MATLAB: Can I use diff function to subtract pixel from 2-pixel away from it? So it not neighbour pixel.

digital image processingimage analysis

Can I use diff function to subtract pixel from 2-pixel away from it? So it not neighbour pixel.

No.

I suggest you use

diff2_col = YourArray(:,3:end) - YourArray(:,1:end-2)

diff2_row = YourArray(3:end,:) - YourArray(1:end-2,:)

depending on which dimension you want to proceed across.

blkproc or blockproc. Once you are within an individual 3x3 block, say B, then B(2,2) = max(B(2,2),255 * any(B(:)==255))

Why did you group in cells in thr first place?

Referencing the content of a cell should be done with curly brackets:

r{1,1}