How to calculate 2x2 Hessian Matrix for each pixel of an image in MATLAB.??

Plz help me how to calculate 2x2 Hessian Matrix for each pixel of an image in MATLAB.?? I have found these equations but I don't know what is "h" & "k" here. plz help me about this..

ux(xi, yj) ≈ 1/2h(ui+1,j − ui−1,j) uy(xi, yj) ≈ 1/2k(ui,j+1 − ui,j−1) uxx(xi, yj) ≈ 1/h^2 (ui+1,j − 2ui,j + ui−1,j) uyy(xi, yj) ≈ 1/k^2 (ui,j+1 − 2ui,j + ui,j−1) ,