OpenCV Running Video from WebCam on different thread

Question

I have 2 webcams and I want to get input from both of them at the same time. Therefore I believe I have to work with threads in c++ which is pthread. When I run my code given below, the webcam turns on for a second and the routine exits. I can't figure out what is wrong in my code.

void *WebCam(void *arg){

    VideoCapture cap(0);
    for (; ; ) {
        Mat frame;
        *cap >> frame;
        resize(frame, frame, Size(640, 480));
        flip(frame, frame, 1);

        imshow("frame", frame);

        if(waitKey(30) >= 0)
            break;
    }
    pthread_exit(NULL);   
}

int main(){
    pthread_t thread1, thread2;
    pthread_create(&thread1, NULL, &WebCam, NULL);
    return 0;
}

this is doe for one webcam just to turn and do streaming. Once this one works than other will be just copy of it.

Answer 1

When you create the thread, it starts running, but your main program, which is still running, just terminates, making the child thread finish too. Try adding this after pthread_create :

pthread_join(thread1, NULL);

By the way, even if you have two cameras, you can avoid the use of threads. I am not sure, but they could be problematic when dealing with the highgui functions ( imshow , waitKey ), because you must make sure they are thread-safe. Otherwise, what will be the result of having two threads calling waitKey at the same time? You could get rid of threads with a design similar to this one:

VideoCapture cap0(0);
VideoCapture cap1(1);

for(;;)
{
  cv::Mat im[2];
  cap0 >> im[0];
  cap1 >> im[1];

  // check which of im[i] is non empty and do something with it
}