Sliding Neighbourhood Operation (Image Filtering)

Hi,

I have a sample image and I would like to apply a 4 by 5 'sliding block' to this image.

In each 4 x 5 block, I would like to use the center pixel of that block to represent this block.

Here is what I tried:

fn = '002.png';
I = imread(fn);

m = 4; 
n = 5;

% the center pixel is floor(([m n]+1)/2)

f = @(x) x(floor(([m n]+1)/2),:);
I2 = nlfilter(I,[4 5],f);

figure
imshow(I2)

What I get is

Unable to perform assignment because the size of the left side is 1-by-1 and the size of the
right side is 2-by-5.

Error in nlfilter (line 76)
        b(i,j) = feval(fun,x,params{:});

I'm guessing the bug is in the 'f', but I don't know how to modify it.