glutDisplayFunc  -  sets the display callback for the cur-
rent window.

glutDisplayFunc
	glutDisplayFunc(func) -> None

void glutDisplayFunc(void (*func)(void));

func      The new display callback function.

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

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

When an overlay is established for a window, but there  is
no  overlay display callback registered, the display call-
back is used for redisplaying both the overlay and  normal
plane  (that is, it will be called if either the redisplay
state or overlay redisplay state is set).  In  this  case,
the layer in use is not implicitly changed on entry to the
display callback.

See  glutOverlayDisplayFunc  to  understand  how  distinct
callbacks for the overlay and normal plane of a window may
be established.

When a window is created, no display callback  exists  for
the  window. It is the responsibility of the programmer to
install a display callback for the window before the  win-
dow  is  shown.  A display callback must be registered for
any window that is shown. If a  window  becomes  displayed
without a display callback being registered, a fatal error
occurs. Passing NULL to glutDisplayFunc is illegal  as  of
GLUT  3.0;  there  is  no  way to ``deregister'' a display
callback (though another callback routine  can  always  be
registered).

Upon  return from the display callback, the normal damaged
state  of  the  window  (returned  by  calling  glutLayer-
Get(GLUT_NORMAL_DAMAGED)  is cleared. If there is no over-
lay display callback registered the overlay damaged  state
of the window (returned by calling glutLayerGet(GLUT_OVER-
LAY_DAMAGED) is also cleared.

glutCreateMenu, glutPostRedisplay, glutOverlayDisplayFunc

Mark J. Kilgard (mjk@nvidia.com)