matlab,image-processing,filter , Lowpass filter non working


Lowpass filter non working

Question:

Tag: matlab,image-processing,filter

I have a noisy image that I am trying to clean using a lowpass filter (code below, modified from here). The image I get as a result is essentially identical to the one I gave as an input.

I'm not an expert, but my conclusion would be that the input image is so noisy that no patterns are found. Do you agree? Do you have any suggestion on how to interpret the result?

Result from the code:

enter image description here

Input image:

enter image description here

Code:

clear; close all;
frame = 20;
size_y = 512;   % This is actually size_x
size_x = 256;   % This is actually size_y
roi=5;thresh=100000;

AA = imread('image.png');
A = zeros(size_x, size_y);
A = AA(1:size_x, 1:size_y);
A(isnan(A)) = 0 ;

B = fftshift(fft2(A));
fabs = abs(B);

figure; imshow(B);

local_extr = ordfilt2(fabs, roi^2, ones(roi));  % find local maximum within 3*3 range

result = (fabs == local_extr) & (fabs > thresh);

[r, c] = find(result);
for i=1:length(r)
    if (r(i)-128)^2+(c(i)-128)^2>thresh   % periodic noise locates in the position outside the 20-pixel-radius circle
        B(r(i)-2:r(i)+2,c(i)-2:c(i)+2)=0;  % zero the frequency components
    end
end

Inew=ifft2(fftshift(B));
figure;
subplot(2,1,1); imagesc(A), colormap(gray); title('Original image');
subplot(2,1,2);imagesc(real(Inew)),colormap(gray); title('Filtered image');

Answer:

For filtering this kind of signal, you can try to use the median filter. It might be more appropriated than a means or Gaussian filter. The median filter is very effective on "salt and paper" noise when the mean just blur the noise.

As the signal seems very noisy, you need to try to find the good size of kernel for the filter. You can also try to increase the contrast of the image (after filtering) in order to see more the difference between the gray levels.


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