design my own 2d filter--matlab

Question

I'm wondering is there a way to define my own 2d filter. Assuming there is a matrix A, I'd like to assign every element of A to the maximum value of its 8 neighbors. For example:

A = [1 2 3 4 5
     6 7 8 9 10
     -1 3 4 8 13
     4 2 0 9 1]

After filtering:

A = [1 2 3 4 5
     6 8 9 10 10
     -1 8 9 13 13
     4 2 0 9 1]

Thanks

Answer 1

You may use imdilate to get you the maximum of a 9x9 neighbhorhood -

%%// Dilate to get the max of a 9x9 neighborhood (including the element itself)
A1 = imdilate(A, true(3));

%%// Since you are looking to keep the boundary elements
m1 = ones(size(A));
m1(2:end-1,2:end-1)=0;
A1 = m1.*A+~m1.*A1

If you are looking to get the maximum of a 8x8 neighbhorhood, i.e. exclude the element itself, use a modified kernel for use into imdilate -

%%// Dilate to get the max of a 8x8 neighborhood (excluding the element itself)
h = true(3);
h(2,2)=false;
A1 = imdilate(A,h);

%%// Since you are looking to keep the boundary elements
m1 = ones(size(A));
m1(2:end-1,2:end-1)=0;
A1 = m1.*A+~m1.*A1

Answer 2

You can create an arbitrary filter with nlfilter, an example here. In your case:

fun = @(x) max(x(:)),
B = nlfilter(YourImage,[3 3],fun);

In the case that you want to exclude the center pixel, replace fun by something like:

fun=@(x) max([x(1:3, 1)' x(1,2) x(3,2) x(1:3,3)'])

Answer 3

You can try the following syntax and use ordfilt2 as a maximum filter:

B = ordfilt2(A, 9, ones(3,3))

Here is a doc page for ordfilt2.