source: trunk/doc/src/examples/videowidget.qdoc

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/*!
    \example multimedia/videowidget
    \title Video Widget Example

    The Video Widget example shows how to implement a video widget using
    QtMultimedia's QAbstractVideoSurface

    \image video-videowidget.png

    \section1 VideoWidgetSurface Class Definition

    \snippet examples/multimedia/videowidget/videowidgetsurface.h 0

    The VideoWidgetSurface class inherits QAbstractVideoSurface and paints
    video frames on a QWidget.  This is a separate class to VideoWidget as both
    QAbstractVideoSurface and QWidget inherit QObject.

    In addition to the functions from QAbstractVideoSurface, VideoWidgetSurface
    has functions for determining the video display rectangle, and painting
    the video.

    \section1 VideoWidgetSurface Class Implementation

    \snippet examples/multimedia/videowidget/videowidgetsurface.cpp 0

    From the supportedPixelFormats() function we return a list of pixel formats
    the surface can paint. The order of the list hints at which formats are
    preferred by the surface.  Assuming a 32-bit RGB backbuffer, we'd expect
    that a 32-bit RGB type with no alpha to be fastest to paint so
    QVideoFrame::Image_RGB32 is first in the list.

    Since we don't support rendering using any special frame handles we don't
    return any pixel formats if handleType is not
    QAbstractVideoBuffer::NoHandle.

    \snippet examples/multimedia/videowidget/videowidgetsurface.cpp 1

    In isFormatSupported() we test if the frame type of a surface format maps
    to a valid QImage format, that the frame size is not empty, and the handle
    type is QAbstractVideoBuffer::NoHandle.  Note that the
    QAbstractVideoSurface implementation of isFormatSupported() will verify
    that the list of supported pixel formats returned by
    \c supportedPixelFormats(format.handleType()) contains the pixel format and
    that the size is not empty so a reimplementation wasn't strictly necessary
    in this case.

    \snippet examples/multimedia/videowidget/videowidgetsurface.cpp 2

    To start our surface we'll extract the image format and size from the
    selected video format and save it for use in the paint() function.  If the
    image format, or size are invalid then we'll set an error and return false.
    Otherwise we'll save the format and confirm the surface has been started,
    by calling QAbstractVideoSurface::start(). Finally since the video size may
    have changed we'll trigger an update of the widget, and video geometry.

    \snippet examples/multimedia/videowidget/videowidgetsurface.cpp 5

    The updateVideoRect() function calculates the region within the widget the
    video occupies.  The \l {QVideoSurfaceFormat::sizeHint()}{size hint} of the
    video format gives a suggested size for the video calculated from the
    \l {QVideoSurfaceFormat::viewport()}{viewport} and
    \l {QVideoSurfaceFormat::pixelAspectRatio()}{pixel aspect ratio}.  If the
    suggested size fits within the widget then we create a new rect of that
    size in the center of the widget.  Otherwise we shrink the size maintaining
    the aspect ratio so that it does fit.

    \snippet examples/multimedia/videowidget/videowidgetsurface.cpp 4

    We can't paint from outside a paint event, so when a new frame is received
    in present() we save a reference to it and force an immediate repaint of
    the video region.  We retain the saved reference to the frame after the
    repaint so that the widget can be repainted between frame changes if
    necessary.

    If the format of the frame doesn't match the surface format we can't paint
    it or very likely any future frames.  So we set an
    \l {QAbstractVideoSurface::UnsupportedFormatError}{UnsupportedFormatError}
    on our surface and stop it immediately.

    \snippet examples/multimedia/videowidget/videowidgetsurface.cpp 6

    The paint() function is called by the video widget to paint the current
    video frame.  Before we draw the frame first we'll check the format for
    the scan line direction and if the scan lines are arranged from bottom to
    top we'll flip the painter so the frame isn't drawn upside down.  Then
    using the image format information saved in the start() function we'll
    construct a new QImage from the current video frame, and draw it to the
    the widget.

    \snippet examples/multimedia/videowidget/videowidgetsurface.cpp 3

    When the surface is stopped we need to release the current frame and
    invalidate the video region.   Then we confirm the surface has been
    stopped by calling QAbstractVideoSurface::stop() which sets the started
    state to false and finally we update so the video widget so paints over
    the last frame.

    \section1 VideoWidget Class Definition

    The VideoWidget class uses the VideoWidgetSurface class to implement a
    video widget.

    \snippet examples/multimedia/videowidget/videowidget.h 0

    The VideoWidget QWidget implementation is minimal with just the sizeHint(),
    paintEvent(), and resizeEvent() functions in addition to the constructor,
    destructor and an instance of VideoWidgetSurface.

    \section1 VideoWidget Class Implementation

    \snippet examples/multimedia/videowidget/videowidget.cpp 0

    In the VideoWidget constructor we set some flags to speed up re-paints a
    little.  Setting the Qt::WA_NoSystemBackground flag and disabling automatic
    background fills will stop Qt from a painting a background that'll be
    completely obscured by the video.  The Qt::WA_PaintOnScreen flag will
    allow us to paint to the screen instead of the back buffer where supported.

    Next we set the background color to black, so that any borders around the
    video are filled in black rather the default background color.

    Finally we construct an instance of the VideoWidgetSurface class.

    \snippet examples/multimedia/videowidget/videowidget.cpp 1

    In the destructor we simply delete the VideoWidgetSurface instance.

    \snippet examples/multimedia/videowidget/videowidget.cpp 2

    We get the size hint for the widget from the video format of the surface
    which is calculated from viewport and pixel aspect ratio of the video
    format.

    \snippet examples/multimedia/videowidget/videowidget.cpp 3

    When the video widget receives a paint event we first check if the surface
    is started,  if not then we simply fill the widget with the background
    color.  If it is then we draw a border around the video region clipped
    by the paint region, before calling paint on the video surface to draw the
    current frame.

    \snippet examples/multimedia/videowidget/videowidget.cpp 4

    The resizeEvent() function is reimplemented to trigger an update of the
    video region when the widget is resized.
*/