视频概述

视频特征

Qt Multimedia offers both high and low level C++ classes for playing and manipulating video data, and QML types for playback and control. Some of these classes also overlap with both camera and audio classes, which can be useful.

视频实现细节

在 C++ 中播放视频

You can use the QMediaPlayer class to decode a video file, and display it using QVideoWidget , QGraphicsVideoItem , or a custom class.

此处范例,使用 QVideoWidget :

player = new QMediaPlayer;
playlist = new QMediaPlaylist(player);
playlist->addMedia(QUrl("http://example.com/myclip1.mp4"));
playlist->addMedia(QUrl("http://example.com/myclip2.mp4"));
videoWidget = new QVideoWidget;
player->setVideoOutput(videoWidget);
videoWidget->show();
playlist->setCurrentIndex(1);
player->play();
							

And an example with QGraphicsVideoItem :

player = new QMediaPlayer(this);
QGraphicsVideoItem *item = new QGraphicsVideoItem;
player->setVideoOutput(item);
graphicsView->scene()->addItem(item);
graphicsView->show();
player->setMedia(QUrl("http://example.com/myclip4.ogv"));
player->play();
							

在 QML 中播放视频

可以使用 VideoOutput to render content that is provided by either a MediaPlayer Camera VideoOutput is a visual component that can be transformed or acted upon by shaders (as the QML 视频着色器效果范例 shows), while all media decoding and playback control is handled by the MediaPlayer .

Alternatively there is also a higher level Video type that acts as a single, visual element to play video and control playback.

处理低级视频帧

Qt Multimedia offers a number of low level classes to make handling video frames a bit easier. These classes are primarily used when writing code that processes video or camera frames (for example, detecting barcodes, or applying a fancy vignette effect), or needs to display video in a special way that is otherwise unsupported.

QVideoFrame class encapsulates a video frame and allows the contents to be mapped into system memory for manipulation or processing, while deriving a class from QAbstractVideoSurface allows you to receive these frames from QMediaPlayer and QCamera .

class MyVideoSurface : public QAbstractVideoSurface
{
    QList<QVideoFrame::PixelFormat> supportedPixelFormats(
            QAbstractVideoBuffer::HandleType handleType = QAbstractVideoBuffer::NoHandle) const
    {
        Q_UNUSED(handleType);
        // Return the formats you will support
        return QList<QVideoFrame::PixelFormat>() << QVideoFrame::Format_RGB565;
    }
    bool present(const QVideoFrame &frame)
    {
        Q_UNUSED(frame);
        // Handle the frame and do your processing
        return true;
    }
};
							

and with an instance of this surface, myVideoSurface , you can set the surface as the 视频输出 for QMediaPlayer .

player->setVideoOutput(myVideoSurface);
							

Several of the built-in Qt classes offer this functionality as well, so if you decode video in your application, you can present it to classes that offer a QVideoRendererControl class, and in QML you can set a custom object for the source of a VideoOutput with either a writable videoSurface property (that the instance will set it's internal video surface to) or a readable mediaObject property with a QMediaObject derived class that implements the QVideoRendererControl 接口。

The following snippet shows a class that has a writable videoSurface property and receives frames through a public slot onNewVideoContentReceived() . These frames are then presented on the surface set in setVideoSurface() .

class MyVideoProducer : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QAbstractVideoSurface *videoSurface READ videoSurface WRITE setVideoSurface)
public:
    QAbstractVideoSurface* videoSurface() const { return m_surface; }
    void setVideoSurface(QAbstractVideoSurface *surface)
    {
        if (m_surface != surface && m_surface && m_surface->isActive()) {
            m_surface->stop();
        }
        m_surface = surface;
        if (m_surface)
            m_surface->start(m_format);
    }
    // ...
public slots:
    void onNewVideoContentReceived(const QVideoFrame &frame)
    {
        if (m_surface)
            m_surface->present(frame);
    }
private:
    QAbstractVideoSurface *m_surface;
    QVideoSurfaceFormat m_format;
};
							

录制视频

You can use the QMediaRecorder class in conjunction with other classes to record video to disk. Primarily this is used with the camera, so consult the 摄像头概述 了解更多信息。

监视视频帧

You can use the QVideoProbe class to access video frames as they flow through different parts of a media pipeline when using other classes like QMediaPlayer , QMediaRecorder or QCamera . After creating the high level media class, you can set the source of the video probe to that instance. This can be useful for performing some video processing tasks (like barcode recognition, or object detection) while the video is rendered normally. You can not affect the video frames using this class, and they may arrive at a slightly different time than they are being rendered.

Here's an example of installing a video probe while recording the camera:

camera = new QCamera;
viewfinder = new QCameraViewfinder();
camera->setViewfinder(viewfinder);
camera->setCaptureMode(QCamera::CaptureVideo);
videoProbe = new QVideoProbe(this);
if (videoProbe->setSource(camera)) {
    // Probing succeeded, videoProbe->isValid() should be true.
    connect(videoProbe, SIGNAL(videoFrameProbed(QVideoFrame)),
            this, SLOT(detectBarcodes(QVideoFrame)));
}
camera->start();
// Viewfinder frames should now also be emitted by
// the video probe, even in still image capture mode.
// Another alternative is to install the probe on a
// QMediaRecorder connected to the camera to get the
// recorded frames, if they are different from the
// viewfinder frames.
							

范例

存在 C++ 和 QML 两种可用范例。

C++ 范例

QML 范例

参考文档编制

C++ 类

QAbstractPlanarVideoBuffer

抽象平面视频数据

QAbstractVideoBuffer

抽象视频数据

QAbstractVideoFilter

表示应用于 VideoOutput 类型接收视频帧的过滤器

QAbstractVideoSurface

视频呈现表面的基类

QVideoFilterRunnable

表示拥有所有图形 计算资源,履行实际过滤 (或计算) 的过滤器实现

QVideoFrame

表示视频数据帧

QVideoProbe

允许监视正播放 (或录制) 视频帧

QVideoSurfaceFormat

指定视频呈现表面的流格式

QML 类型

MediaPlayer

把媒体回放添加到场景

Playlist

针对要播放的指定媒体列表

PlaylistItem

定义 Playlist 播放列表项

Video

展示指定视频的方便类型

VideoOutput

渲染视频或摄像头取景器