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06.29.2007 at 02:55PM PDT, ID: 22667835
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Resize image by FFT

Tags: image, matlab, resize, fft
Hello everyone:
I have another question about FFT resize image.
I have an image and calculate its FFT, question is if I enlarge or reduce images size, how can I get its new FFT base on previous FFT? I do not want to re-calculate FFT again.
Matlab code would be good for testing! Thanks in advance!

Chao
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Question Stats
Zone: Programming
Question Asked By: seaviewfc
Solution Provided By: chinu1310
Participating Experts: 4
Solution Grade: A
Views: 52
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06.29.2007 at 03:08PM PDT, ID: 19393926

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06.29.2007 at 03:24PM PDT, ID: 19393998

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07.03.2007 at 01:20AM PDT, ID: 19409531

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07.03.2007 at 04:02AM PDT, ID: 19410064

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07.28.2007 at 04:29AM PDT, ID: 19584256

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10.30.2007 at 05:17AM PDT, ID: 20176301

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06.29.2007 at 03:08PM PDT, ID: 19393926
here is example code of how to enlarge an image using an FFT:

% Copyright: Joseph O'Ruanaidh 2003, All Rights Reserved
% Disclaimer: not to be used for air traffic control

function y=fftResize(x, l)

ff=fft2(x);
[m,n]=size(ff);

ml = floor((l-1)*m/2)*2
mm = m + 2*ml
nl = floor((l-1)*n/2)*2
nn = n + 2*nl

% what will soon be the new transform
qq = zeros(mm, nn);

% The following indices copy from the original FFT to the new FFT if mod(m, 2)==0
    xr =  [1:m/2 ml+m/2+1 2*ml+((m/2+2):m)]
else
    xr = [1:((m+1)/2) 2*ml+(((m+3)/2):m)]
end
if mod(n, 2)==0
    yr = [1:n/2 nl+n/2+1 2*nl+((n/2+2):n)]
else
    yr = [1:((n+1)/2)  2*nl+(((n+3)/2):n)]
end

% Do the copying and rescale the indices
qq(xr, yr) = ff*(mm*nn)/(m*n);

% And don't forget the most important part
y=ifft2(qq);


Accepted Solution
 
06.29.2007 at 03:24PM PDT, ID: 19393998
chinu1310: Thank you for reply! I tested on that code before. Only enlarge is not enough for this question. I need more explanation theoretically.
 
07.03.2007 at 01:20AM PDT, ID: 19409531
An intensity in the frequency domain is basically a histogram bin.
Consider what it means to resize an image:
Doubling its size will effectively halve all frequencies.
Halving its size will effectively double all frequencies.
Also, the "histogram bins" must be rescaled according to the new total "energy" in the image (the next to last code line above).
Assisted Solution
 
07.03.2007 at 04:02AM PDT, ID: 19410064
Thanks Ugumba: Sorry about my poor math.
I wrote some code, it doesn't work.

im = imread('1.bmp');
im = rgb2gray(im);
t1 = fft2(double(im));
t2 = imresize(t1,0.5,'bilinear');
figure,imshow(real(ifft2(t2)),[])

Help me please!
 
07.28.2007 at 04:29AM PDT, ID: 19584256
Ugumba: if you double the size, i think what you double, for a same image, the sample frequency, so low frequencies are the same but higher frequencies are absent, because we cant reconstruct details that weren't present at original image.

For doubling from FFT, what i would do create a double size all Zeros FFT table and fill the low frequency coefficients by copying from the original FFT, and keep zero the high frequency coefficients.
For halving, just filter with a cut frequency of pi/2  the first half coefficients with a FIR filter to remove aliasing, then truncate the FFT to half the size (in the case of 2D FFT, a quarter the size).
 
10.30.2007 at 05:17AM PDT, ID: 20176301
No comment has been added to this question in more than 21 days, so it is now classified as abandoned.

I will leave the following recommendation for this question in the Cleanup topic area:
   Split: chinu1310 {http:#19393926} & Ugumba {http:#19409531}

Any objections should be posted here in the next 4 days. After that time, the question will be closed.

Venabili
EE Cleanup Volunteer
 
 
11.03.2007 at 09:46PM PDT, ID: 20209580
Forced accept.

Computer101
EE Admin
 
 
 
20080236-EE-VQP-29