isosurface { //-------------------------------------------------------------
  function{
     f_helix2(
        x,y,z,
        0,    // not used,
        12,    // period,      turns on the length of 2*pi 
        0.38, // minor radius,
        0.50, // major radius,
        0,    // not used, 
        0.5,  // cross section type, (0.0 to 1.0 = square ... rounded to circle
              //                 over 2.0 to 3.0 = rounded to diamond and concave diamond
        0     // cross section rotation angle
      )
  }          
 contained_by {box {<-1,-1,-1>,<1,1,1>}}
 max_gradient 8
 
 texture{ pigment{ color rgb <1,1,1>*1.1} 
          finish { specular 0.4 phong 0.7 }
        }

 scale 0.9
 rotate<0,0,0>
 translate< 0,1.2,0>
} // end of isosurface -------------------------------------------------------
