If img is a logic say where ones define edge pixels and zeros define non-edge pixels then you are correct nnz(img): counts the number of non-zero entries and therefore the number of edge pixels this would be equivalent to say: sum(img(:)) If img is a logic then you can also count the non-edge pixels using the nnz command, e.g. : nnz(~img) Where ~ is NOT. Another method would be to say that the non-edge pixels are simply: numel(img)-nnz(img) I.e. the number of elements (all) minus the edge pixels.
Good luck,
Kevin
