How to use 'center pixel' in a sliding block to represent this block using 'nlfilter'?

Hi,

I am dealing with all the coloured images rather than greyscale images. So a lot of the filtering algorithms appear slightly more complicated to me on coloured images than greyscale images.

I would like to use the center pixel in a sliding block (say, 9 by 9 block) to represent this block. How to achieve it using 'nlfilter'??????

Also, in terms of f = @(x), how should I write it?????? Noticing that the center pixel is determined by

floor(([m n]+1)/2)