shown the positions

Hi everyone,

Right now I got the camera to grab the person shown in jit.pwindow, but then how can i define the x, y position of a person in the screen?

For example,
I think first i have to detect the center of the screen as (0,0) (no matter how big is the screen)
then, if the person is moving towards right side, the x should go up, more than 0 (right?) and if the person is going to the left then the x should go down to negative… same idea for up and down.. is this sounds right?

Thank you everyone for helping me in advance :)