Question

I am trying to process a video by playing with its frames via the methods of OpenCV library and display these frames with imshow method of opencv/highgui with no problem here.

But when it comes to display them in real-time with a Qt-gui app, I couldn’t managed.

The program gets the path of the video from openfile dialog and starts to grap frames of the video within a while loop and process them. After process step, when I try to display these processed frames on a QLabel, the QLabel of gui-app doesn’t display anything (along the length of the video) but only the last frame at the end of the video, nothing else. I tried to refresh/update the label for each frame but it didn’t work.

Do I miss something to do? Or is there any more convenient method for it?

Necessary part of my code is below. Thanks in advance.

bool stop=false;
cv::VideoCapture capture("a.avi");
cv::Mat cur_frame;

while (!stop) {    
             //...               
             capture.read(cur_frame);                
             //process steps..               
             QImage img= QImage((const unsigned char*)(cur_frame.data),
                                 cur_frame.cols,cur_frame.rows,
                                 QImage::Format_RGB888);
             ui->label->setPixmap(QPixmap::fromImage(img));
             // resize the label to fit the image
             ui->label->resize(ui->label->pixmap()->size());
             //...
             cv::waitkey(50);
             }

Answer 1

Qt can only update the UI when control has returned to the event loop. You can try calling qApp->processEvents() in your loop but that may not be the optimum approach.