I created a code which does filtering with convulution but it gives an error

A = imread('lena.jpg'); 

F = uint8([-1 -1 -1;0 0 0;1 1 1]);
B = uint8(padarray(A,[1 1]));
Output = uint8(zeros(size(A)));

 for i = 1:size(B,1)-2
    for j = 1:size(B,2)-2
        X = B(i:i+2,j:j+2).*F;
        Output(i,j) = sum(X(:));
    end
 end

imshow(Output)

but it doesn't work correctly. What is my mistake?

[Rik]

Why do you want to replace the efficient convolution with a nested loop?

Answer 1

[Chunru]

The image has 3 channels and is a 3D array. You need to loop over the 3rd dimension as well. Use a different image below.

A = imread('onion.png'); 
F = uint8([-1 -1 -1;0 0 0;1 1 1]);
B = uint8(padarray(A,[1 1]));
Output = uint8(zeros(size(A)));
for k=1:size(B,3)
    for i = 1:size(B,1)-2
        for j = 1:size(B,2)-2
            
            X = B(i:i+2,j:j+2, k).*F;
            Output(i,j, k) = sum(X(:));
        end
    end
end
subplot(121)
imshow(A)
subplot(122)
imshow(Output)

[Demet Bayraktar]

it worked perfectly when i use different image. thank you for your help