MATLAB: Function output wrong size

blur image

I have to write a function that blurs an image file.
The idea is to take an input 'img', then using the value of each individual pixel, take the mean of a submatrix with that pixel as the centre of 2*w+1 sub matrix and use that value to replace the pixels in the same position in a generated new 'blurred' image. I hope that makes sense.
This is what I've written so far which appears to work but is apparently outputting a 1×9 vector with the wrong result and I can't figure out why or how to fix it. Any help appreciated. 
function output = blur(img,w)
[row, col] = size(img);
img = double(img);
temp_img = zeros(row,col, 'uint8');
for i = 1:row
    for j = 1:col
        r1 = max(1,i-w);
        r2 = min(row,i+w);
        c1 = max(1,j-w);
        c2 = min(col,j+w);
        temp_matrix = img(r1:r2,c1:c2);
        val_2 = mean(temp_matrix(:));
        temp_img(r1:r2,c1:c2) = val_2;
    end
end
output = uint8(temp_img);
end

Best Answer

I don't understand why you would store the result val_2 in multiple positions, instead of only in i,j.
Another thing I don't understand is why you don't use convn (or conv2 or imfilter) to do this. Those will flip the kernel, but since your kernel should be flat (and therefore symetrical), that doesn't matter.
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