X.Org X servers support an X protocol extension called Composite. This extension allows windows to be drawn into pixmaps instead of directly onto the screen. In conjunction with the Damage and Render extensions, this allows a program called a composite manager to blend windows together to draw the screen.

Performance will be degraded significantly if the RenderAccel option is disabled in xorg.conf. See Appendix B, X Config Options for more details.

When the NVIDIA X driver is used and the Composite extension is enabled, NVIDIA's OpenGL implementation interacts properly with the Damage and Composite X extensions. This means that OpenGL rendering is drawn into offscreen pixmaps and the X server is notified of the Damage event when OpenGL renders to the pixmap. This allows OpenGL applications to behave properly in a composited X desktop.

If the Composite extension is enabled on an X server older than X11R6.9.0, then GLX will be disabled. You can force GLX on while Composite is enabled on pre-X11R6.9.0 X servers with the AllowGLXWithComposite X configuration option. However, GLX will not render correctly in this environment. Upgrading your X server to X11R6.9.0 or newer is recommended.

You can enable the Composite X extension by running nvidia-xconfig --composite. Composite can be disabled with nvidia-xconfig --no-composite. See the nvidia-xconfig(1) man page for details.

If you are using an OpenGL-based composite manager, you may also need the DisableGLXRootClipping option to obtain proper output.

The Composite extension also causes problems with other driver components:

This NVIDIA SunOS driver supports OpenGL rendering to 32-bit ARGB windows. 32-bit visuals are only available on screens with depths 24 or 30. If you are an application developer, you can use these new visuals in conjunction with a composite manager to create translucent OpenGL applications:

    int attrib[] = {
        GLX_RENDER_TYPE, GLX_RGBA_BIT,
        GLX_DRAWABLE_TYPE, GLX_WINDOW_BIT,
        GLX_RED_SIZE, 1,
        GLX_GREEN_SIZE, 1,
        GLX_BLUE_SIZE, 1,
        GLX_ALPHA_SIZE, 1,
        GLX_DOUBLEBUFFER, True,
        GLX_DEPTH_SIZE, 1,
        None };
    GLXFBConfig *fbconfigs, fbconfig;
    int numfbconfigs, render_event_base, render_error_base;
    XVisualInfo *visinfo;
    XRenderPictFormat *pictFormat;

    /* Make sure we have the RENDER extension */
    if(!XRenderQueryExtension(dpy, &render_event_base, &render_error_base)) {
        fprintf(stderr, "No RENDER extension found\n");
        exit(EXIT_FAILURE);
    }

    /* Get the list of FBConfigs that match our criteria */
    fbconfigs = glXChooseFBConfig(dpy, scrnum, attrib, &numfbconfigs);
    if (!fbconfigs) {
        /* None matched */
        exit(EXIT_FAILURE);
    }

    /* Find an FBConfig with a visual that has a RENDER picture format that
     * has alpha */
    for (i = 0; i < numfbconfigs; i++) {
        visinfo = glXGetVisualFromFBConfig(dpy, fbconfigs[i]);
        if (!visinfo) continue;
        pictFormat = XRenderFindVisualFormat(dpy, visinfo->visual);
        if (!pictFormat) continue;

        if(pictFormat->direct.alphaMask > 0) {
            fbconfig = fbconfigs[i];
            break;
        }

        XFree(visinfo);
    }

    if (i == numfbconfigs) {
        /* None of the FBConfigs have alpha.  Use a normal (opaque)
         * FBConfig instead */
        fbconfig = fbconfigs[0];
        visinfo = glXGetVisualFromFBConfig(dpy, fbconfig);
        pictFormat = XRenderFindVisualFormat(dpy, visinfo->visual);
    }

    XFree(fbconfigs);

When rendering to a 32-bit window, keep in mind that the X RENDER extension, used by most composite managers, expects "premultiplied alpha" colors. This means that if your color has components (r,g,b) and alpha value a, then you must render (a*r, a*g, a*b, a) into the target window.

More information about Composite can be found at http://freedesktop.org/Software/CompositeExt