isosurface { //-------------------------------------------------------------

  function { x*x + z*z - y*y - 0.25  }

  threshold 0
  accuracy 0.0001
  max_gradient 5
  contained_by {box {<-2,-1,-2>, < 2, 1, 2>}}
  //open
  texture { pigment{ color rgb<0.0,0.5,1.0>}
            finish { phong 1}
	  }
  scale 1.0
  rotate <-15, 25, 0>
  translate <-0, 1.4, 0.5>
} // end of isosurface -----------------------------------------------------