I'm working on an example where I'm getting a logical error possibly. I want to replace every element of a matrix by the minimum of its neighborhood.

I've written the following piece of code. But it's not running and giving an error which I'm unable to figure out.

z = rgb2gray(imread('gantrycrane.png')); figure, imshow(z)

for i = 2:263

N = 0;

for j = 2:399

for k = (i-1)+N:(i+1)+N

for l = j-1:j+1

if(z(k,l)>z(i,j))

z(i,j)=z(k,l);

end

end

end

N = N+1;

end

end

Answer 1

[Simon Chan]

The value of N will be very large in the loop and gives an error.

Try the following to replace each element of the matrix by by the minimum value of its neighborhood (including the element itself).

for r = 2:263
    for c = 2:399
        region = z(r-1:r+1,c-1:c+1);                % Extract the neighborhood for each element
        z(r,c) = min(min(region));
    end
end

[Muhammad Hamza Shahid]

I tried the above piece of code. It just blackened the whole image.

[Muhammad Hamza Shahid]

The actual problem states that:

Simple Image Filter. Load up and show MATLAB’s image ‘gantrycrane.png’ using the following line: z = rgb2gray(imread(’gantrycrane.png’)); figure, imshow(z) Create a new image where every element of matrix z is replaced by the minimum of its neighbourhood values. The neighbourhood includes the central element and the surrounding 8 elements. Exclude the top and bottom rows of elements, as well the left and the right edges. Use nested loops. Repeat the process, but this time replace the value with the maximum within the neighbourhood. The resultant images should look like the ones in Figure 3.7.

[Simon Chan]

The code should work as follows:

clear; clc;
z=rgb2gray(imread('gantrycrane.png'));
[Ny,Nx]=size(z);
zmin = uint8(zeros(Ny,Nx));
for r = 2:Ny-1
    for c = 2:Nx-1
        region = z(r-1:r+1,c-1:c+1);
        zmin(r,c) = min(min(region));               % use max(max(region)) to find maximum
    end
end
imshowpair(z, zmin, 'montage')

[Muhammad Hamza Shahid]

OMG, you made my day sir. You are 1000X times better than my DIP instructor. Thank you so much!

[Simon Chan]

Thanks for your comment, please accept the answer if you consider my reply useful.

[Muhammad Hamza Shahid]

It's my first time on Mathworks, I didn't know how it worked. Otherwise, I'll accept it 100 times.