Low pass filter for image

I want to create a lowpass filter (circle!) for an 110 x 160 pixel image.

The cut-off frequency should be 0.13/pixel.

What I tried so far:

bricks = im2double(imread("bricks.jpg"));
y1 = fft2(bricks);

[N, M] = size(bricks);
u = -1/2:1/M:1/2-1/M;
v = -1/2:1/N:1/2-1/N;
[U, V] = meshgrid(u, v);
d = U.*U/(0.13*0.13) + V.*V/(0.13*0.13);
mask = d <= 1;

yt1 = fftshift(y1);
yt1 = yt1.*mask;
low_filtered_image = ifft2(ifftshift(yt1), 'symmetric');

The spectrum of my filter and the filtered spectrum look like this:

That gives me the following result for my lowpass filtered image:

But I am not sure if this is correct.

Answer 1

[Image Analyst]

Use fft2, then zero out anything beyond 0.13. I'll leave it to you to figure out what index that is in the spectral image.

This looks like a homework problem. If you have any questions ask your instructor or read the link below to get started:

How do I get help on homework questions on MATLAB Answers? - MATLAB Answers - MATLAB Central

Obviously we can't give you the full solution because you're not allowed to turn in our code as your own.

[Dominik Deml]

But the return value of fft2 is not the frequency. So how can I compare the return value to a frequency?

[Image Analyst]

The return value of fft2 is the frequency image. Read up on it.

[Dominik Deml]

I know that this is the frequency image. Anyway, I found this post https://stackoverflow.com/questions/64036198/matlab-how-to-apply-low-pass-and-high-pass-filters-using-fft2-and-fftshift . But this does not solve my problem because 0.13 is too low.

[Image Analyst]

Then use a higher value so that less of the spectral image is erased. See attached demos.