How does Mean Filter work?

I created a small code to see how does Mean Filter work.

How does it get values?? What is the equation?

A = [ 1 2 3; 4 5 6; 7 8 9]
h = ones(3,3)
imfilter(A,h)

    12    21    16
    27    45    33
    24    39    28

I get this as output, I don't understand how exactly 12,21,16,27 is calculated using imfilter command with 3 by 3 filter.

[Beena Bethel]

Its taking the sum of the neighboring pixels in the example shown here and not the average. I did not understand how whether it takes sum or average of its neighboring pixels....

[Walter Roberson]

This is taking the sum, according to the weighting stored in h. Except for the edge cases you can use

imfilter(A, h/numel(h))

to get the average.

Getting the edge cases right is a little tougher:

imfilter(A, h) ./ imfilter(ones(size(A)), h)

The denominator is effectively counting the number of pixels that are in the neighborhood -- the number of pixels whose value contributed to the sum.

Answer 1

[Walter Roberson]

>> conv2(A,fliplr(flipud(h)))

ans =

     1     3     6     5     3
     5    12    21    16     9
    12    27    45    33    18
    11    24    39    28    15
     7    15    24    17     9

>> conv2(A,fliplr(flipud(h)), 'same')

ans =

      12    21    16
      27    45    33
      24    39    28

>> imfilter(A,h)

ans =

      12    21    16
      27    45    33
      24    39    28

http://www.mathworks.com/help/matlab/ref/conv2.html