title: How to find the difference between one frame to another for a specified part of an image only
body: <p>I'm using OpenCV for my project on video feature tracking. I need to create a mask which is the difference between one frame to the next one. I know to do this, we can use the cv.absdiff function. The problem is I want the mask to contain only the difference for a specified small part of the frame, i.e. not the entire image. I'm not sure how to go about doing this.</p>