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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 :



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 :


Qt Code: 
  1. startwebcamAct = new QAction(tr("&Start Webcam"), this);
  2.  connect(startwebcamAct, SIGNAL(triggered()), this, SLOT(start_webcam()));
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Then, the "start_webcam" function uses a QTimer as follows :


Qt Code: 
  1. void ImageViewer::start_webcam()
  2. {
  3.   webcam_on = 1;
  4.   invertWebcam = 0;
  5.   close_current();
  6.   updateStatusWebcam(webcam_on);
  7.   // Init capture
  8.   capture = cvCaptureFromCAM(0);
  9.   first_image = cvQueryFrame(capture);
  10.   // Init current qimage
  11.   current_qimage = QImage(QSize(first_image->width,first_image->height),QImage::Format_RGB32);
  12.   resize(first_image->width,first_image->height);
  13.   // start QTimer
  14.   timer = new QTimer(this);
  15.   connect(timer,SIGNAL(timeout()),this,SLOT(query_frame()));
  16.   timer->start(0);  
  17. }
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with "query_frame" function (where I use repaint() ) :

Qt Code: 
  1. void ImageViewer::query_frame()
  2. {
  3.   webcam_off = 0;
  4.   IplImage* frame = cvQueryFrame(capture);
  5.   int w = frame->width;
  6.   int h = frame->height;
  7.  
  8.   if (matrixMode)
  9.   {
  10.     current_image = cvCreateImage(cvGetSize(frame),8,3);
  11.     cvCvtColor(frame,current_image,CV_BGR2RGB);
  12.     int cvIndex = 0;
  13.     int cvLineStart = 0;
  14.     unsigned char red,green,blue;
  15.  
  16.     for(int j = 0; j < h; j++)
  17.     {
  18.       cvIndex = cvLineStart;
  19.       for(int i = 0; i < w; i++)
  20.       {
  21.         red = 0;
  22.         green = current_image->imageData[cvIndex+1];
  23.         blue = 0;
  24.  
  25.         current_qimage.setPixel(i,j,qRgb(red, green, blue));
  26.         cvIndex += 3;
  27.       }
  28.       cvLineStart += current_image->widthStep;
  29.     }
  30.   }
  31.  gaussianfilter(webcam_off);
  32.   border_detect(webcam_off);
  33.   if (invertWebcam)
  34.     invertvalues(webcam_off);
  35.   cvReleaseImage(&current_image);
  36.   repaint();
  37. }
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The repaint() function calls (immediatly ?) paintEvent :


Qt Code: 
  1. void ImageViewer::paintEvent(QPaintEvent*)
  2. {
  3.  
  4. QPainter painter(this);
  5. painter.drawImage(QRect(0,0,current_qimage.width(),current_qimage.height()),current_qimage);
  6. }
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At the execution, if I maximize the GUI window, video is shown correctly ; here's an example of this good display :




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 ?