Finding the largest difference between any two pixels within a set of 5 contiguous pixels

I have a matrix of 22 x 22 cells each containing a number. For a given cell, I need to find the largest difference between any two cells within a set of five contiguous pixels (ie. that cell and the one above, below, left and right of it). I then need to step forward one cell from the original cell and again analyse the next set of five cells, and repeat this for every column in the matrix.

Answer 1

[Walter Roberson]

Use imdilate() with [0 1 0; 1 1 1; 0 1 0] to get the largest value, and imerode() with the same structuring element to get the smallest value, and subtract.

imdilate() corresponds to maximum over the selected pixels, and imerode() corresponds to minimum over the selected pixels.

[mcl1993]

How do I refer to the initial specific cell to start looking at that subsection of 5 pixels?

[Walter Roberson]

You do not need to. The morphological processing will do all possibilities.

   M = randi(10,22,22);
   pixel_diff = imdilate(M,[0 1 0;1 1 1;0 1 0]) - imerode(M,[0 1 0;1 1 1;0 1 0]);

If you prefer something more explicit, and do not mind that this version does not handle the edges, then

   M = randi(10,22,22);

   R = 2:size(M,1)-1; C = 2:size(M,2)-1;
   PT = M(R-1, C);   %row above, same column
   PL = M(R,C-1);    %same row, previous column
   PC = M(R, C);     %here
   PR = M(R, C+1);   %same row, next column
   PB = M(R+1, C);   %next row, same column
   stack = cat(3, PT, PL, PC, PR, PB);
   Mmax = max(stack, [], 3);
   Mmin = min(stack, [], 3);
   pixel_diff2 = Mmax - Mmin;

   isequal( pixel_diff(2:end-1, 2:end-1), pixel_diff2)

[mcl1993]

I have tried to run this code, and get the error 'undefined function or variable 'pixel_diff'.

[mcl1993]

Thanks! The first part worked perfectly

[Walter Roberson]

The last line there, the reference to pixel_diff is a reference to the computation of the short version with imerode and imdilate, and the isequal() is just there to prove that the computations come out the same, to give evidence that the imerode / imdilate version can be trusted.