Matlab: Matrix Neighbour Extraction

Question

I have a large number of images which I\'ve broken down into segments such that their matrices look like:

img = [ 1 1 1 1 1 2 2 2 3 3 3 3  
        1 1 1 1 2         


        

Answer 1

A simple (but probably not maximally efficient) solution is to dilate each region mask to pick neighbors:

labels = unique(img);
nLabels = length(labels);
neighbors = cell(nLabels,1);

for iLabel = 1:nLabels
   msk = img == labels(iLabel);
   adjacentPixelMask = imdilate(msk,true(3)) & ~msk;
   neighbors{iLabel} = unique(img(adjacentPixelMask));
end

neighbors{1}
ans =
     2
     4
     5