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+/****************************************************************************
+**
+** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
+** Contact: Nokia Corporation (qt-info@nokia.com)
+**
+** This file is part of the documentation of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** No Commercial Usage
+** This file contains pre-release code and may not be distributed.
+** You may use this file in accordance with the terms and conditions
+** contained in the either Technology Preview License Agreement or the
+** Beta Release License Agreement.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 2.1 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL included in the
+** packaging of this file. Please review the following information to
+** ensure the GNU Lesser General Public License version 2.1 requirements
+** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
+**
+** In addition, as a special exception, Nokia gives you certain
+** additional rights. These rights are described in the Nokia Qt LGPL
+** Exception version 1.0, included in the file LGPL_EXCEPTION.txt in this
+** package.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3.0 as published by the Free Software
+** Foundation and appearing in the file LICENSE.GPL included in the
+** packaging of this file. Please review the following information to
+** ensure the GNU General Public License version 3.0 requirements will be
+** met: http://www.gnu.org/copyleft/gpl.html.
+**
+** If you are unsure which license is appropriate for your use, please
+** contact the sales department at http://qt.nokia.com/contact.
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+/*!
+ \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.
+*/