I'd really like to attach to my web camera, process the frames I get out of it, and then display the processed frame in a window with some super-imposed markings.

The strategy I've been trying is to have a QGraphicsView with a QGraphicsScene that has a QGraphicsPixmap added. I then use a custom VideoSurface to grab frames. Right now I'm trying to just transfer the frames to the QGraphicsView (no extra markings or processing) but if I call setPixmap the program will crash just after the frame handler returns. If I skip the call to setPixmap, it doesn't crash. Could anyone point me in the right direction?

Qt Code: 
  1. MainWindow::MainWindow(QWidget *parent) :
  2.     QMainWindow(parent),
  3.     ui(new Ui::MainWindow),
  4.     frame(640, 480)
  5. {
  6.     ui->setupUi(this);
  7.     QList<QCameraInfo> cameras = QCameraInfo::availableCameras();
  8.     cam = new QCamera(cameras.first(), this);
  9.     cam->load();
  10.     frame_grabber = new CameraFrameGrabber();
  11.     cam->setViewfinder(frame_grabber);
  12.     cam->setCaptureMode(QCamera::CaptureViewfinder);
  13.     connect(frame_grabber, SIGNAL(frameAvailable(QImage)), this, SLOT(handleFrame(QImage)));
  14.     cam->start();
  15.     frame_timer.start();
  16.     scene = new QGraphicsScene();
  17.     pix = new QGraphicsPixmapItem(frame);
  18.     scene->addItem(pix);
  19.     ui->gv_viewfinder->setScene(scene);
  20. }
  21.  
  22. MainWindow::~MainWindow()
  23. {
  24.     delete ui;
  25. }
  26.  
  27. void MainWindow::handleFrame(const QImage& img)
  28. {
  29.     frame_times.push_back(frame_timer.elapsed());
  30.     frame_timer.restart();
  31.     if (frame_times.size() > 10) frame_times.pop_front();
  32.     int frame_count = frame_times.size();
  33.     int frame_total = 0;
  34.     for(int i = 0; i < frame_count; ++i)
  35.     {
  36.         frame_total += frame_times[i];
  37.     }
  38.     double fps = 0;
  39.     if (frame_count)
  40.     {
  41.         fps = (frame_total / frame_count);
  42.     }
  43.     ui->sb_fps->setValue(fps);
  44.  
  45.     bool good_pix = frame.convertFromImage(img);
  46.     if (good_pix) pix->setPixmap(frame);
  47.     this->update();
  48. }
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Qt Code: 
  1. bool CameraFrameGrabber::present(const QVideoFrame &frame)
  2. {
  3.     if (frame.isValid()) {
  4.         QVideoFrame cloneFrame(frame);
  5.         cloneFrame.map(QAbstractVideoBuffer::ReadOnly);
  6.         const QImage image(cloneFrame.bits(),
  7.                            cloneFrame.width(),
  8.                            cloneFrame.height(),
  9.                            QVideoFrame::imageFormatFromPixelFormat(cloneFrame .pixelFormat()));
  10.         emit frameAvailable(image);
  11.         cloneFrame.unmap();
  12.         return true;
  13.     }
  14.     return false;
  15. }
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