There's a few examples here: http://docs.gimp.org/en/plug-in-convmatrix.html http://docs.gimp.org/en/plug-in-convmatrix.html Try the "edge detection" example w

There's a few examples here: http://docs.gimp.org/en/plug-in-convmatrix.html http://docs.gimp.org/en/plug-in-convmatrix.html Try the "edge detection" example with -3 instead of -4, and it looks very similar to yours.