glutPostOverlayRedisplay, glutPostWindowOverlayRedisplay -
marks the overlay of the current or  specified  window  as
needing to be redisplayed.

glutPostOverlayRedisplay
	glutPostOverlayRedisplay() -> None
glutPostWindowOverlayRedisplay
	glutPostWindowOverlayRedisplay(win) -> None

void glutPostOverlayRedisplay(void);
void glutPostWindowOverlayRedisplay(int win);

Mark the overlay of current window as needing to be redis-
played.  The next iteration through glutMainLoop, the win-
dow's  overlay  display  callback  (or  simply the display
callback if no overlay  display  callback  is  registered)
will  be  called  to redisplay the window's overlay plane.
Multiple calls to glutPostOverlayRedisplay before the next
display  callback opportunity (or overlay display callback
opportunity if one is registered) generate only  a  single
redisplay. glutPostOverlayRedisplay may be called within a
window's display or overlay display  callback  to  re-mark
that window for redisplay.

Logically,  overlay  damage  notification  for a window is
treated as a glutPostOverlayRedisplay on the damaged  win-
dow. Unlike damage reported by the window system, glutPos-
tOverlayRedisplay will not set to true the overlay's  dam-
aged  status  (returned  by glutLayerGet(GLUT_OVERLAY_DAM-
AGED).

If the window you want to post an overlay redisplay on  is
not already current (and you do not require it to be imme-
diately made current),  using  glutPostWindowOverlayRedis-
play  is  more efficient that calling glutSetWindow to the
desired window and then calling  glutPostOverlayRedisplay.

If  you are doing an interactive effect like rubberbanding
in the overlay, it is a good idea to structure  your  ren-
dering  to  minimize  flicker  (most  overlays are single-
buffered).  Only clear the overlay if you  know  that  the
window  has  been damaged.  Otherwise, try to simply erase
what you last drew and redraw it in an  updated  position.
Here is an example overlay display callback used to imple-
ment overlay rubberbanding:

  void
  redrawOverlay(void)
  {
    static int prevStretchX, prevStretchY;

    if (glutLayerGet(GLUT_OVERLAY_DAMAGED)) {
      /* Damage means we need a full clear. */
      glClear(GL_COLOR_BUFFER_BIT);
    } else {

      /* Undraw last rubber-band. */
      glIndexi(transparent);
      glBegin(GL_LINE_LOOP);
      glVertex2i(anchorX, anchorY);
      glVertex2i(anchorX, prevStretchY);
      glVertex2i(prevStretchX, prevStretchY);
      glVertex2i(prevStretchX, anchorY);
      glEnd();
    }
    glIndexi(red);
    glBegin(GL_LINE_LOOP);
    glVertex2i(anchorX, anchorY);
    glVertex2i(anchorX, stretchY);
    glVertex2i(stretchX, stretchY);
    glVertex2i(stretchX, anchorY);
    glEnd();
    prevStretchX = stretchX;
    prevStretchY = stretchY;
  }

Notice how glutLayerGet(GLUT_OVERLAY_DAMAGED) is  used  to
determine  if  a clear needs to take place because of dam-
age; if a clear is unnecessary, it is faster to just  draw
the last rubberband using the transparent pixel.

When  the  application  is  through with the rubberbanding
effect, the best way to get ride of the rubberband  is  to
simply hide the overlay by calling glutHideOverlay.

glutPostRedisplay, glutEstablishOverlay, glutLayerGet

Mark J. Kilgard (mjk@nvidia.com)