Calculate the image gradient

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Donghui Sun 2013-9-23

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/87917-calculate-the-image-gradient

评论： Matt J 2017-3-1

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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?

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naser telesi 2017-3-1

if Gx is zero then let Gx = 1 to avoid divide by zero problem

Matt J 2017-3-1

if Gx is zero then let Gx = 1 to avoid divide by zero problem

But theta will be wrong...

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Matt J 2013-9-23

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Use atan2 instead

theta=atan2(Gy,Gx);

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Donghui Sun 2013-9-23

A big thanks.

Image Analyst 2013-9-23

Like I said, imgradient() already gives that to you as Gdir - see the help. No need to do it yourself with atan2.

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Image Analyst 2013-9-23

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https://ww2.mathworks.cn/matlabcentral/answers/87917-calculate-the-image-gradient#answer_97526

Why do it yourself? Why not just use imgradient()?

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Donghui Sun 2013-9-23

Haha, I just want to try another method. In addition, the gradient direction in imgradient() contains angles in degrees within the range [-180 180]. I need the angle in radians.

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