How can I calculate the curve angle at specific points on the contour of Handwriting connected component

Hi everybody I would like to calculate the curve angle at each point on contour. as shown in the attached figure, I want to calculate the angle (theta 1) formed by the line connecting Pi and Pi-e and the X axis and the angle (theta2) formed by the line connecting Pi and Pi+e and the X axis. having those theta1 and theta2 I can compute the curve angle at Pi. Can anyone help me in calculating theta1 and theta2. thanks in advance.

[Walter Roberson]

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