interactive ground piano

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Hello,
I have been working with opencv for few weeks and fortunately am able to do some detections and trackings with it, but I have no idea what should I do for below project.I would appreciate any help from forum users.
Project overview:
A piano projected on ground with video projector, made in Unreal engine (this part is done). I want to detect users legs or body with a camera when they put it over piano keys and play the key. in C++ I need to declare regions for whole screen (where piano keys are located) to detect when users leg enters that spacial region!
how can I declare multiple regions to detect object movement or entrance?
Thanks in advance.