FFT of an image

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Rachel
Rachel 2012-6-18
Hello,
I am a new Matlab user trying to compute the FFT of a set of images using the following code:
I=imread('imagename.pbm');
F=fft2(double(I));
S=fftshift(F);
L-log2(S);
A=abs(L);
imagesc(A)
The code works fine and produces a 2D power spectrum of my image and during subsequent processing I histogram the data in A according to distance form the centre. I have two questions:
  1. How do I read this output? The majority of the data cluster around one point on the x-axis...is the corresponding value on the x-axis in cycles/deg? If not how do I get this information from the analysis?
  2. How do I read in multiple images at a time so that I don't have to do this analysis for each individual image?
Thank you so much for your help!!
Rachel
  2 个评论
Walter Roberson
Walter Roberson 2012-6-18
Note: you will probably want
S = fftshift( fftshift(F), 2 );
as otherwise you only move the origin to the center of an edge rather than to the center of the array.
Dr. Seis
Dr. Seis 2012-6-18
doing "fftshift" twice is not necessary:
>> a = rand(4,4)
a =
0.4669 0.3845 0.6051 0.5502
0.0665 0.6463 0.9255 0.3866
0.3338 0.2953 0.9869 0.0707
0.7681 0.4264 0.7705 0.7959
>> fftshift(a)
ans =
0.9869 0.0707 0.3338 0.2953
0.7705 0.7959 0.7681 0.4264
0.6051 0.5502 0.4669 0.3845
0.9255 0.3866 0.0665 0.6463

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Dr. Seis
Dr. Seis 2012-6-18
See my answer here for a coded up description of what the 2D Fourier transform does using a random 2D image as an example (should help with your first question):
http://www.mathworks.com/matlabcentral/answers/24965-fft2-function-in-matlab
As to your second question, why wouldn't you do the analysis on an image by image basis? Or is it you want to read in multiple images at one time and then perform some operation on each of them in a similar way?

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