How can I find theta on Matlab?

Hello!

I'm working on a Tennis Ball Machine Project. The project aims to change the idea of tennis ball being shot at random on the tennis court, but to have the capability to intellectually interact with the player giving a tougher training exercise.
By utilizing image processing (Visual Servoing) , the project once completed will have a fully automated “Tennis robot” that detects where the player is on the court and intelligently interacts with him/her.

Anyway, the part I'm working on right now is to detect the player in the tennis court.
For now the camera only detect the player and give its coordinates on the screen in pixels.
However, I want to find the player location in real time (in the real court), this can be done by finding the the theta in Matlab between the norm and the player.

My question is how can I find theta?

Check the attachment for better clarification.

Thanks ..