Make ifft2 to give real output

Question

Mona Mahboob Kanafi 2015-5-20

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https://ww2.mathworks.cn/matlabcentral/answers/217750-make-ifft2-to-give-real-output

评论： Ralf Mackenbach 2019-4-10

I'm trying to generate a randomly rough surface(isotropic surface). Suppose I have the absolute values of fourier components of the surface in "H" and H has symmetry with respect to center of the array. Now, I need to define some random phase for the fourier series which are between -pi to pi (due to fftshift) as in:

phi = -pi + (pi+pi)*rand(n); % random phase

Now, I don't know what to do to make the output of my surface real..

Here is the gist of the code I used so far:

[a,b] = pol2cart(phi,H);
H_complex = complex(a,b); % the complex fourier transform composed of H & phi
z = ifft2(ifftshift((H_complex)));

If I need to apply conjugate symmetry to the fourier components, how should I do it? The output is complex and I need a real surface.

Please help! Thanks a lot.

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Answer 1

Mona Mahboob Kanafi 2015-5-21

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编辑：Mona Mahboob Kanafi 2015-5-21

在 MATLAB Online 中打开

I found my answer, so maybe useful for someone else as well. I need to apply conjugate symmetry condition for 2D fourier transform, both in magnitude values and phase components. I applied it as below for a n*m matrix:

"H" is the magnitude:

H(1,1) = 0;
H(1,m/2+1) = 0;
H(n/2+1,m/2+1) = 0;
H(n/2+1,1) = 0;
H(2:end,2:m/2) = rot90(H(2:end,m/2+2:end),2);
H(1,2:m/2) = rot90(H(1,m/2+2:end),2);
H(n/2+2:end,1) = rot90(H(2:n/2,1),2);
H(n/2+2:end,m/2+1) = rot90(H(2:n/2,m/2+1),2);

Similar operation but with negative sign applies to "phi" (phase):

phi(1,1) = 0;
phi(1,m/2+1) = 0;
phi(n/2+1,m/2+1) = 0;
phi(n/2+1,1) = 0;
phi(2:end,2:m/2) = -rot90(phi(2:end,m/2+2:end),2);
phi(1,2:m/2) = -rot90(phi(1,m/2+2:end),2);
phi(n/2+2:end,1) = -rot90(phi(2:n/2,1),2);
phi(n/2+2:end,m/2+1) = -rot90(phi(2:n/2,m/2+1),2);

You can refer to this link for a good example of 2D fourier transform on a matrix and how the results should look like: http://fourier.eng.hmc.edu/e101/lectures/image_processing/node6.html

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Ralf Mackenbach 2019-4-10

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Small correction to this. The condition that

H(1,1) = 0;
H(1,m/2+1) = 0;
H(n/2+1,m/2+1) = 0;
H(n/2+1,1) = 0;

Does not need to hold necessarily. So a better (examplary) version would be

% Choose even m and n
m = 100 ;
n = 100 ;
% Choose average value of surface
z0 = 1 ;
% Fill random examplary matrices
H   = rand(n,m) ;
phi = 2*pi*rand(n,m) ;
% Apply symmetry to matrices
H(1,1)              = z0*m*n ;
H(2:end,2:m/2)      = rot90(H(2:end,m/2+2:end),2);
H(1,2:m/2)          = rot90(H(1,m/2+2:end),2);
H(n/2+2:end,1)      = rot90(H(2:n/2,1),2);
H(n/2+2:end,m/2+1)  = rot90(H(2:n/2,m/2+1),2);
phi(1,1)            = 0;
phi(1,m/2+1)        = 0;
phi(n/2+1,m/2+1)    = 0;
phi(n/2+1,1)        = 0;
phi(2:end,2:m/2)    = -rot90(phi(2:end,m/2+2:end),2);
phi(1,2:m/2)        = -rot90(phi(1,m/2+2:end),2);
phi(n/2+2:end,1)    = -rot90(phi(2:n/2,1),2);
phi(n/2+2:end,m/2+1)= -rot90(phi(2:n/2,m/2+1),2);
% Convert to polar
[a,b] = pol2cart(phi,H);
H_complex = complex(a,b);
% Inverse and plot
S = ifft2(H_complex, 'symmetric') ;
surf(S)

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Answer 2

Thomas Koelen 2015-5-20

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use abs() or real()? depending on if you want only the real number or the absolute value.

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Mona Mahboob Kanafi 2015-5-20

编辑：Mona Mahboob Kanafi 2015-5-20

Thanks Thomas, I need the output to be real based on a solid theory. Since, from the beginning, I have to define the absolute values of the fft so that it gives a surface with specified standard deviation (root mean square roughness). Real values, don't have the same std and absolute values have the same std, but not a correct shape! Absolute values are only positive while my surface must have a zero mean. This is not an option for me at the moment.

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