opencv face detection

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hi I want to convert the detected face rectangle (the original face detection sample)into a 3D coordinates . I have the intrinsic parameters of my camera, how can I determine the depth Z? using the projection equation? how can i get the face depth ?

x = fx X / Z + u

y = fy Y / Z + v

I understand that fx fy and u v are intrinsic parameters, x y are given by the detected face rectangle.what does the X,Y represents?