source: trunk/doc/src/examples/ahigl.qdoc@ 357

/****************************************************************************
**
** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
** Contact: Qt Software Information (qt-info@nokia.com)
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL$
** Commercial Usage
** Licensees holding valid Qt Commercial licenses may use this file in
** accordance with the Qt Commercial License Agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and Nokia.
**
** 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 qt-sales@nokia.com.
** $QT_END_LICENSE$
**
****************************************************************************/

/*!
    \example qws/ahigl
    \title OpenGL for Embedded Systems Example

    \section1 Introduction

    This example demonstrates how you can use OpenGL for Embedded
    Systems (ES) in your own screen driver and \l{add your graphics
    driver to Qt for Embedded Linux}. In \l{Qt for Embedded Linux},
    painting is done in software, normally performed in two steps:
    First, each client renders its windows onto its window surface in
    memory using a paint engine. Then the server uses the screen
    driver to compose the window surface images and copy the
    composition to the screen.  (See the \l{Qt for Embedded Linux
    Architecture} documentation for details.)

    This example is not for the novice. It assumes the reader is
    familiar with both OpenGL and the screen driver framework
    demonstrated in the \l {Accelerated Graphics Driver Example}.

    An OpenGL screen driver for Qt for Embedded Linux can use OpenGL ES
    in three ways. First, the \l{QWSServer}{Qt for Embedded Linux server}
    can use the driver to compose multiple window images and then show the
    composition on the screen. Second, clients can use the driver to
    accelerate OpenGL painting operations using the QOpenGLPaintEngine
    class. Finally, clients can use the driver to do OpenGL operations
    with instances of the QGLWidget class. This example implements all
    three cases.

    The example uses an implementation of OpenGL ES from 
    \l {http://ati.amd.com}{ATI} for the 
    \l {http://ati.amd.com/products/imageon238x/}{Imageon 2380}. The
    OpenGL include files gl.h and egl.h must be installed to compile
    the example, and the OpenGL and EGL libraries must be installed
    for linking. If your target device is different, you must install
    the include files and libraries for that device, and you also
    might need to modify the example source code, if any API signatures
    in your EGL library differ from the ones used here.

    After compiling and linking the example source, install the
    screen driver plugin with the command \c {make install}. To
    start an application that uses the plugin, you can either set the
    environment variable \l QWS_DISPLAY and then start the
    application, or just start the application with the \c -display
    switch, as follows:

    \snippet doc/src/snippets/code/doc_src_examples_ahigl.qdoc 0

    The example driver also implements an animated transition effect
    for use when showing new windows or reshowing windows that have
    been minimized. To enable this transition effect, run the
    application with \c {-display ahigl:effects}.

    \section1 The Class Definitions

    The example comprises three main classes plus some helper classes.
    The three main classes are the plugin (QAhiGLScreenPlugin), which
    is defined in qscreenahiglplugin.cpp, the screen driver
    (QAhiGLScreen), which is defined in qscreenahigl_qws.h, and the
    window surface (QAhiGLWindowSurface), which is defined in
    qwindowsurface_ahigl_p.h. The "Ahi" prefix in these class names
    stands for \e {ATI Handheld Interface}. The example was written
    for the ATI Imageon 2380, but it can also be used as a template
    for other ATI handheld devices.

    \section2 The Plugin Class Definition

    The screen driver plugin is class QAhiGLScreenPlugin.

    \snippet examples/qws/ahigl/qscreenahiglplugin.cpp 0

    QAhiGLScreenPlugin is derived from class QScreenDriverPlugin,
    which in turn is derived from QObject.

    \section2 The Screen Driver Class Definitions

    The screen driver classes are the public class QAhiGLScreen and
    its private implementation class QAhiGLScreenPrivate. QAhiGLScreen
    is derived from QGLScreen, which is derived from QScreen. If your
    screen driver will only do window compositions and display them,
    then you can derive your screen driver class directly from
    QScreen.  But if your screen driver will do accelerated graphics
    rendering operations with the QOpenGLPaintEngine, or if it will
    handle instances of class QGLWidget, then you must derive your
    screen driver class from QGLScreen.

    \snippet examples/qws/ahigl/qscreenahigl_qws.h 0

    All functions in the public API of class QAhiGLScreen are virtual
    functions declared in its base classes. hasOpenGL() is declared in
    QGLScreen. It simply returns true indicating our example screen
    driver does support OpenGL operations.  The other functions in the
    public API are declared in QScreen.  They are called by the
    \l{QWSServer}{Qt for Embedded Linux server} at the appropriate times.

    Note that class QScreen is a documented class but class QGLScreen
    is not. This is because the design of class QGLScreen is not yet
    final.

    The only data member in class QAhiGLScreen is a standard d_ptr,
    which points to an instance of the driver's private implementation
    class QAhiGLScreenPrivate. The driver's internal state is stored
    in the private class. Using the so-called d-pointer pattern allows
    you to make changes to the driver's internal design without
    breaking binary compatibility.

    \snippet examples/qws/ahigl/qscreenahigl_qws.cpp 0

    Class QAhiGLScreenPrivate is derived from QObject so that it can
    use the Qt signal/slot mechanism. QAhiGLScreen is not a QObject,
    so it can't use the signal/slot mechanism. Signals meant for our
    screen driver are received by slots in the private implementation
    class, in this case, windowEvent() and redrawScreen().

    \section2 The Window Surface Class Definitions

    The window surface classes are QAhiGLWindowSurface and its private
    implementation class QAhiGLWindowSurfacePrivate. We create class
    QAhiGLWindowSurface so the screen driver can use the OpenGL paint
    engine and the OpenGL widget, classes QOpenGLPaintEngine and
    QGLWidget.  QAhiGLWindowSurface is derived from the more general
    OpenGL window surface class, QWSGLWindowSurface, which is derived
    from QWSWindowSurface.

    \snippet examples/qws/ahigl/qwindowsurface_ahigl_p.h 0

    In addition to implementing the standard functionality required by
    any new subclass of QWSWindowSurface, QAhiGLWindowSurface also
    contains the textureId() function used by QAhiGLScreen.

    The same d-pointer pattern is used in this window surface class.
    The private implementation class is QAhiGLWindowSurfacePrivate. It
    allows making changes to the state variables of the window surface
    without breaking binary compatibility.

    \snippet examples/qws/ahigl/qwindowsurface_ahigl.cpp 0

    In this case, our private implementation class has no member
    functions except for its constructor. It contains only public data
    members which hold state information for the window surface.

    \section2 The Helper Classes

    The example screen driver maintains a static \l {QMap} {map} of
    all the \l {QWSWindow} {windows} it is showing on the screen.
    Each window is mapped to an instance of struct WindowInfo.

    \snippet examples/qws/ahigl/qscreenahigl_qws.cpp 2

    As each new window is created, an instance of struct WindowInfo is
    allocated and inserted into the window map. WindowInfo uses a
    GLuint to identify the OpenGL texture it creates for the window.
    Note that the example driver, in addition to drawing windows using
    OpenGL, also supports drawing windows in the normal way without
    OpenGL, but it uses an OpenGL texture for the rendering operations
    in either case.  Top-level windows that are drawn without OpenGL
    are first rendered in the normal way into a shared memory segment,
    which is then converted to a OpenGL texture and drawn to the
    screen.

    To animate the window transition effect, WindowInfo uses an
    instance of the helper class ShowAnimation.  The animation is
    created by the windowEvent() slot in QAhiGLScreenPrivate, whenever
    a \l {QWSServer::WindowEvent} {Show} window event is emitted by
    the \l {QWSServer} {window server}. The server emits this signal
    when a window is shown the first time and again later, when the
    window is reshown after having been minimized.

    \snippet examples/qws/ahigl/qscreenahigl_qws.cpp 1

    Class ShowAnimation is derived from the QTimeLine class, which is
    used for controlling animations. QTimeLine is a QObject, so
    ShowAnimation can use the Qt signal/slot mechanism. We will see
    how the timeline's \l {QTimeLine::valueChanged()} {valueChanged()}
    and \l {QTimeLine::finished()} {finished()} signals are used to
    control the animation and then destroy the instance of
    ShowAnimation, when the animation ends. The ShowAnimation
    constructor needs the pointer to the screen driver's private
    implementation class so it can set up these signal/slot
    connections.

    \section1 The Class Implementations

    \section2 The Plugin Class Implementation

    QAhiGLScreenPlugin is a straightforward derivation of
    QScreenDriverPlugin.  It reimplements \l{QScreenDriverPlugin::}{keys()}
    and \l{QScreenDriverPlugin::}{create()}. They are
    called as needed by the \l{QWSServer}{Qt for Embedded Linux server.}
    Recall that the server detects that the ahigl screen driver has
    been requested, either by including "ahigl" in the value for the
    environment variable QWS_DISPLAY, or by running your application
    with a command line like the following.

    \snippet doc/src/snippets/code/doc_src_examples_ahigl.qdoc 1

    The server calls \l {QScreenDriverPlugin::} {keys()}, which
    returns a \l {QStringList} containing the singleton "ahigl"
    matching the requested screen driver and telling the server that
    it can use our example screen driver. The server then calls \l
    {QScreenDriverPlugin::} {create()}, which creates the instance of
    QAhiGLScreen.

    \snippet examples/qws/ahigl/qscreenahiglplugin.cpp 1

    In the code snippet above, the macro Q_EXPORT_PLUGIN2 is used to export
    the plugin class, QAhiGLScreen, for the qahiglscreen plugin.
    Further information regarding plugins and how to create them
    can be found at \l{How to Create Qt Plugins}.

    \section2 The Screen Driver Class Implementations

    The plugin creates the singleton instance of QAhiGLScreen. The
    constructor is passed a \c displayId, which is used in the base
    class QGLScreen to identify the server that the screen driver is
    connected to. The constructor also creates its instance of
    QAhiGLScreenPrivate, which instantiates a QTimer. The timeout()
    signal of this timer is connected to the redrawScreen() slot so
    the timer can be used to limit the frequency of actual drawing
    operations in the hardware.

    The public API of class QAhiGLScreen consists of implementations
    of virtual functions declared in its base classes. The function
    hasOpenGL() is declared in base class QGLScreen. The others are
    declared in base class QScreen.

    The \l {QScreen::}{connect()} function is the first one called by
    the server after the screen driver is constructed. It initializes
    the QScreen data members to hardcoded values that describe the ATI
    screen. A better implementation would query the hardware for the
    corresponding values in its current state and use those. It asks
    whether the screen driver was started with the \c effects option
    and sets the \c doEffects flag accordingly.

    \snippet examples/qws/ahigl/qscreenahigl_qws.cpp 7

    The \l {QScreen::}{initDevice()} function is called by the server
    after \l {QScreen::}{connect()}. It uses EGL library functions to
    initialize the ATI hardware. Note that some data structures used
    in this example are specific to the EGL implementation used, e.g.,
    the DummyScreen structure.

    \snippet examples/qws/ahigl/qscreenahigl_qws.cpp 8

    Note the signal/slot connection at the bottom of initDevice(). We
    connect the server's QWSServer::windowEvent() signal to the
    windowEvent() slot in the screen driver's private implementation
    class.  The windowEvent() slot handles three window events,
    QWSServer::Create, QWSServer::Destroy, and QWSServer::Show.

    \snippet examples/qws/ahigl/qscreenahigl_qws.cpp 5

    The function manages instances of the helper classes associated
    with each window.  When a QWSServer::Create event occurs, it means
    a new top-level \l {QWSWindow} {window} has been created. In this
    case, an instance of helper class WindowInfo is created and
    inserted into the window map with the pointer to the new \l
    {QWSWindow} {window} as its key. When a QWSServer::Destroy event
    occurs, a window is being destroyed, and its mapping is removed
    from the window map. These two events are straightforward. The
    tricky bits happen when a QWSServer::Show event occurs. This case
    occurs when a window is shown for the first time and when it is
    reshown after having been minimized. If the window transition
    effect has been enabled, a new instance of the helper class
    ShowAnimation is created and stored in a QPointer in the window's
    instance of WindowInfo. The constructor of ShowAnimation
    automatically \l {QTimeLine::start()} {starts} the animation of
    the transition effect.

    \snippet examples/qws/ahigl/qscreenahigl_qws.cpp 3

    To ensure that a ShowAnimation is not deleted until its animation
    ends, the \l {QTimeLine::finished()} {finished()} signal is
    connected to the \l {QObject::deleteLater()} {deleteLater()} slot.
    When the animation ends, the finished() signal is emitted and the
    deleteLater() slot deletes the ShowAnimation. The key here is that
    the pointer to the ShowAnimation is stored in a QPointer in the
    WindowInfo class. This QPointer will also be notified when the
    ShowAnimation is deleted, so the QPointer in WindowInfo can null
    itself out, if and only if it is still pointing to the instance
    of ShowAnimation being deleted.

    The \l {QTimeLine::valueForTime()} {valueForTime()} function in
    QTimeLine is reimplemented in ShowAnimation to return time values
    that represent a curved path for the window transition effect.

    \snippet examples/qws/ahigl/qscreenahigl_qws.cpp 4

    valueForTime() is called internally, when the time interval it
    computed during the previous call has elapsed. If it computes a
    next time value that is different from the one computed
    previously, the \l {QTimeLine::valueChanged()} {valueChanged()}
    signal is emitted.  The ShowAnimation constructor shown above
    connects this signal to the redrawScreen() slot in the screen
    driver's private implementation class. This is how the animation
    actually happens.

    The screen driver's implementation of \l {QScreen::}
    {exposeRegion()} is where the main work of the screen driver is
    meant to be done, i.e., updating the screen. It is called by the
    \l {QWSServer} {window system} to update a particular window's
    region of the screen. But note that it doesn't actually update the
    screen, i.e., it doesn't actually call redrawScreen() directly,
    but starts the updateTimer, which causes redrawScreen() to be
    called once for each updateTimer interval. This means that all
    calls to exposeRegion() during an updateTimer interval are handled
    by a single call to redrawScreen(). Thus updateTimer can be used
    to limit the frequency of screen updates.

    \snippet examples/qws/ahigl/qscreenahigl_qws.cpp 13

    The call to the private function invalidateTexture() destroys
    the window's existing texture (image). This ensures that a new
    texture will be created for the window, when redrawScreen() is
    eventually called.

    But there is a caveat to using updateTimer to limit the frequency
    of screen updates. When the driver's animated transition effect
    for new windows is enabled and a new window is being shown for the
    first time or reshown after having been minimized, an instance of
    ShowAnimation is created to run the animation. The valueChanged()
    signal of this ShowAnimation is also connected to the
    redrawScreen() slot, and QTimeLine, the base class of our
    ShowAnimation, uses its own, internal timer to limit the speed of
    the animation. This means that in the driver as currently written,
    if the window transition effect is enabled (i.e. if the plugin is
    started, with \c {-display ahigl:effects}), then redrawScreen()
    can be called both when the update timer times out and when the
    ShowAnimation timer times out, so the screen might get updated
    more often than the frequency established by the update timer.
    This may or may not be a bug, depending on your own hardware, if
    you use this example as a template for your own OpenGL driver.

    The screen driver's private function redrawScreen() constructs
    the window compositions. It is called only by the function of the
    same name in the screen driver's private implementation class.

    \snippet examples/qws/ahigl/qscreenahigl_qws.cpp 6

    Recall that this redrawScreen() in the private implementation
    class is a slot function connected to two signals, the \c
    timeout() signal of the updateTimer in the private implementation
    class, and the valueChanged() signal of the helper class
    ShowAnimation. Thus, the screen is only ever updated when a
    timeout of one of the two timers occurs. This is important for two
    reasons. First, the screen is meant to be updated no more than
    once per updateTimer interval. Second, however, if the animated
    window transition effect is requested, the screen might be updated
    more often than that, and this might be a bug if the hardware
    can't handle more frequent updates.

    The redrawScreen() in QAhiGLScreen begins by using standard
    OpenGL to fill the screen with the background color.

    \snippet examples/qws/ahigl/qscreenahigl_qws.cpp 10

    Next it iterates over the list of all \l {QWSWindow} {client
    windows} obtained from the \l {QWSServer} {server}, extracting
    from each window its instance of QWSWIndowSurface, then using that
    window surface to create an OpenGL texture, and finally calling
    the helper function drawWindow() to draw the texture on the
    screen.

    \snippet examples/qws/ahigl/qscreenahigl_qws.cpp 11

    Note the call to glBindTexture() immediately before the call to
    drawWindow(). This call binds the identifer \c GL_TEXTURE_2D to
    the texture we have just created. This makes our texture
    accessible to functions in the OpenGL libraries. If you miss that
    point, digging into the internals of drawWindow() won't make much
    sense.

    Finally, the cursor is added to the window composition, and in the
    last statement, the whole thing is displayed on the screen.

    \snippet examples/qws/ahigl/qscreenahigl_qws.cpp 12

    The call to \c drawWindow(win,progress), in addition to passing a
    pointer to the window to be redrawn, also passes the \c progress
    parameter obtained by calling \l {QTimeLine::currentValue()} on
    the window's instance of ShowAnimation. Recall that the current
    value of the timeline is updated internally by a timer local to
    the timeline, and the redrawScreen() slot is called whenever the
    current value changes. The progress value will only be used if
    the animated transition effect has been enabled. These extra calls
    of redrawScreen() may cause the screen to be updated more often
    than the rate determined by updateTimer. This must be taken
    into account, if you set your updateTimer to timeout at the
    maximum screen update frequency your hardware can handle.

    The drawWindow() function is not shown here and not explained
    further, but the call to drawWindow() is the entry point to a
    hierarchy of private helper functions that execute sequences of
    OpenGL and EGL library calls. The reader is assumed to be familiar
    enough with the OpenGL and EGL APIs to understand the code in
    these helper functions on his own. Besides drawWindow(), the list
    of these helper functions includes drawQuad(), drawQuadWavyFlag(),
    the two overloadings of drawQuad_helper() (used by drawQuad() and
    drawQuadWacyFlag()), and setRectCoords().

    Note the two different ways the window's texture can be created in
    redrawScreen(). If the window surface is an OpenGL window surface
    (QAhiGLWindowSurface described below), the texture is obtained
    from the window surface directly by calling its textureId()
    function. But when the window surface is not an OpenGL one, the
    static function createTexture() is called with the window
    surface's \l {QImage} {image} to copy that image into an OpenGL
    texture. This is done with the EGL functions glTexImage2D() and
    glTexSubImage2D(). createTexture() is another function that
    should be understandable for exsperienced OpenGL users, so it is
    not shown or explained further here.

    The two implementations of \l {QScreen::}{createSurface()} are for
    instantiating new window surfaces. The overloading with the widget 
    parameter is called in the client.

    \snippet examples/qws/ahigl/qscreenahigl_qws.cpp 14

    If the parameter is an \l {QGLWidget} {OpenGL widget}, or, when it
    isn't an OpenGL widget but its size is no bigger than 256 x 256,
    we instantiate our subclass QAhiGLWindowSurface.  Otherwise, we
    instantiate a QWSWindowSurface. The size contraint is a
    limitation of the OpenGL ES libraries we are using for our ATI
    device.

    Note the test at the top of the function asking if our application
    process is the \l {QApplication::GuiServer} {server}.  We only
    create instances of QAhiGLWindowSurface if our client is in the
    server process. This is because of an implementation restriction
    required by the OpenGL library used in the example. They only
    support use of OpenGL in the server process. Hence a client can
    use the QAhiGLWindowSurface if the client is in the server
    process.

    The other overloading of createSurface() is called by the
    server to create a window surface that will hold a copy of a
    client side window surface.

    \snippet examples/qws/ahigl/qscreenahigl_qws.cpp 15

    This overloading accepts a QString parameter identifying the type
    of window surface to instantiate. QAhiGLWindowSurface is
    instantiated if the parameter is \c ahigl. Otherwise, a normal
    QWSWindowSurface is instantiated. The client's window surface
    communicates its image data to the server's window surface through
    shared memory.

    The implementation of \l {QScreen::}{setMode()}, is a stub in this
    example. It would normally reset the frame buffer's resolution.
    Its parameters are the \e width, \e height, and the bit \e depth
    for the frame buffer's new resolution. If you implement setMode()
    in your screen driver, remember that it must emit a signal to warn
    other applications to redraw their frame buffers with the new
    resolution.  There is no significance to setMode() not being
    implemented in this example. It simply wasn't implemented.
    However, the stub had to be included because QScreen declares
    setMode() to be pure virtual.

    Before the application exits, the server will call \l {QScreen::}
    {shutdownDevice()} to release the hardware resources. This is also
    done using EGL library functions.

    \snippet examples/qws/ahigl/qscreenahigl_qws.cpp 9

    The server will also call \l {QScreen::}{disconnect()}, but this
    function is only a stub in this example.

    \section2 The window Surface Class Implementations    

    QAhiGLScreen creates instances of QAhiGLWindowSurface in its two
    createSurface() functions, and there are two constructors for
    QAhiGLWindowSurface that correspond to these two versions of
    createSurface(). The constructor accepting a \l {QWidget} {widget}
    parameter is called by the client side version of createSurface(),
    and the constructor without the \l {QWidget} {widget} parameter is
    called by the server side version. There will be a window surface
    constructed on the server side for each one constructed on the
    client side.

    \snippet examples/qws/ahigl/qwindowsurface_ahigl.cpp 1
    \codeline
    \snippet examples/qws/ahigl/qwindowsurface_ahigl.cpp 2

    The constructors create an instance of QAhiGLWindowSurfacePrivate,
    the private implementation class, which contains all the state
    variables for QAhiGLWindowSurface. The client side constructor
    also creates an instance of QWSGLPaintDevice, the OpenGL paint
    device, for return by \l {QWSWindowSurface::} {paintDevice()}.
    This ensures that all \l {QPainter}s used on this surface will use
    an OpenGL enabled QPaintEngine. It is a bit of jiggery pokery,
    which is required because \l {QWSWindowSurface::} {paintDevice()}
    is declared pure virtual. Normally, the client side constructor
    will be called with an \l {QGLWidget}{OpenGL widget}, which has
    its own \l {QWidget::} {paintEngine()} function that returns the
    global static OpenGL paint engine, but because the constructor
    also accepts a normal \l {QWidget}{widget}, it must be able to
    find the OpenGL paint engine in that case as well, so since \l
    {QWSWindowSurface::} {paintDevice()} must be implemented anyway,
    the constructor creates an instance of QWSGLPaintDevice, which can
    always return the global static pointer to QOpenGLPaintEngine.

    The OpenGL library implementation used for this example only
    supports one OpenGL context. This context is therefore shared
    among the single instance of QAhiGLScreen and all instances of
    QAhiGLWindowSurface. It is passed to both constructors.

    This example uses the OpenGL frame buffer object extension, which
    allows for accelerating OpenGL painting operations. Using this
    OpenGL extension, painting operations are performed in a frame
    buffer object, which QAhiGLScreen later uses to construct window
    compositions on the screen. Allocation of the frame buffer object
    is performed in \l {QWindowSurface::} {setGeometry()}. A safer way
    to use this extension would be to first test to see if the
    extension is supported by your OpenGL library, and use a different
    approach if it is not.

    \snippet examples/qws/ahigl/qwindowsurface_ahigl.cpp 3

    Since there can be several instances of the QAhiGLWindowSurface, we need
    to make sure that the correct framebuffer object is active before painting.
    This is done by reimplementing \l QWindowSurface::beginPaint():

    \snippet examples/qws/ahigl/qwindowsurface_ahigl.cpp 4

    Finally we need to make sure that whenever a widget grows beyond the size
    supported by this driver (256 x 256), the surface is deleted and a new
    standard surface is created instead. This is handled by reimplementing
    \l QWSWindowSurface::isValid():    

    \snippet examples/qws/ahigl/qwindowsurface_ahigl.cpp 5
*/