let on_key ~key:k ~x:x ~y:y =
( match k with
| Glut.KEY_PAGE_UP -> tz := !tz +. 0.1
| Glut.KEY_PAGE_DOWN -> tz := !tz -. 0.1
| Glut.KEY_UP -> rx := !rx +. 10.
| Glut.KEY_DOWN -> rx := !rx -. 10.
| Glut.KEY_LEFT -> ry := !ry +. 10.
| Glut.KEY_RIGHT -> ry := !ry -. 10.
| _ -> ()
) ;
Glut.postRedisplay ()