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_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);
         }

       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.

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,
       glutPostOverlayRedisplay, glutShowOverlay, glutOverlayDisplayFunc

AUTHOR

       Mark J. Kilgard (mjk@nvidia.com)