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glutEstablishOverlay(3GLUT)                         GLUT                         glutEstablishOverlay(3GLUT)



NAME
       glutEstablishOverlay - establishes an overlay (if possible) for the current window.

SYNTAX
       #include <GLUT/glut.h>

       void glutEstablishOverlay(void);

DESCRIPTION
       glutEstablishOverlay  establishes an overlay (if possible) for the current window. The requested dis-play display
       play mode for the overlay is determined by the initial display mode.   glutLayerGet(GLUT_OVERLAY_POS-SIBLE) glutLayerGet(GLUT_OVERLAY_POSSIBLE)
       SIBLE)  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 ter-minate terminate
       minate 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 dis-carded. discarded.
       carded.

       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  estab-lished. established.
       lished.

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 pos-sibly possibly
       sibly  projection matrices of both the normal plane and the overlay.  For example, your reshape call-back callback
       back 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(3G), glutRemoveLayer(3G), glutCreateWindow(3G), glutPostOverlayRedisplay(3G), glutShowOverlay(3G), glutOver-layDisplayFunc(3G) glutOverlayDisplayFunc(3G)
       layDisplayFunc(3G)


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



GLUT                                                 3.7                         glutEstablishOverlay(3GLUT)