glutestablishoverlay(3glut) [redhat man page]
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 win- dow with the current initial display mode. Do not attempt to establish an overlay when one is not possible; GLUT will terminate the pro- gram. If glutEstablishOverlay is called when an overlay already exists, the existing overlay is first removed, and then a new overlay is estab- lished. 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@nvidia.com) GLUT
3.7 glutEstablishOverlay(3GLUT)
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glutPostOverlayRedisplay(3GLUT) GLUT glutPostOverlayRedisplay(3GLUT) NAME
glutPostOverlayRedisplay, glutPostWindowOverlayRedisplay - marks the overlay of the current or specified window as needing to be redis- played. SYNTAX
void glutPostOverlayRedisplay(void); void glutPostWindowOverlayRedisplay(int win); DESCRIPTION
Mark the overlay of current window as needing to be redisplayed. The next iteration through glutMainLoop, the window's overlay display callback (or simply the display callback if no overlay display callback is registered) will be called to redisplay the window's overlay plane. Multiple calls to glutPostOverlayRedisplay before the next display callback opportunity (or overlay display callback opportunity if one is registered) generate only a single redisplay. glutPostOverlayRedisplay may be called within a window's display or overlay display callback to re-mark that window for redisplay. Logically, overlay damage notification for a window is treated as a glutPostOverlayRedisplay on the damaged window. Unlike damage reported by the window system, glutPostOverlayRedisplay will not set to true the overlay's damaged status (returned by glutLayerGet(GLUT_OVER- LAY_DAMAGED). If the window you want to post an overlay redisplay on is not already current (and you do not require it to be immediately made current), using glutPostWindowOverlayRedisplay is more efficient that calling glutSetWindow to the desired window and then calling glutPostOverlayRe- display. EXAMPLE
If you are doing an interactive effect like rubberbanding in the overlay, it is a good idea to structure your rendering to minimize flicker (most overlays are single-buffered). Only clear the overlay if you know that the window has been damaged. Otherwise, try to simply erase what you last drew and redraw it in an updated position. Here is an example overlay display callback used to implement overlay rubberband- ing: void redrawOverlay(void) { static int prevStretchX, prevStretchY; if (glutLayerGet(GLUT_OVERLAY_DAMAGED)) { /* Damage means we need a full clear. */ glClear(GL_COLOR_BUFFER_BIT); } else { /* Undraw last rubber-band. */ glIndexi(transparent); glBegin(GL_LINE_LOOP); glVertex2i(anchorX, anchorY); glVertex2i(anchorX, prevStretchY); glVertex2i(prevStretchX, prevStretchY); glVertex2i(prevStretchX, anchorY); glEnd(); } glIndexi(red); glBegin(GL_LINE_LOOP); glVertex2i(anchorX, anchorY); glVertex2i(anchorX, stretchY); glVertex2i(stretchX, stretchY); glVertex2i(stretchX, anchorY); glEnd(); prevStretchX = stretchX; prevStretchY = stretchY; } Notice how glutLayerGet(GLUT_OVERLAY_DAMAGED) is used to determine if a clear needs to take place because of damage; if a clear is unneces- sary, it is faster to just draw the last rubberband using the transparent pixel. When the application is through with the rubberbanding effect, the best way to get ride of the rubberband is to simply hide the overlay by calling glutHideOverlay. SEE ALSO
glutPostRedisplay, glutEstablishOverlay, glutLayerGet AUTHOR
Mark J. Kilgard (mjk@nvidia.com) GLUT
3.7 glutPostOverlayRedisplay(3GLUT)