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This will be a very basic algorithm just to evaluate your use case. It can be improved a lot.

(i) In your case, the first step is to identify whether there is a change or not between 2 frames. It can be identified by using a simple StandardDeviation measurement. Set a threshold for acceptable difference in deviation.

Mat prevFrame, currentFrame;

for(;;)
{
    //Getting a frame from the video capture device.
    cap >> currentFrame;

    if( prevFrame.data )
    {
         //Finding the standard deviations of current and previous frame.
         Scalar prevStdDev, currentStdDev;
         meanStdDev(prevFrame, Scalar(), prevStdDev);
         meanStdDev(currentFrame, Scalar(), currentStdDev);

          //Decision Making.
          if(abs(currentStdDev - prevStdDev) < ACCEPTED_DEVIATION)
          {
               Save the images and break out of the loop.
          }

    }

    //To exit from the loop, if there is a keypress event.
    if(waitKey(30)>=0)
        break;

    //For swapping the previous and current frame.
    swap(prevFrame, gray);
}

(ii) The first step will only identify the change in frames. In order to locate the position where the change occured, find the difference between the two saved frames using AbsDiff. Using this difference image mask, find the contours and finally mark the region with a bounding rectangle.

Hope this answers your question.

This will be a very basic algorithm just to evaluate your use case. It can be improved a lot.

Mat prevFrame, currentFrame;

for(;;)
{
    //Getting a frame from the video capture device.
    cap >> currentFrame;

    if( prevFrame.data )
    {
         //Finding the standard deviations of current and previous frame.
         Scalar prevStdDev, currentStdDev;
         meanStdDev(prevFrame, Scalar(), prevStdDev);
         meanStdDev(currentFrame, Scalar(), currentStdDev);

          //Decision Making.
          if(abs(currentStdDev - prevStdDev) < ACCEPTED_DEVIATION)
          {
               Save the images and break out of the loop.
          }
}     
    }

    //To exit from the loop, if there is a keypress event.
    if(waitKey(30)>=0)
        break;

    //For swapping the previous and current frame.
    swap(prevFrame, gray);
currentFrame);
}

Hope this answers your question.