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/****************************************************************************
**
** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
** Contact: Nokia Corporation (qt-info@nokia.com)
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** This file is part of the documentation of the Qt Toolkit.
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**
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** additional rights. These rights are described in the Nokia Qt LGPL
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** If you have questions regarding the use of this file, please contact
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** $QT_END_LICENSE$
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****************************************************************************/
/*!
\example video/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/video/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/video/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/video/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/video/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/video/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/video/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/video/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/video/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/video/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/video/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/video/videowidget/videowidget.cpp 1
In the destructor we simply delete the VideoWidgetSurface instance.
\snippet examples/video/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/video/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/video/videowidget/videowidget.cpp 4
The resizeEvent() function is reimplemented to trigger an update of the
video region when the widget is resized.
*/
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