Corrupted video with OpenCV reading

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Recently, I have done a graphical user interface in C++/Qt/OpenCV on Debian 6.0. It achieves image processing (Canny Edge) on a webcam video streaming.

At the execution, the video is corrupted. The most annoying is this bug is not reproducible, it appears randomly.

Here's an example of corrupted video :

http://i.stack.imgur.com/r2R64.png

This problem seems not to come from the webcam because a simple OpenCV display code shows a good video.

Into my Qt code, I use a main class inherited from QMainWindow. The "start_webcam" button is linked to the "start_webcam" slot by :

Code:

---

startwebcamAct = new QAction(tr("&Start Webcam"), this);
 connect(startwebcamAct, SIGNAL(triggered()), this, SLOT(start_webcam()));

---

Then, the "start_webcam" function uses a QTimer as follows :

Code:

---

void ImageViewer::start_webcam()
{
  webcam_on = 1;
  invertWebcam = 0;
  close_current();
  updateStatusWebcam(webcam_on);
  // Init capture
  capture = cvCaptureFromCAM(0);
  first_image = cvQueryFrame(capture);
  // Init current qimage
  current_qimage = QImage(QSize(first_image->width,first_image->height),QImage::Format_RGB32);
  resize(first_image->width,first_image->height);
  // start QTimer
  timer = new QTimer(this);
  connect(timer,SIGNAL(timeout()),this,SLOT(query_frame()));
  timer->start(0);  
}

---

with "query_frame" function (where I use repaint() ) :

Code:

---

void ImageViewer::query_frame()
{
  webcam_off = 0;
  IplImage* frame = cvQueryFrame(capture);
  int w = frame->width;
  int h = frame->height;
 
  if (matrixMode)
  {
    current_image = cvCreateImage(cvGetSize(frame),8,3);
    cvCvtColor(frame,current_image,CV_BGR2RGB);
    int cvIndex = 0;
    int cvLineStart = 0;
    unsigned char red,green,blue;
 
    for(int j = 0; j < h; j++)
    {
      cvIndex = cvLineStart;
      for(int i = 0; i < w; i++)
      {
        red = 0;
        green = current_image->imageData[cvIndex+1];
        blue = 0;
 
        current_qimage.setPixel(i,j,qRgb(red, green, blue));
        cvIndex += 3;
      }
      cvLineStart += current_image->widthStep;
    }
  }
 gaussianfilter(webcam_off);
  border_detect(webcam_off);
  if (invertWebcam)
    invertvalues(webcam_off);
  cvReleaseImage(&current_image);
  repaint();
}

---

The repaint() function calls (immediatly ?) paintEvent :

Code:

---

void ImageViewer::paintEvent(QPaintEvent*)
{
 
QPainter painter(this);
painter.drawImage(QRect(0,0,current_qimage.width(),current_qimage.height()),current_qimage);
}

---

At the execution, if I maximize the GUI window, video is shown correctly ; here's an example of this good display :

http://i.imgur.com/jJltyT1.png


This lag seems to be a conflict between the QTimer into "start_webcam()" and the "paintEvent" function but I am not sure.

I tried to use update() instead of repaint() but the problem remains.

Anyone could give me a clue on this kind of bug ?

With this lag, I really suspect a no-synchronization between the call of query_frame and the call of paintEvent into this.

Could you help me to force the synchronization in order to, when I call paintEvent with repaint(), have the image displayed once and after, have
the next call of query_frame by QTimer ?

Thanks !