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discrete image gradient

The two finite-difference approximations to an image's partial derivatives.

The discrete image gradient \nabla I=(I_x,I_y) approximates \partial I/\partial x and \partial I/\partial y via finite differences (e.g. the central difference I_x\approx(I(x+1,y)-I(x-1,y))/2), computed by convolution since no true derivative exists on discrete pixel data.

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