Find the contours of a screen, as well as its diagonals to find the midpoint [closed]

I would like to use a picture of a screen to detect its contours, as well as its middle and all this starting from sketch in Python.

I found that Hough transform could be used to find the contour lines of the screen, but I do not know what methods could be useful to find the midpoint.

The steps of my little project are :

Step 1: Contour the screen

Step 2 : Diagonals of the screen

Step 3 : Find the midpoint in a kind of reference frame with coordinates.

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