[Octave-bug-tracker] [bug #63094] FFT2 produces a result that differs from MATLAB

[anonymous]

URL:
  <https://savannah.gnu.org/bugs/?63094>

                 Summary: FFT2 produces a result that differs from MATLAB
                 Project: GNU Octave
               Submitter: None
               Submitted: Fri 23 Sep 2022 12:15:38 PM UTC
                Category: Octave Function
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Incorrect Result
                  Status: None
             Assigned to: None
         Originator Name: STEPHEN E RANKIN
        Originator Email: s.e.rankin@outlook.com
             Open/Closed: Open
                 Release: 6.4.0
         Discussion Lock: Any
        Operating System: GNU/Linux


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Follow-up Comments:


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Date: Fri 23 Sep 2022 12:15:38 PM UTC By: Anonymous
The result of trying to differentiate a Gaussian by FFT fails on Octave
running on Ubuntu 22.04, but works on MATLAB with the same code:

    size2 = 101;
    A = 10;
    sigma = 10;
    gauss = zeros(size2);

    for y = 1:size2
        for x = 1:size2
            gauss(y,x)=A*exp(-1/(sigma^2)*((y-size2/2)^2+(x- 
            size2/2)^2)); %gaussiana
        end
    end



    hf = fftshift(fft2(gauss));
    [U,V] = meshgrid(-50:50, -50:50);
    hf2 = (exp(1i*2*pi*U/101) - 1).*(exp(1i*2*pi*V/101) - 1).*hf;
    out_freq = real(ifft2(ifftshift(hf2)));
    out_freq = out_freq(1:end-1,1:end-1);
    out_spatial = diff(diff(gauss, 1, 1), 1, 2);
    figure;
    subplot(1,2,1);
    imshow(out_spatial, []);
    title('Derivative from spatial domain');
    subplot(1,2,2);
    imshow(out_freq, []);
    title('Derivative from frequency domain');








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File Attachments:


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Date: Fri 23 Sep 2022 12:15:38 PM UTC  Name: octave.jpg  Size: 19KiB   By:
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<http://savannah.gnu.org/bugs/download.php?file_id=53743>
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Date: Fri 23 Sep 2022 12:15:38 PM UTC  Name: matlab.jpg  Size: 15KiB   By:
None

<http://savannah.gnu.org/bugs/download.php?file_id=53744>

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