y-axis are still inverted despite using fftshift

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Nishant Pathak
Nishant Pathak on 20 Nov 2021
Commented: Dave B on 20 Nov 2021
I have used the Matlab codes for simulating a phase screen. However, as you can see in the figure, y-axis are still inverted despite using fftshift. Please resolve this issue.
M = 3001;
a=3;
d = (M-1)/2; fx = (-d:d);
kx = (ones(2*d+1,1)*fx).^2; K = sqrt(kx+transpose(kx));
D = K.^(-a);
D(d+1,d+1) = 0;
R = randn(2*d+1)+1i*randn(2*d+1);
PS = (fft2(fftshift(sqrt(D).*R)));
figure(1); imagesc(abs(PS)); colormap(gray);title('Phase screen');

Answers (2)

Dave B
Dave B on 20 Nov 2021
Edited: Dave B on 20 Nov 2021
Your code doesn't run and you didn't include an image, but in general, when you call imagesc it will plot in ij coordinates, meaning that 0,0 is at the top left corner of the axes. To switch to traditional xy coordinates, simple use:
axis xy
(after calling image)
Alternatively, you can control the YDir property on the axes:
set(gca,'YDir','normal')
  2 Comments
Dave B
Dave B on 20 Nov 2021
I just added my answer to the bottom of your code, is this not what you're looking for?
M = 3001;
a=3;
d = (M-1)/2;
fx = (-d:d);
kx = (ones(2*d+1,1)*fx).^2;
K = sqrt(kx+transpose(kx));
D = K.^(-a);
D(d+1,d+1) = 0;
R = randn(2*d+1)+1i*randn(2*d+1);
PS = (fft2(fftshift(sqrt(D).*R)));
figure(1); imagesc(abs(PS));
colormap(gray);
title('Phase screen');
axis xy

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VBBV
VBBV on 20 Nov 2021
Edited: VBBV on 20 Nov 2021
M = 3001;sz = 4;cx = 6;
d = (M-1)/2; fx = (-d:d);
kx = (ones(2*sz+1,1)*cx).^2; K = sqrt(kx+transpose(kx));a = 2;
D = K.^(-a);
D(d+1,d+1) = 0;
R = randn(d+1)+1i*randn(d+1);
PS = fft2(fftshift(sqrt(D).*R));
figure(1); imagesc(abs(PS)); colormap(gray);title('Phase screen');
ax = gca; ax.YDir = 'Normal'
You could also use handle to current axes and set the direction as Normal

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