Video tracking as a mouse

Hi, all.
I played a lot with video tracking patches here. I am trying to make something interesting. It is quite fun.

However, my knowledge of video tracking is quite narrow. only I can play with some examples. that’s all.

here is my question for video tracking. When people want to pick only one from a lot of objects to play at the same render window by tracking of shadow, how could I masure for this? is there any example? in other way, how can I simply pick one object with hand shadow as a mouse?

I could see a lot of example. but they are all different purpose not from this issue. :(

thank you very much.