MATLAB: Calculate the image gradient

image gradientImage Processing Toolbox

I use the function imgradientxy to get the Gx and Gy firstly. Then obtain the direction of the gradient.
some codes like:
    % code
b = zeros(100);
b(1:4:end,:) = 1;
[Gx, Gy] = imgradientxy(b);
theta = atan(Gy./Gx);
The image only has some horizontal lines. So The Gx is all zeros and the theta have a lot NAN. What should I do to get the gradient for this image and for other?

Best Answer

Use atan2 instead
 theta=atan2(Gy,Gx);
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