Question

I have a vector of type cvMat into which I have been storing frames taken from my computer’s webcam. After storing 100 frames, I would like to play the frames back. If record is my vector of cvMats, I thought this might be done as so:

cvNamedWindow("play-back",CV_WINDOW_AUTOSIZE);
cvMoveWindow("play-back",100,100);
for (vector<Mat>::iterator iter = record.begin(); iter != record.end();++iter) {
   imshow("play-back",*iter);
}

When executed, the program seems to work well enough for storing the cvMats and getting input from the webcam, but when I attempt to get playback, the program seems to execute that portion of code very quickly — so quickly, in fact, that I don’t have time to appreciate the results. How might I be able to improve this code so that the playback is not so rushed?

Answer 1

You need to give control to OpenCV to actually render each image in the window and hold it for a brief period of time before switching to the next. You should add a call to cvWaitKey with a delay of, say 41ms (approximately 24 fps). Then you can check cvWaitKey’s return value so that the user can stop the playback. Something like this:

cvNamedWindow("play-back",CV_WINDOW_AUTOSIZE);
cvMoveWindow("play-back",100,100);
for (vector<Mat>::iterator iter = record.begin(); iter != record.end();++iter) {
   imshow("play-back",*iter);
   if( cvWaitKey(41) == 27 ) // ESC
       break;
}