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/****************************************************************************
**
** ...
**
** Copyright (C) 2005-2008 Trolltech ASA. All rights reserved.
**
** This file is part of the Qt GUI Toolkit.
**
** This file may be used under the terms of the GNU General
** Public License versions 2.0 or 3.0 as published by the Free
** Software Foundation and appearing in the files LICENSE.GPL2
** and LICENSE.GPL3 included in the packaging of this file.
** Alternatively you may (at your option) use any later version
** of the GNU General Public License if such license has been
** publicly approved by Trolltech ASA (or its successors, if any)
** and the KDE Free Qt Foundation.
**
** Please review the following information to ensure GNU General
** Public Licensing requirements will be met:
** http://trolltech.com/products/qt/licenses/licensing/opensource/.
** If you are unsure which license is appropriate for your use, please
** review the following information:
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** or contact the sales department at sales@trolltech.com.
**
** This file may be used under the terms of the Q Public License as
** defined by Trolltech ASA and appearing in the file LICENSE.QPL
** included in the packaging of this file. Licensees holding valid Qt
** Commercial licenses may use this file in accordance with the Qt
** Commercial License Agreement provided with the Software.
**
** This file is provided "AS IS" with NO WARRANTY OF ANY KIND,
** INCLUDING THE WARRANTIES OF DESIGN, MERCHANTABILITY AND FITNESS FOR
** A PARTICULAR PURPOSE. Trolltech reserves all rights not granted
** herein.
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**********************************************************************/
/*!
\page opengl-x11-overlays.html
\title How to use X11 overlays with the Qt OpenGL extension
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, consult the highly
recommended book \e{OpenGL Programming for the X Window System} (Mark
Kilgard, Addison Wesley Developers Press 1996).
\warning From version 5.0 onwards, 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. See the
\link opengl-overlay-example.html overlay\endlink example program. 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 destroy 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 deep TrueColor visual in the main plane,
and an 8 bit PseudoColor (default) visual in the overlay plane.
\endlist
The provided example program \link opengl-overlay-example.html X11
overlay\endlink will check for these and report if anything is wrong.
See \link #x11visuals About X11 Visuals\endlink, below for more
information.
\section1 How it works
Given the above, a QGLWidget will by default use 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 destroying the image in the OpenGL window. In other words,
we can use all the drawing capabilities of QPainter to draw the
annotations, rubberbands, etc. For the typical use of overlays,
this is much easier than using OpenGL for rendering the 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. In the example program
\link opengl-overlay-example.html X11 overlay\endlink, the file
\c main.cpp contains a routine that returns a QColor containing the
transparent color. For the overlay widget, you will typically want to
set the background color to the transparent color, so that the OpenGL
image shows through except where explicitly overpainted.
Note: to use this technique, you must not use the "ManyColor" or
"TrueColor" ColorSpec for QApplication, because this will force
the normal Qt widgets to use a TrueColor visual, which will typically
be in the main plane, not in the overlay plane as desired.
\target x11visuals
\section1 About X11 visuals
The utilities directory contains two small programs that can help you
determine the capabilities of your X server. These programs are from
the OpenGL book mentioned above, see utilities/NOTICE for copyright
information. The full set of example programs from this book is
available at \l{ftp://ftp.sgi.com/pub/opengl/opengl_for_x/}.
\c glxvisuals will list all the GL-capable visuals the X server
provides, together with the depth and other GL-specific information
for each. Note especially the column "lvl"; a number in this column
means the visual is in an overlay plane.
\c sovinfo will list all available visuals, and provides special
transparency information for overlay visuals.
The \link opengl-overlay-example.html X11 overlay\endlink example
program will output what visual is used for the normal Qt widgets, and
what visual is used by the QGLWidget.
*/
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