Video Widget Example

VideoWidgetSurface Class Implementation

 QList<QVideoFrame::PixelFormat> VideoWidgetSurface::supportedPixelFormats(
         QAbstractVideoBuffer::HandleType handleType) const
 {
     if (handleType == QAbstractVideoBuffer::NoHandle) {
         return QList<QVideoFrame::PixelFormat>()
                 << QVideoFrame::Format_RGB32
                 << QVideoFrame::Format_ARGB32
                 << QVideoFrame::Format_ARGB32_Premultiplied
                 << QVideoFrame::Format_RGB565
                 << QVideoFrame::Format_RGB555;
     } else {
         return QList<QVideoFrame::PixelFormat>();
     }
 }

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.

 bool VideoWidgetSurface::isFormatSupported(
         const QVideoSurfaceFormat &format, QVideoSurfaceFormat *similar) const
 {
     Q_UNUSED(similar);

     const QImage::Format imageFormat = QVideoFrame::imageFormatFromPixelFormat(format.pixelFormat());
     const QSize size = format.frameSize();

     return imageFormat != QImage::Format_Invalid
             && !size.isEmpty()
             && format.handleType() == QAbstractVideoBuffer::NoHandle;
 }

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 supportedPixelFormats(format.handleType()) contains the pixel format and that the size is not empty so a reimplementation wasn't strictly necessary in this case.

 bool VideoWidgetSurface::start(const QVideoSurfaceFormat &format)
 {
     const QImage::Format imageFormat = QVideoFrame::imageFormatFromPixelFormat(format.pixelFormat());
     const QSize size = format.frameSize();

     if (imageFormat != QImage::Format_Invalid && !size.isEmpty()) {
         this->imageFormat = imageFormat;
         imageSize = size;
         sourceRect = format.viewport();

         QAbstractVideoSurface::start(format);

         widget->updateGeometry();
         updateVideoRect();

         return true;
     } else {
         return false;
     }
 }

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.

 void VideoWidgetSurface::updateVideoRect()
 {
     QSize size = surfaceFormat().sizeHint();
     size.scale(widget->size().boundedTo(size), Qt::KeepAspectRatio);

     targetRect = QRect(QPoint(0, 0), size);
     targetRect.moveCenter(widget->rect().center());
 }

The updateVideoRect() function calculates the region within the widget the video occupies. The size hint of the video format gives a suggested size for the video calculated from the viewport and 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.

 bool VideoWidgetSurface::present(const QVideoFrame &frame)
 {
     if (surfaceFormat().pixelFormat() != frame.pixelFormat()
             || surfaceFormat().frameSize() != frame.size()) {
         setError(IncorrectFormatError);
         stop();

         return false;
     } else {
         currentFrame = frame;

         widget->repaint(targetRect);

         return true;
     }
 }

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 UnsupportedFormatError on our surface and stop it immediately.

 void VideoWidgetSurface::paint(QPainter *painter)
 {
     if (currentFrame.map(QAbstractVideoBuffer::ReadOnly)) {
         const QTransform oldTransform = painter->transform();

         if (surfaceFormat().scanLineDirection() == QVideoSurfaceFormat::BottomToTop) {
            painter->scale(1, -1);
            painter->translate(0, -widget->height());
         }

         QImage image(
                 currentFrame.bits(),
                 currentFrame.width(),
                 currentFrame.height(),
                 currentFrame.bytesPerLine(),
                 imageFormat);

         painter->drawImage(targetRect, image, sourceRect);

         painter->setTransform(oldTransform);

         currentFrame.unmap();
     }
 }

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.

 void VideoWidgetSurface::stop()
 {
     currentFrame = QVideoFrame();
     targetRect = QRect();

     QAbstractVideoSurface::stop();

     widget->update();
 }

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.

 class VideoWidget : public QWidget
 {
     Q_OBJECT
 public:
     VideoWidget(QWidget *parent = 0);
     ~VideoWidget();

     QAbstractVideoSurface *videoSurface() const { return surface; }

     QSize sizeHint() const;

 protected:
     void paintEvent(QPaintEvent *event);
     void resizeEvent(QResizeEvent *event);

 private:
     VideoWidgetSurface *surface;
 };

 VideoWidget::VideoWidget(QWidget *parent)
     : QWidget(parent)
     , surface(0)
 {
     setAutoFillBackground(false);
     setAttribute(Qt::WA_NoSystemBackground, true);
     setAttribute(Qt::WA_PaintOnScreen, true);

     QPalette palette = this->palette();
     palette.setColor(QPalette::Background, Qt::black);
     setPalette(palette);

     setSizePolicy(QSizePolicy::MinimumExpanding, QSizePolicy::MinimumExpanding);

     surface = new VideoWidgetSurface(this);
 }

 VideoWidget::~VideoWidget()
 {
     delete surface;
 }

 QSize VideoWidget::sizeHint() const
 {
     return surface->surfaceFormat().sizeHint();
 }

 void VideoWidget::paintEvent(QPaintEvent *event)
 {
     QPainter painter(this);

     if (surface->isActive()) {
         const QRect videoRect = surface->videoRect();

         if (!videoRect.contains(event->rect())) {
             QRegion region = event->region();
             region.subtract(videoRect);

             QBrush brush = palette().background();

             foreach (const QRect &rect, region.rects())
                 painter.fillRect(rect, brush);
         }

         surface->paint(&painter);
     } else {
         painter.fillRect(event->rect(), palette().background());
     }
 }

 void VideoWidget::resizeEvent(QResizeEvent *event)
 {
     QWidget::resizeEvent(event);

     surface->updateVideoRect();
 }