glutPostOverlayRedisplay(3GLUT)GLUglutPostOverlayRedisplay(3GLUT)

NAME
glutPostOverlayRedisplay - marks the overlay of the cur-
rent window as needing to be redisplayed.

SYNTAX
void glutPostOverlayRedisplay(void);

DESCRIPTION
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 glutLayer-
Get(GLUT_OVERLAY_DAMAGED).

EXAMPLE
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.

SEE ALSO
glutPostRedisplay, glutEstablishOverlay, glutLayerGet

AUTHOR
Mark J. Kilgard (mjk@sgi.com)

GLUT 3.2 2