Video Overview

Video Features

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.

Video Implementation Details

Playing Video in C++

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

Here's an example of using 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();

Playing Video in QML

You can use VideoOutput to render content that is provided by either a MediaPlayer or a Camera. The VideoOutput is a visual component that can be transformed or acted upon by shaders (as the QML Video Shader Effects Example 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.

Working with Low Level Video Frames

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.

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 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 video output 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 interface.

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;
  };

Recording Video

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 Camera Overview for more information.

Monitoring Video Frames

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.

Examples

There are both C++ and QML examples available.

C++ Examples

QML Examples

Reference Documentation

C++ Classes

QML Types