How to draw goal line - the waypoint

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Hi guys ! I've started my journey with OpenCV and Python not so long ago and I need your guidance. I would like to make project for my own purpose. I'm talikng about simple "goal line technology". I would like to be able to count when center of the ball passed certain point-goal line. By using many tutorials around the web I've created simple ball tracking program that track the ball position and also calculate its center. So my idea is to find a way to draw a line/goal line and calcualte distance between center of the ball and the line and if it is equal to 0 or -value to count it as a goal. But I dont't know how to approach this problem. I'm looking forward for any help. Cheers!