glutOverlayDisplayFunc - sets the overlay display callback
for the current window.

glutOverlayDisplayFunc
	glutOverlayDisplayFunc(func) -> None

void glutOverlayDisplayFunc(void (*func)(void));

func      The new overlay display callback function.

glutDisplayFunc sets the overlay display callback for  the
current  window. The overlay display callback is function-
ally the same as the window's display callback except that
the overlay display callback is used to redisplay the win-
dow's overlay.

When GLUT determines that the overlay plane for the window
needs  to be redisplayed, the overlay display callback for
the window is called.  Before the  callback,  the  current
window  is set to the window needing to be redisplayed and
the layer in use is set to the overlay. The  overlay  dis-
play  callback  is  called  with no parameters. The entire
overlay region should be redisplayed in  response  to  the
callback  (this includes ancillary buffers if your program
depends on their state).

GLUT determines when the overlay display  callback  should
be  triggered  based  on  the  window's  overlay redisplay
state. The overlay redisplay state for  a  window  can  be
either  set explicitly by calling glutPostOverlayRedisplay
or implicitly as the result of window damage  reported  by
the  window system. Multiple posted overlay redisplays for
a window are coalesced by GLUT to minimize the  number  of
overlay display callbacks called.

Upon return from the overlay display callback, the overlay
damaged state of the window (returned by calling  glutLay-
erGet(GLUT_OVERLAY_DAMAGED) is cleared.

The  overlay display callback can be deregistered by pass-
ing NULL to glutOverlayDisplayFunc.  The  overlay  display
callback is initially NULL when an overlay is established.
See glutDisplayFunc to understand how the display callback
alone is used if an overlay display callback is not regis-
tered.

glutDisplayFunc, glutPostOverlayRedisplay

Mark J. Kilgard (mjk@nvidia.com)