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/****************************************************************************
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**
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** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
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** All rights reserved.
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** Contact: Nokia Corporation (qt-info@nokia.com)
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**
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** This file is part of the documentation of the Qt Toolkit.
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**
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** $QT_BEGIN_LICENSE:LGPL$
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** No Commercial Usage
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** This file contains pre-release code and may not be distributed.
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** You may use this file in accordance with the terms and conditions
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** contained in the Technology Preview License Agreement accompanying
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** this package.
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**
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** GNU Lesser General Public License Usage
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** Alternatively, this file may be used under the terms of the GNU Lesser
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** General Public License version 2.1 as published by the Free Software
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** Foundation and appearing in the file LICENSE.LGPL included in the
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** packaging of this file. Please review the following information to
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** ensure the GNU Lesser General Public License version 2.1 requirements
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** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
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**
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** In addition, as a special exception, Nokia gives you certain additional
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** rights. These rights are described in the Nokia Qt LGPL Exception
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** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
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**
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** If you have questions regarding the use of this file, please contact
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** Nokia at qt-info@nokia.com.
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**
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**
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**
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**
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**
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**
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**
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**
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** $QT_END_LICENSE$
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**
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****************************************************************************/
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/*!
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\example multimedia/videowidget
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\title Video Widget Example
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The Video Widget example shows how to implement a video widget using
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QtMultimedia's QAbstractVideoSurface
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\image video-videowidget.png
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\section1 VideoWidgetSurface Class Definition
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\snippet examples/multimedia/videowidget/videowidgetsurface.h 0
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The VideoWidgetSurface class inherits QAbstractVideoSurface and paints
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video frames on a QWidget. This is a separate class to VideoWidget as both
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QAbstractVideoSurface and QWidget inherit QObject.
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In addition to the functions from QAbstractVideoSurface, VideoWidgetSurface
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has functions for determining the video display rectangle, and painting
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the video.
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\section1 VideoWidgetSurface Class Implementation
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\snippet examples/multimedia/videowidget/videowidgetsurface.cpp 0
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From the supportedPixelFormats() function we return a list of pixel formats
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the surface can paint. The order of the list hints at which formats are
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preferred by the surface. Assuming a 32-bit RGB backbuffer, we'd expect
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that a 32-bit RGB type with no alpha to be fastest to paint so
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QVideoFrame::Image_RGB32 is first in the list.
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Since we don't support rendering using any special frame handles we don't
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return any pixel formats if handleType is not
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QAbstractVideoBuffer::NoHandle.
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\snippet examples/multimedia/videowidget/videowidgetsurface.cpp 1
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In isFormatSupported() we test if the frame type of a surface format maps
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to a valid QImage format, that the frame size is not empty, and the handle
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type is QAbstractVideoBuffer::NoHandle. Note that the
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QAbstractVideoSurface implementation of isFormatSupported() will verify
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that the list of supported pixel formats returned by
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\c supportedPixelFormats(format.handleType()) contains the pixel format and
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that the size is not empty so a reimplementation wasn't strictly necessary
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in this case.
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\snippet examples/multimedia/videowidget/videowidgetsurface.cpp 2
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To start our surface we'll extract the image format and size from the
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selected video format and save it for use in the paint() function. If the
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image format, or size are invalid then we'll set an error and return false.
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Otherwise we'll save the format and confirm the surface has been started,
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by calling QAbstractVideoSurface::start(). Finally since the video size may
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have changed we'll trigger an update of the widget, and video geometry.
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\snippet examples/multimedia/videowidget/videowidgetsurface.cpp 5
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The updateVideoRect() function calculates the region within the widget the
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video occupies. The \l {QVideoSurfaceFormat::sizeHint()}{size hint} of the
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video format gives a suggested size for the video calculated from the
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\l {QVideoSurfaceFormat::viewport()}{viewport} and
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\l {QVideoSurfaceFormat::pixelAspectRatio()}{pixel aspect ratio}. If the
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suggested size fits within the widget then we create a new rect of that
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size in the center of the widget. Otherwise we shrink the size maintaining
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the aspect ratio so that it does fit.
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\snippet examples/multimedia/videowidget/videowidgetsurface.cpp 4
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We can't paint from outside a paint event, so when a new frame is received
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in present() we save a reference to it and force an immediate repaint of
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the video region. We retain the saved reference to the frame after the
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repaint so that the widget can be repainted between frame changes if
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necessary.
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If the format of the frame doesn't match the surface format we can't paint
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it or very likely any future frames. So we set an
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\l {QAbstractVideoSurface::UnsupportedFormatError}{UnsupportedFormatError}
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on our surface and stop it immediately.
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\snippet examples/multimedia/videowidget/videowidgetsurface.cpp 6
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The paint() function is called by the video widget to paint the current
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video frame. Before we draw the frame first we'll check the format for
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the scan line direction and if the scan lines are arranged from bottom to
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top we'll flip the painter so the frame isn't drawn upside down. Then
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using the image format information saved in the start() function we'll
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construct a new QImage from the current video frame, and draw it to the
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the widget.
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\snippet examples/multimedia/videowidget/videowidgetsurface.cpp 3
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When the surface is stopped we need to release the current frame and
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invalidate the video region. Then we confirm the surface has been
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stopped by calling QAbstractVideoSurface::stop() which sets the started
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state to false and finally we update so the video widget so paints over
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the last frame.
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\section1 VideoWidget Class Definition
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The VideoWidget class uses the VideoWidgetSurface class to implement a
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video widget.
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\snippet examples/multimedia/videowidget/videowidget.h 0
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The VideoWidget QWidget implementation is minimal with just the sizeHint(),
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paintEvent(), and resizeEvent() functions in addition to the constructor,
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destructor and an instance of VideoWidgetSurface.
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\section1 VideoWidget Class Implementation
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\snippet examples/multimedia/videowidget/videowidget.cpp 0
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In the VideoWidget constructor we set some flags to speed up re-paints a
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little. Setting the Qt::WA_NoSystemBackground flag and disabling automatic
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background fills will stop Qt from a painting a background that'll be
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completely obscured by the video. The Qt::WA_PaintOnScreen flag will
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allow us to paint to the screen instead of the back buffer where supported.
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Next we set the background color to black, so that any borders around the
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video are filled in black rather the default background color.
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Finally we construct an instance of the VideoWidgetSurface class.
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\snippet examples/multimedia/videowidget/videowidget.cpp 1
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In the destructor we simply delete the VideoWidgetSurface instance.
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\snippet examples/multimedia/videowidget/videowidget.cpp 2
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We get the size hint for the widget from the video format of the surface
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which is calculated from viewport and pixel aspect ratio of the video
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format.
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\snippet examples/multimedia/videowidget/videowidget.cpp 3
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When the video widget receives a paint event we first check if the surface
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is started, if not then we simply fill the widget with the background
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color. If it is then we draw a border around the video region clipped
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by the paint region, before calling paint on the video surface to draw the
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current frame.
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\snippet examples/multimedia/videowidget/videowidget.cpp 4
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The resizeEvent() function is reimplemented to trigger an update of the
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video region when the widget is resized.
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*/
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