You are contradicting yourself in your question here. First you want to detect motion in a live stream, which by itself is not all that hard to do, but then you want to constrain it to the detection of motion for humans only and you also want keep it simple.
An impossible task. If you are able to work that out so it will detect humans only and not moving animals nor cars and machinery you'd be able to make a lot of money with that. Just in case you are not getting what I'm driving at consider this:
What about humanoid robots?
What about a human in the guise of a robot, animal or whatever non human appearance you can think of.
Cheers!
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