G'MIC exercises

Software G'MIC
616

To work on a neighbourhood of pixels (N), I would do

foo_k : check ${1=1}>=1
  f "begin(const boundary=1; const S=$1*2+1);
    ref(crop(x-$1,y-$1,0,c,S,S,1,1),N)"

How about a non-rectangular neighbourhood; e.g. an x shape?

xmarksthespot