I'm having some trouble using a QGraphicsVideoItem with a GStreamer pipeline as source.

I'm trying to show the video on a QMdiSubWindow, and using a QVideoWidget it works, but i need to draw some boxes on top on the video and followed this forum topic.

Using a local file instead of the GStreamer pipeline shows the video with no problems.

To avoid an XY problem, TLDR: trying to draw rectangles on top of a video inside of a QMdiSubWindow whose source is a GStreamer pipeline.

Here's the code for my subclassed subwindow

videoSubWindow::videoSubWindow(int shmID, int width, int height, QWidget* parent): QMdiSubWindow(), shmID(shmID), player(nullptr), videoWidget(nullptr) {
    std::cerr << __func__ << "\n";

    std::string playerLaunch = "gst-pipeline: shmsrc socket-path=/tmp/shm" + std::to_string(shmID) +
        " ! video/x-raw, format=(string)I420, width=(int)" + std::to_string(width) + 
        ", height=(int)" + std::to_string(height) + ", framerate=(fraction)30/1 ! queue ! xvimagesink name=qtvideosink";

    videoWidget = new roiVideoWidget(this);
    scene = new QGraphicsScene(videoWidget);
    item = new QGraphicsVideoItem;
    videoWidget->setScene(scene);
    scene->addItem(item);
    player = new QMediaPlayer;
    player->setVideoOutput(item);
    player->setMedia(QUrl(playerLaunch.c_str()));
    //player->setMedia(QUrl::fromLocalFile("testvideo.mp4"));
    
    setWidget(videoWidget);
        
    setAttribute(Qt::WA_DeleteOnClose);
}

Thanks in advance for any help.

PS: Using Qt 5.12 on Ubuntu 20.04 and GCC 10