What is the most efficient way to display decoded video frames in Qt?

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Thanks for the answers, but I finally revisited this problem and came up with a rather simple solution that gives good performance. It involves deriving from QGLWidget and overriding the paintEvent() function. Inside the paintEvent() function, you can call QPainter::drawImage(...) and it will perform the scaling to a specified rectangle for you using hardware if available. So it looks something like this:

class QGLCanvas : public QGLWidget
{
public:
    QGLCanvas(QWidget* parent = NULL);
    void setImage(const QImage& image);
protected:
    void paintEvent(QPaintEvent*);
private:
    QImage img;
};

QGLCanvas::QGLCanvas(QWidget* parent)
    : QGLWidget(parent)
{
}

void QGLCanvas::setImage(const QImage& image)
{
    img = image;
}

void QGLCanvas::paintEvent(QPaintEvent*)
{
    QPainter p(this);

    //Set the painter to use a smooth scaling algorithm.
    p.setRenderHint(QPainter::SmoothPixmapTransform, 1);

    p.drawImage(this->rect(), img);
}

With this, I still have to convert the YUV 420P to RGB32, but ffmpeg has a very fast implementation of that conversion in libswscale. The major gains come from two things:

  • No need for software scaling. Scaling is done on the video card (if available)
  • Conversion from QImage to QPixmap, which is happening in the QPainter::drawImage() function is performed at the original image resolution as opposed to the upscaled fullscreen resolution.

I was pegging my processor on just the display (decoding was being done in another thread) with my previous method. Now my display thread only uses about 8-9% of a core for fullscreen 1920×1200 30fps playback. I’m sure it could probably get even better if I could send the YUV data straight to the video card, but this is plenty good enough for now.

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