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/*!

    \page x11overlays.html

    \title How to Use X11 Overlays with Qt

    X11 overlays are a powerful mechanism for drawing

    annotations etc., on top of an image without destroying it, thus saving

    a great deal of image rendering time. For more information, see the highly

    recommended book \e{OpenGL Programming for the X Window System} (Mark

    Kilgard, Addison Wesley Developers Press 1996).

    \warning The Qt OpenGL Extension includes direct support for the

    use of OpenGL overlays. For many uses of overlays, this makes the

    technique described below redundant. The following is a discussion

    on how to use non-QGL widgets in overlay planes.

    In the typical case, X11 overlays can easily be used together with the

    current version of Qt and the Qt OpenGL Extension. The following

    requirements apply:

    \list 1

    \i Your X server and graphics card/hardware must support overlays.

       For many X servers, overlay support can be turned on with

       a configuration option; consult your X server installation

       documentation.

    \i Your X server must (be configured to) use an overlay visual as the

       default visual. Most modern X servers do this, since this has the

       added advantage that pop-up menus, overlapping windows etc., will

       \e not affect underlying images in the main plane, thereby

       avoiding expensive redraws.

    \i The best (deepest) visual for OpenGL rendering is in the main

       plane. This is the normal case. Typically, X servers that support

       overlays provide a 24-bit \c TrueColor visual in the main plane,

       and an 8-bit \c PseudoColor (default) visual in the overlay plane.

    \endlist

    Assuming that the requirements mentioned above are met, a

    QGLWidget will default to using the main plane visual, while all

    other widgets will use the overlay visual. Thus, we can place a

    normal widget on top of the QGLWidget, and do drawing on it,

    without affecting the image in the OpenGL window. In other words,

    we can use all the drawing capabilities of QPainter to draw

    annotations, rubberbands, etc. For the typical use of overlays,

    this is much easier than using OpenGL for rendering annotations.

    An overlay plane has a specific color called the transparent

    color. Pixels drawn in this color will not be visible; instead

    the underlying OpenGL image will show through.

    To use this technique, you must not use the

    QApplication::ManyColor or QApplication::TrueColor color

    specification for QApplication, because this will force the

    normal Qt widgets to use a \c TrueColor visual, which will

    typically be in the main plane, not in the overlay plane as

    desired.

*/