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

glutEstablishOverlay
	glutEstablishOverlay() -> None

void glutEstablishOverlay(void);

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.

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_COLORMAP_SIZE);
    white = (transparent + 2) % glutGet(GLUT_WINDOW_COLORMAP_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 workstation)!0);
  }

If  you  setup  an  overlay  and  you  install  a  reshape

callback,  you  need  to update the viewports and possibly
projection matrices of both the normal plane and the over-
lay.   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.

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.

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

Mark J. Kilgard (mjk@nvidia.com)