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Re: Help-octave Digest, Vol 126, Issue 4

From: Ozzy Lash
Subject: Re: Help-octave Digest, Vol 126, Issue 4
Date: Tue, 6 Sep 2016 10:51:01 -0500

On Tue, Sep 6, 2016 at 9:37 AM, Yaowang Li <address@hidden> wrote:
Dear Dave, 

Thank you very much for your response.

I understand the Octave index starting from 1, not 0. and I did not find the what I want. That is why I ask this question. 

On the purpose of making it clearer. I enclosed my script and the example image. Hope to help you to find where is wrong. 

Best Regards,


On Tue, Sep 6, 2016 at 9:39 AM, Dave Cottingham <address@hidden> wrote:
Yaowang writes:

For a real function, for example, an image (u), Its Fourier transform
should follow the Friedel symmetry, I mean the U(s) = U*(-s). Here,
U(s) is the Fourier transform of u, and is a complex number. U*(s) is
its conjugate. I tested two images in imagej, and it works. However, it
is not happen in Octave, the real part is not equal and the imaginary
part is not opposite. They should have the magnitude and opposite
phase. That is what is my question. that is what I did in Octave.


%Fourier transform

Thank you very much. 

Best Regards,


The element for s = (0, 0) is stored at index (1, 1), and the indexing is modulo the size of the array.

For example, suppose your image is of size 64 x 64. Then s = (1, 2) is at index (2, 3), and s = (-1, -2) is at index (64, 63). If you use this indexing rule, you should see the complex conjugate relation you expect.

 - Dave Cottingham

Help-octave mailing list

Since your image is 256x256, I think that the fft matrix that you have created after using the fftshift function contains values for the fft for indices ranging from -128 to 127.  So that would put the values corresponding to index 0 at (129,129) instead of (128,128) and sould make it so that the values in the first row and column have no symmetry pairs.


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