3x3 Average filter in matlab

Question

I've written code to smooth an image using a 3x3 averaging filter, however the output is strange, it is almost all black. Here's my code. ``` function [filtered_img] = average_filter(noisy_img) [m,n] = size(noisy_img); filtered_img = zeros(m,n); for i = 1:m-2 for j = 1:n-2 sum = 0; for k = i:i+2 for l = j:j+2 sum = sum+noisy_img(k,l); end end filtered_img(i+1,j+1) = sum/9.0; end end end ``` I call the function as follows: ``` img=imread('img.bmp'); filtered = average_filter(img); imshow(uint8(filtered)); ``` I can't see anything wrong in the code logic so far, I'd appreciate it if someone can spot the problem.

Accepted Answer

Assuming you're working with grayscal images, you should replace the inner two for loops with :

```
filtered_img(i+1,j+1) = mean2(noisy_img(i:i+2,j:j+2));
```

Does it change anything?

EDIT: don't forget to reconvert it to uint8!!

```
filtered_img = uint8(filtered_img);
```

Edit 2: the reason why it's not working in your code is because `sum` is saturating at 255, the upper limit of uint8. `mean` seems to prevent that from happening

3x3 Average filter in matlab

Solution

Problem