視頻概述

視頻特徵

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.

視頻實現細節

在 C++ 中播放視頻

可以使用 QMediaPlayer 類解碼視頻文件，和顯示它使用 QVideoWidget , QGraphicsVideoItem ，或自定義類。

此處範例，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();

和範例采用 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 中播放視頻

可以使用 VideoOutput 渲染的內容提供通過 MediaPlayer 或 Camera 。 VideoOutput is a visual component that can be transformed or acted upon by shaders (as the QML 視頻著色器效果範例 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.

處理低級視頻幀

Qt Multimedia 提供瞭許多低級類以使處理視頻幀變得更容易一些。這些類首要用於編寫處理視頻或攝像頭幀的代碼 (例如：檢測條碼或應用花哨暈影效果)，或需要以其它不受支持的特殊方式顯示視頻。

The 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 允許接收這些幀從 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 視頻輸齣 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;
};

錄製視頻

可以使用 QMediaRecorder class in conjunction with other classes to record video to disk. Primarily this is used with the camera, so consult the 攝像頭概述瞭解更多信息。

監視視頻幀

可以使用 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.

範例

存在 C++ 和 QML 兩種可用範例。

C++ 範例

QML 範例

參考文檔編製

C++ 類

QAbstractPlanarVideoBuffer	抽象平麵視頻數據
QAbstractVideoBuffer	抽象視頻數據
QAbstractVideoFilter	錶示應用於通過 VideoOutput 類型接收視頻幀的過濾器
QAbstractVideoSurface	視頻呈現錶麵的基類
QVideoFilterRunnable	錶示擁有所有圖形計算資源，履行實際過濾 (或計算) 的過濾器實現
QVideoFrame	錶示視頻數據幀
QVideoProbe	允許監視視頻幀播放或錄製
QVideoSurfaceFormat	指定視頻呈現錶麵的流格式

QML 類型

MediaPlayer	把媒體迴放添加到場景
Playlist	針對要播放的指定媒體列錶
PlaylistItem	定義 Playlist 項
Video	展示指定視頻的方便類型
VideoOutput	渲染視頻或攝像頭取景器