Thanks, guys, for replying!!
I will try do rotoscoping because the background was a green chalkboard — BUT you can even see the wall (white) at the bottom of the board (wow…I just realize how amateur I am) lol O_0
Two more question, guys.
How do I do rotoscoping in After Effects? And after I do that, what should I do if I want to put a background (say, jpeg) on the footage?
Thanks very much 🙂