// --------------------------------------- parametric surface ------------- 
#declare a = 1.0;                       // conical spiral
#declare b = 8;
#declare c = 0.2;
#declare n = 5;
parametric{
 function { a*(1-0.5*v/pi)*sin(n*v+0.5*pi)*(1-cos(u))
            + c*sin(n*v+0.5*pi) }
 function { b*0.5*v/pi + a*(1-0.5*v/pi)*sin(u) }
 function { a*(1-0.5*v/pi)*cos(n*v+0.5*pi)*(1-cos(u))
            + c*cos(n*v+0.5*pi) }
  <0,0>,<2*pi,2*pi>  // start, end of (u,v)
  contained_by {box {<-1,-1,-1>*2*pi,v1,b/3,1>*2*pi}}
  max_gradient 10
  accuracy 0.0035
  precompute 18 x,y,z
  texture{ pigment{ color rgb<1,1,1>}
           finish { phong 0.5}}
  scale 0.125
  rotate <0, 90, 0>
} // end of parametric ----------------------------------------------------