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What module should I use for streaming C++ QImage objects into QML?



  • Hi! I'm just start learning qt for developing desktop application. In particular, the front-end is implemented in QML and the back-end is implemented in C++.

    My Qt6 app uses camera device. To control the camera device, I need to use the external C++ library provided by the camera vendor, so I can't use QCamera class of the QT Multimedia module.
    So I made my own Camera class for handling streaming, which converts new raw frame to QImage and pushed it into a one-way queue. Whenever a new QImage is pushed in the queue, a signal emitted.
    Now I want to create a slot that is connected to this signal and renders the QImage to QML.

    I've searched many forums, but I still can't figure it out. Some Qt5 examples use QAbstractVideoSurface, which seems to be no longer valid in Qt6. Some examples use QQuickImageProvider, but that doesn't seem like a good way to keep updating images.

    Any advice will be appreciated


  • Lifetime Qt Champion

    Hi and welcome to devnet,

    Beside the controlling that you do with that external library, is the camera itself usable through QtMultimedia ?



  • @SGaist
    I think no.
    Just to add a little background, the camera communicates via GigE and the external library is ArenaSDK from Lucid Vision Labs, and the camera device can be controlled with the GenICam API. Also I need to handle multiple streaming cameras.

    I think when I import a physical camera device as an object from the external library, this camera device will not be looked up with QMediaDevices::videoInputs() .
    If I implement a custom camera class that inherits from QCameraDevice and conforms to the GenICam API, I might be able to use QtMultimedia. But it doesn't seem to provide QCameraDevice as an abstract class.


    By the way, I found a Qt5 example provided by the external library; Create a QVideoFrame using QImage and take it as an argument of QAbstractVideoSurface::present() .

    void pushImage(Arena::IImage* pImage)
    {
    	if (m_isStreaming)
    	{
    		QVideoFrame videoFrame(
    			QImage(
    				(uchar*)(pImage->GetData()),
    				pImage->GetWidth(),
    				pImage->GetHeight(),
    				QImage::Format::Format_RGB32));
    		m_pSurface->present(videoFrame);
    	}
    }
    

    Widgets are used instead of QML in this example, but I've seen some Qt5 examples in which QAbstractVideoSurface is binded into QML's VideoOutput.

    Only now did the question become a bit clearer.

    1. Constructing QVideoFrame with QImage is deprecated in Qt6. Is there still a way to do it in Qt6?
    2. If not, is there any other way to put QImage in VideoOutput?

  • Lifetime Qt Champion

    With Qt 5, you could have gone with a custom plugin or QMediaObject but I currently don't know for Qt 6.

    Maybe something to bring to mailing list. Did you already check the bug tracker ?



  • This post is deleted!


  • @wooseokyourself If you have the QImage then copy the bits to QVideoFrame, something like:

    QImage image(...);
    QVideoFrameFormat video_frame_format(image.size(), QVideoFrameFormat::pixelFormatFromImageFormat(image.format()));
    QVideoFrame video_frame(video_frame_format);
    if(video_frame.map(QVideoFrame::ReadWrite))){
        std::memcpy(video_frame.bits(0), image.bits(), video_frame.mappedBytes(0));
        video_frame.unmap(); 
    }
    


  • @eyllanesc Thanks. Your solution works for converting QImage to QVideoFrame.



  • @SGaist Thanks for reply. But I don't understand what you meant "mailing list" and "bug tracker".
    Thanks for @eyllanesc 's answer, I could convert QImage to QVideoFrame, but the next bigger problem is how to send C++'s QVideoFrame to QML's VideoOutput. So I didn't use QVideoFrame, but subclassing QQuickPaintedItem for QML's image object and bind it with my custom C++ image provider which update new QImage by using signal-slot.
    I'm not sure it's best optimized solution, but quite fast as much as I expected.



  • @wooseokyourself See this example: https://stackoverflow.com/questions/69432427/how-to-use-qvideosink-in-qml-in-qt6/69432938#69432938. In your case it is not necessary to use QImage, just use:

    QSize size(pImage->GetWidth(), pImage->GetHeight());
    QVideoFrameFormat video_frame_format(size, QVideoFrameFormat::pixelFormatFromImageFormat(QImage::Format_RGB32));
    QVideoFrame video_frame(video_frame_format);
    if(video_frame.map(QVideoFrame::ReadWrite))){
        std::memcpy(video_frame.bits(0), pImage->GetData(), video_frame.mappedBytes(0));
        video_frame.unmap(); 
    }
    

  • Lifetime Qt Champion

    @wooseokyourself said in What module should I use for streaming C++ QImage objects into QML?:

    @SGaist Thanks for reply. But I don't understand what you meant "mailing list" and "bug tracker".

    The interest mailing list where you can find Qt's developers/maintainers.

    The bug tracker is well "the place where the bugs are tracked".


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