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/****************************************************************************
**
** Copyright (C) 2015 The Qt Company Ltd.
** Contact: http://www.qt.io/licensing/
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
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** accordance with the commercial license agreement provided with the
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** information use the contact form at http://www.qt.io/contact-us.
**
** GNU Free Documentation License Usage
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file. Please review the following information to ensure
** the GNU Free Documentation License version 1.3 requirements
** will be met: http://www.gnu.org/copyleft/fdl.html.
** $QT_END_LICENSE$
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****************************************************************************/
/*!
\page x11overlays.html
\title How to Use X11 Overlays with Qt
\ingroup best-practices
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.
*/
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