What method is used to detect this rectangle?

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Rotating Target changed distances computed with SolvePnP

I'm trying to detect the pose of a hexagon shape with a similar frame around it but I'm not able to isolate out everything that is going on in the background. Perhaps another issue I have is that I'm really only familiar with Python and there are no comments in this code (and most others in the Github/samples directory). I have followed each of the tutorial lessons for OpenCV Python on opencv-python-tutroals.readthedocs.org and PythonProgramming.net.

Can the author or anyone help identify the method?