Image substraction

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I am working on a college project, I am trying to detect damage on the surface of a marble. I am working with OpenCV 4.3.0 I have converted to HSV carried out erode and dilation and isolated the marble and drawn a circle around it. What I would like to try is to save just what is in the circle and the use substraction with the current image in the circle to highlight the difference or defect. Is it possible to save just the section of a frame inside circle. The target marble will be rotating so all parts are seen, so the location within the frame will move. Any suggestions welcome, I intend to run on a Pi4 in real time.