Implementing IMFILTER in matlab

matlab

Solution

The size of `r` should be the same as the original I think. And I don't understand why you convert to single precision using `single`. Anyway, I think you want to do the following:

%# Let's first create a small test image from the built-in peppers image
original = im2double(imread('peppers.png'));
original = original(1:5,1:8,1);

filter = 1/9 * [-1 -1 -1 ; -1 17 -1 ; -1 -1 -1];
s = size(original);
r = zeros(s);

for i = 2:s(1)-1
    for j = 2:s(2)-1
        temp = original(i-1:i+1,j-1:j+1) .* filter;
        r(i,j) = sum(temp(:));
    end
end

The result is as follows:

r =

         0         0         0         0         0         0         0         0
         0    0.2336    0.2157    0.2514    0.2436    0.2257    0.2344         0
         0    0.2453    0.2444    0.2671    0.2693    0.2418    0.2240         0
         0    0.2741    0.2728    0.2397    0.2505    0.2375    0.2436         0
         0         0         0         0         0         0         0         0

And with `imfilter`, it is:

r2 = imfilter(original, filter)

r2 =

    0.3778    0.3325    0.3307    0.3442    0.3516    0.3312    0.3163    0.3856
    0.3298    0.2336    0.2157    0.2514    0.2436    0.2257    0.2344    0.3386
    0.3434    0.2453    0.2444    0.2671    0.2693    0.2418    0.2240    0.3512
    0.3272    0.2741    0.2728    0.2397    0.2505    0.2375    0.2436    0.3643
    0.3830    0.3181    0.3329    0.3403    0.3508    0.3272    0.3412    0.4035

As you see, the results are the same except the ones on the borders. There are a few strategies to compute the ones on the borders as mirroring the image to the out of the borders, keeping them the same, etc. Please read the documentation of `imfilter` and choose one strategy.

Note that I didn't flipped `filter` here since the filter is symmetric in both directions. And I recommend you to avoid loops! There are nested loops of depth four in your code!

Lastly, you can use 2-D convolution to do the same as `imfilter`:

r3 = conv2(original, filter, 'same');

r3 =

    0.3778    0.3325    0.3307    0.3442    0.3516    0.3312    0.3163    0.3856
    0.3298    0.2336    0.2157    0.2514    0.2436    0.2257    0.2344    0.3386
    0.3434    0.2453    0.2444    0.2671    0.2693    0.2418    0.2240    0.3512
    0.3272    0.2741    0.2728    0.2397    0.2505    0.2375    0.2436    0.3643
    0.3830    0.3181    0.3329    0.3403    0.3508    0.3272    0.3412    0.4035

Problem

I am trying to filter an image with out using `imfilter`. I should get the same results as `imfilter` but I keep getting diffrent results. Can someone tell me where I went wrong? ``` orignal=imread('obj6__17.png'); filter=1/9*[-1 -1 -1 ; -1 17 -1 ; -1 -1 -1]; s=size(orignal); r=zeros(s(1)); temp = zeros(3); for i= 2: s(1)-1 for j = 2: s(2)-1 for n= 1: 3 for m= 1:3 temp(n,m)=orignal(i+2-n,j+2-m)*filter(n,m); end end r(i,j)=sum(single(sum(temp))); end end ```

Original source