Why does fsamp2 perform fftshift on output coefficients?

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skykeithtr 2020-7-2

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https://ww2.mathworks.cn/matlabcentral/answers/558178-why-does-fsamp2-perform-fftshift-on-output-coefficients

编辑： skykeithtr 2020-7-2

Why does matlab internal function fsamp2 perform fftshift on the output filter coefficients? After swapped, the output cannot be a direct input to filter2 function, right?

Following are an example and the concerned part of codes in fsamp2.m.

% For example, to create a low-pass filter LPF with frequency response Hd
[f1,f2] = freqspace(25,'meshgrid');
Hd = zeros(25,25); d = sqrt(f1.^2 + f2.^2) < 0.5; % (cutoff at 0.5)
Hd(d) = 1;
% Here's my thought of creating a filter:
LPF = fftshift(Hd); % convolution kernel
LPF = rot90(LPF,2); % correlationn kernel
signal_filtered = filter2(LPF,signal);
% But if I use fsamp2:
h = fsamp2(Hd);
% According to the fsamp2 function below (mainly the line commented with "QUESTION"), 
% h is not the same with LPF and cannot be a direct input to filter2 function.
% However, h can be used with freqz2(h) directly because freqz2 swaps h back.
function h = fsamp2(f1,f2,hd,siz)
if nargin==1, % Uniform spacing case (fast)
  hd = f1;
  hd = rot90(fftshift(rot90(hd,2)),2); % Inverse fftshift
  h = fftshift(ifft2(hd)); % QUESTION: ifft2(hd) already outputs the desired coefficients, why use fftshift then?
elseif nargin==2 || nargin==3,
    % etc...
else
    % etc...
end
% etc
h = rot90(h,2); % Rotate for use with filter2
end % end function

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Sulaymon Eshkabilov 2020-7-2

It is used to swap the right halves of your computed vector values.

skykeithtr 2020-7-2

Yes, but I just can't see why it needs to be swapped. I have edited the question to discuss whether it would be an appropriate input to other functions like filter2 and imfilter.

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