Optimizing QPainter drawing & Converting QVideoFrame straight to QPixMap

Question

Some background info about my issue. My goal is to optimize drawing of images coming from webcam, the images come as QVideoFrame and are currently loaded in to QImage and drawn from there. This solution works fine, but drawing QImage is very slow on X11. Drawing one image takes about 20ms which doesn't sound like much but when you do this for every frame this cut's the framerate of the camerafeed to half.

I did some research and testing, drawing QPixMaps in X11 can be done about 10 times faster than drawing QImages.

This is how the drawing process is done currently

if(mVFcurrentFrame.map(QAbstractVideoBuffer::ReadOnly))
{
QImage image(mVFcurrentFrame.bits(), mVFcurrentFrame.width(), mVFcurrentFrame.height(), mVFcurrentFrame.bytesPerLine(), imageFormat);
painter->drawImage(0,0,image); //Takes about 20ms
mVFcurrentFrame.unmap();     
}

What i have tried so far:

Converting the QImage to QPixMap, this works but the conversion is as slow as painting the Qimage
Loading the QVideoFrame straight to QPixMap with QPixMap::loadFromData(), can't make it work.

So my question is, can i convert QVideoFrame straight to QPixMap and draw it instead of using QImage and how would you do the QVideoFrame to QPixmap conversion without using QImage in between?

I have tried using QPixMap::loadFromData() method to load the video frame but so far i have been unable to make it work.

If this isn't possible could i thread the QImage to QPixMap conversion or optimize the drawing in some other way?

On Qt 4.5 and newer, [conversions between `QImage` and `QPixmap` are fast](https://blog.qt.digia.com/blog/2008/10/22/so-long-and-thanks-for-the-blit/) *as long as the image has the correct format*. This is because a pixmap has its memory shared with the server, if the server is local. Most likely your image's format doesn't match the screen format. — Kuba hasn't forgotten Monica, May 12 '14 at 13:55

score 0 · Answer 1 · edited Jan 17 '18 at 02:40

This is my problem too. camera frames are shown very slowly in QLabel. my code is here:

QCamera *camera = new QCamera(this);
camera->setCaptureMode(QCamera::CaptureViewfinder);
QVideoProbe *videoProbe = new QVideoProbe(this);
bool ret = videoProbe->setSource(camera);
if (ret) {
    connect(videoProbe, SIGNAL(videoFrameProbed(const QVideoFrame &)),
            this, SLOT(present(const QVideoFrame &)));
}
camera->start();

...
...

bool MainWindow::present(const QVideoFrame &frame)
{

    QVideoFrame cloneFrame(frame);
    if(cloneFrame.map(QAbstractVideoBuffer::ReadOnly))
    {
        QImage img(
                cloneFrame.size(), QImage::Format_ARGB32);
                qt_convert_NV21_to_ARGB32(cloneFrame.bits(),
                (quint32 *)img.bits(),
                cloneFrame.width(),
                cloneFrame.height());

        label->setPixmap(QPixmap::fromImage(img));

        cloneFrame.unmap();
    }

    return true;
}

Optimizing QPainter drawing & Converting QVideoFrame straight to QPixMap

1 Answers1