How to ensure the order of the 4 points of the polygon is always correct？How to find the bottom or top border's points？

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the same question is here: https://stackoverflow.com/questions/52678376/how-to-detect-top-edge-of-qr-code-in-python-and-pyzbar/52681707

i can not get the correct angle of rotation, because the position of the P1 and P2 is not correct sometimes. what can i do？ thank you very much！