@Override
 public int compare(Coord3d o1, Coord3d o2) {
   if (o1.equals(o2)) return 0;
   else {
     double d1 = camera.getEye().distance(o1);
     double d2 = camera.getEye().distance(o2);
     if (d1 < d2) return 1;
     else return -1;
   }
 }