glutestablishoverlay(3)

NAME

glutEstablishOverlay - establishes an overlay (if possi
ble) for the current window.

SYNTAX

void glutEstablishOverlay(void);

DESCRIPTION

glutEstablishOverlay establishes an overlay (if possible)
for the current window. The requested display mode for the
overlay is determined by the initial display mode. glut
LayerGet(GLUT_OVERLAY_POSSIBLE) can be called to determine
if an overlay is possible for the current window with the
current initial display mode. Do not attempt to establish
an overlay when one is not possible; GLUT will terminate
the program.

If glutEstablishOverlay is called when an overlay already
exists, the existing overlay is first removed, and then a
new overlay is established. The state of the old overlay's
OpenGL context is discarded.

The initial display state of an overlay is shown, however
the overlay is only actually shown if the overlay's window
is shown.

Implicitly, the window's layer in use changes to the over
lay immediately after the overlay is established.

EXAMPLE

Establishing an overlay is a bit involved, but easy once
you get the hang of it. Here is an example:

int overlaySupport;
int transparent, red, white;

glutInitDisplayMode(GLUT_SINGLE | GLUT_INDEX);
overlaySupport = glutLayerGet(GLUT_OVERLAY_POSSIBLE);
if (overlaySupport) {

glutEstablishOverlay();
glutHideOverlay();
transparent = glutLayerGet(GLUT_TRANSPARENT_INDEX);
glClearIndex(transparent);
red = (transparent + 1) % glutGet(GLUT_WINDOW_COL

ORMAP_SIZE);
white = (transparent + 2) % glutGet(GLUT_WINDOW_COL

ORMAP_SIZE);
glutSetColor(red, 1.0, 0.0, 0.0); /* Red. */
glutSetColor(white, 1.0, 1.0, 1.0); /* White. */
glutOverlayDisplayFunc(redrawOverlay);
glutReshapeFunc(reshape);

} else {

printf("Sorry, no nifty overlay (try an SGI worksta

tion)!0);

}

If you setup an overlay and you install a reshape call
back, you need to update the viewports and possibly pro
jection matrices of both the normal plane and the overlay.
For example, your reshape callback might look like this:

void
reshape(int w, int h)
{

if (overlaySupport) {

glutUseLayer(GLUT_OVERLAY);
/* Setup overlay to have X style coordinate system.

*/
glViewport(0, 0, w, h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0, w, 0, h);
glScalef(1, -1, 1);
glTranslatef(0, -h, 0);
glMatrixMode(GL_MODELVIEW);
glutUseLayer(GLUT_NORMAL);

}
glViewport(0, 0, w, h);

}

See the glutOverlayDisplayFunc man page for an example
showing one way to write your overlay display callback.

X IMPLEMENTATION NOTES

GLUT for X uses the SERVER_OVERLAY_VISUALS convention is
used to determine if overlay visuals are available. While
the convention allows for opaque overlays (no trans
parency) and overlays with the transparency specified as a
bitmask, GLUT overlay management only provides access to
transparent pixel overlays.

Until RGBA overlays are better understood, GLUT only sup
ports color index overlays.

SEE ALSO

glutUseLayer, glutRemoveLayer, glutCreateWindow, glutPos
tOverlayRedisplay, glutShowOverlay, glutOverlayDisplayFunc

AUTHOR

Mark J. Kilgard (mjk@nvidia.com)

GLUT 3.7 glutEstablishOverlay(3GLUT)