how to find derivative of an image with respect to x and y axis ?

i have matlab code for an image matrix as given below and i have to find derivative along x and y axis then how to get?

m=1000; n=m; f1=20; f2=0; for i=1:m; for j=1:n; a(i,j)=exp(1i*2*pi*(f1*j/n+f2*i/m)); end end subplot(2,2,2),imshow(a,[]);

[Rik]

You should probably have a read here.

You should also have a look at the documentation for diff and meshgrid

doc diff
doc meshgrid