%
% illustrates the gradient
%

[x, y] = meshgrid([-3:0.1:3]);
[u, v] = meshgrid([-3:0.5:3]);
z = exp(-0.5*(x.^2 + y.^2));
w = exp(-0.5*(u.^2 + v.^2));
[pu, pv] = gradient(w, .1, .1);

figure;
subplot(121);
surfc(x, y, z);

subplot(122);
contour(x, y, z);
hold on;
quiver(u, v, pu, pv);
axis image;