function ZC = zero_crossings(I)

%ZC = zeros(size(I));

I_left = I(:,1:end-1);
I_right = I(:,2:end);
I_top = I(1:end-1,:);
I_bottom = I(2:end,:);

ZC_horz = sign(I_left) ~= sign(I_right);
ZC_horz(:,end+1) = 0;
ZC_vert = sign(I_top) ~= sign(I_bottom);
ZC_vert(end+1,:) = 0;
ZC = ZC_horz | ZC_vert;