Question: I'm attempting to detect if my capture camera gets unplugged. My assumption was that a call to `cvQueryFrame` would return `NULL`, however it continues to return the last valid frame. 

Does anyone know of how to detect camera plug/unplug events with OpenCV? This seems so rudimentary...what am I missing?

Answer: 
There is no API function to do that, unfortunately.

However, my suggestion is that you **create another thread** that simply calls cvCaptureFromCAM() and check it's result (inside a loop). If the camera get's disconnected then it should return NULL.

I'll paste some code just to illustrate my idea:

```
// This code should be executed on another thread!
while (1)
{
  CvCapture* capture = NULL;
  capture = cvCaptureFromCAM(-1); // or whatever parameter you are already using
  if (!capture)
  {
    std::cout << "!!! Camera got disconnected !!!!" << std::endl;
    break;
  }

  // I'm not sure if releasing it will have any affect on the other thread
  cvReleaseCapture(&capture); 
}

```
I think that I have a good workaround for this problem. I create an auxiliary Mat array with zeros with the same resolution like the output from camera. I assign it to Mat array to which just after is assign the frame captured from camera and at the end I check the norm of this array. If it is equal zero it means that there was no new frame captured from camera.

```
VideoCapture cap(0);
if(!cap.isOpened()) return -1;

Mat frame;
cap >> frame;
Mat emptyFrame = Mat::zeros(CV_CAP_PROP_FRAME_WIDTH,     , CV_32F);

for(;;)
{
   frame = emptyFrame;
   cap >> frame;
   if (norm(frame) == 0) break;
}

```