosurface, but it does it by evaluating the functions
directly, rather than by using trace.

I think that his twovar.inc does the same thing for isosurfaces that are
a function of only two variables.

Ingo's macros are at: <http://members.home.nl/seedseven/>
My example of using param.inc is at 
                      <http://www.econym.demon.co.uk/isotut/param.htm>

If the isosurface that you want to meshify can't be manipulated into
either of these two formats, then things become rather trickier. I
strongly suspect that it wouldn't be possible to produce an algorithm
that could meshify all possible isosurfaces using trace. You may have to
use code that depends on prior knowledge of certain properties of the
particular surface you are trying to meshify. For example, if you happen
to know that the entire isosurface can be directly observed from a
single location or direction, then you can trace the surface by emitting
trace rays from there, but if your surface is highly convoluted, then
it's tricky to decide where to start the trace rays.

Since you were talking about height_fields in your other postings,
here's some code that takes a height_field-like isosurface and meshifies
it. With this particular isosurface, I happen to know that I can trace
vertically downwards (-y) from a point high above the surface
(<X,100,Y>) and can represent the surface by a square XY grid. The mesh
is green and the original isosurface red in this rendering.


#include "functions.inc"
global_settings {assumed_gamma 1.0}
camera { location  <2, 3, -6> look_at <0, 0, 0>}
light_source {<-100,200,-100> colour rgb 1}


// Declare the isosurface
#declare S=
isosurface {
  function { y-f_noise3d(x,0,z) }
        max_gradient 1.4
        contained_by{box {<-4.01,-1,-4.01><4.01,1,4.01>}}
        open
        pigment {rgb x}
}

// Render the isosurface
object {S}




#declare Divs=40; // Number of subdivisions
// obtain arrays of points and normals on the surface
#declare A=array[Divs][Divs]
#declare N=array[Divs][Divs]
#declare Norm=<0,0,0>;

#declare i=0;
#while (i<Divs)
  #declare j=0;
  #while (j<Divs)
  
    #declare X=-4+i*8/(Divs-1);
    #declare Z=-4+j*8/(Divs-1);
    
    #declare A[i][j]=trace(S, <X,100,Z>, -y , Norm);
    #declare N[i][j]=Norm;
    
    #declare j=j+1;
  #end
  #declare i=i+1;
#end


// create the mesh2
#declare M = mesh2 {
  vertex_vectors {Divs*Divs
    #declare i=0; #while (i<Divs) #declare j=0; #while (j<Divs)    
      ,A[i][j]
    #declare j=j+1;#end #declare i=i+1;#end
  }
  
  normal_vectors {Divs*Divs
    #declare i=0; #while (i<Divs) #declare j=0; #while (j<Divs)    
      ,N[i][j]
    #declare j=j+1;#end #declare i=i+1;#end
  }
  
  face_indices {(Divs-1)*(Divs-1)*2
    #declare i=0; #while (i<Divs-1) #declare j=0; #while (j<Divs-1)    
      #declare k=i+j*Divs;
      ,<k,k+1,k+Divs>, <k+1,k+1+Divs,k+Divs>
    #declare j=j+1;#end #declare i=i+1;#end
  }
  pigment {rgb y}
  no_shadow
}  


//render the mesh
object {M translate y}

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