glutEstablishOverlay

glutEstablishOverlay(3GLUT)          GLUT          glutEstablishOverlay(3GLUT)



NAME
       glutEstablishOverlay - establishes an overlay (if possible) 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.
       glutLayerGet(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 overlay
       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);
         }

       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 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 transparency) 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 supports color
       index overlays.

SEE ALSO
       glutUseLayer, glutRemoveLayer, glutCreateWindow,
       glutPostOverlayRedisplay, glutShowOverlay, glutOverlayDisplayFunc

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



GLUT                                  3.2          glutEstablishOverlay(3GLUT)