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How can I plot phase and magnitude of Fourier transform of a 2D image in MATLAB? I am using angle and abs and then use imshow, but I get a black image.
what is the use of fftshift in this plotting?

Luis Mendo
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amanda
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2 Answers2

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F = fft2(I); where I is the input
F = fftshift(F); % Center FFT

F = abs(F); % Get the magnitude
F = log(F+1); % Use log, for perceptual scaling, and +1 since log(0) is undefined
F = mat2gray(F); % Use mat2gray to scale the image between 0 and 1

imshow(F,[]); % Display the result

Try this. Code taken from:How to plot a 2D FFT in Matlab?

Community
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lakshmen
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  • I've seen this gode but I didn't understand why we use log and fftshift why we didn't just show that without using this? – amanda Mar 07 '14 at 05:57
  • Thanks lakesh you saved me! I've spent Hours searching for that log(F+1) :D Here is a clarification to be in safe side when you prepare the input I if you read it by f = imread(imPath); you will need this F = fft2(double(f)); as the image has been read in unit8 format so they should be converted to double arrays before doing the FFTs. Quoted from http://matlabgeeks.com/tips-tutorials/how-to-do-a-2-d-fourier-transform-in-matlab/#comment-5137 – N0rA Jul 02 '14 at 15:33
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From your comment, you need to remove the DC offset. Something like:

imagesc(abs(fftshift(fft2(I - mean(I(:))))));
ChuNan
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