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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.
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** $QT_BEGIN_LICENSE:LGPL$
** No Commercial Usage
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** contained in the Technology Preview License Agreement accompanying
** this package.
**
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** 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 1.1, included in the file LGPL_EXCEPTION.txt in this
** package.
**
** If you have questions regarding the use of this file, please contact
** Nokia at qt-info@nokia.com.
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** $QT_END_LICENSE$
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****************************************************************************/
/*!
\example qws/dbscreen
\title Double Buffered Graphics Driver Example
The Double Buffered Graphics Driver example shows how to write your own
double buffered graphics driver and add it to Qt for Embedded Linux.
Similar to the \l{Accelerated Graphics Driver Example}, there are three steps
to writing and implementing this graphics driver:
\list 1
\o \l {Step 1: Creating a Custom Graphics Driver}
{Creating a Custom Graphics Driver}
\o \l {Step 2: Implementing the Back Buffer}
{Implementing the Back Buffer}
\o \l {Step 3: Creating the Driver Plugin}
{Creating the Driver Plugin}
\endlist
After compiling the example code, install the graphics driver plugin with
the command \c {make install}. To start an application using the graphics
driver, you can either set the environment variable \l QWS_DISPLAY and
then run the application, or you can just run the application using the
\c -display switch.
Note that this is a minimal example and this driver will not work well
with widgets painting themself directly to the screen (e.g. widgets with
the Qt::WA_PaintOnScreen window attribute set). Also, the example requires
the Linux framebuffer to be set up correctly and with the correct device
permissions. For further information, refer to
\l{Testing the Linux Framebuffer}.
\section1 Step 1: Creating a Custom Graphics Driver
Usually, a custom graphics driver is created by subclassing the QScreen
class, the base class for implementing screen or graphics drivers in
Qt for Embedded Linux. In this example, however, we subclass the QLinuxFbScreen
class instead, to ensure that our driver uses the Linux framebuffer.
For our graphics driver, the \c DBScreen class, we reimplement five
functions belonging to QScreen:
\list
\o \l{QScreen::initDevice()}{initDevice()},
\o \l{QScreen::shutdownDevice()}{shutdownDevice()},
\o \l{QScreen::blit()}{blit()},
\o \l{QScreen::solidFill()}{solidFill()}, and
\o \l{QScreen::exposeRegion()}{exposeRegion()}.
\endlist
\snippet examples/qws/dbscreen/dbscreen.h 0
In addition to the abovementioned functions, there is a private instance
of QPainter and QImage - \c painter, used for drawing operations on
the back buffer, and \c image, the back buffer itself.
\section1 Step 2: Implementing the Back Buffer
The graphics driver must carry out three main functions:
\list 1
\o Allocate the back buffer on startup and deallocate it on shutdown.
\o Draw to the back buffer instead of directly to the screen
(which is what QLinuxFbScreen does).
\o Copy the back buffer to the screen whenever a screen update is
done.
\endlist
\section2 Device initializing and shutdown
We first reimplement \c initDevice() and \c shutdownDevice().
The \c initDevice() function initializes the framebuffer. We reimplement
this function to enable accelerated drivers to set up the graphic card.
For this example, we first call the super class' implementation to set up
the Linux framebuffer. If this call returns \c false, we return \c false.
Otherwise, we initialize the screen cursor with
QScreenCursor::initSoftwareCursor() as well as instantiate \c image and
\c painter. Then, we return \c true.
\snippet examples/qws/dbscreen/dbscreen.cpp 0
The \c shutdownDevice() function's default implementation only hides the
mouse cursor. Hence, we reimplement it to carry out the necessary cleanup
before the Qt for Embedded Linux server exits.
\snippet examples/qws/dbscreen/dbscreen.cpp 1
Again, we call the super class implementation to shutdown the Linux
framebuffer prior to deleting \c image and \c painter.
\section2 Drawing to the back buffer
We move on to the drawing functions - \c solidFill() and \c blit(). In
QLinuxFbScreen, these functions draw directly to the Linux framebuffer;
but in our driver we reimplement them to draw to the back buffer instead.
\snippet examples/qws/dbscreen/dbscreen.cpp 2
The \c solidFill() function is called from \c exposeRegion() to fill the
given \c region of the screen with the specified \c color. In this
example, we use \c painter to fill rectangles in \c image, the back
buffer, according to the given region.
\snippet examples/qws/dbscreen/dbscreen.cpp 3
The \c blit() function is also called from \c exposeRegion() to copy the
given QRegion object, \c region, in the given QImage object, \c image, to
the QPoint object specified by \c topLeft. Once again we use \c painter
to draw in the back buffer, \c image.
\section2 Displaying the buffer on the screen
The \c exposeRegion() function is called by the Qt for Embedded Linux server
whenever a screen update is required. The given \c region is the screen
region that needs to be updated and \c changing is is the index into
QWSServer::clientWindows() of the window that caused the update.
\snippet examples/qws/dbscreen/dbscreen.cpp 4
In our implementation, we first call the super class implementation to
ensure that \c solidFill() and \c blit() will be called correctly. This
causes the changed areas to be updated in the back buffer. We then call
the super class' implementation of \c blit() to copy the updated region
from the back buffer into the Linux framebuffer.
\section1 Step 3: Creating the Driver Plugin
Qt provides a high level API for writing Qt extentions. One of the plugin
base classes provided is QScreenDriverPlugin, which we use in this example
to create our screen driver plugin.
\snippet examples/qws/dbscreen/dbscreendriverplugin.cpp 0
There are only two functions to reimplement:
\list
\o \l{QScreenDriverPlugin::create()}{create()} - creates a driver
matching the given key
\o \l{QScreenDriverPlugin::create()}{keys()} - returns a list of
valid keys representing the drivers supported by the plugin
\endlist
\snippet examples/qws/dbscreen/dbscreendriverplugin.cpp 1
\codeline
\snippet examples/qws/dbscreen/dbscreendriverplugin.cpp 2
Our plugin will only support one driver, \c dbscreen.
Lastly, we export the plugin.
\snippet examples/qws/dbscreen/dbscreendriverplugin.cpp 3
For detailed information about the Qt plugin system see
\l{How to Create Qt Plugins.}
*/
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