DFT coordinate system

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In the implementation of DFT(for 2D) in opencv what coordinate system they use(Cartesian coordinate system or 2D array like coordinate system)?

(-1, 1)(0, 1)(1, 1)
(-1, 0)(0, 0)(1, 0)
(-1,-1)(0,-1)(1,-1)

(0,0)(0,1)(0,2)
(1,0)(0,1)(0,2)
(2,0)(0,1)(0,2)