The goal of creating a pure low level boundary detector that should label regions in the same way as a human would is ambitious. Humans probably make use of some top-down knowledge when marking boundaries on pictures, so I am not sure if a gradient based approach like this one will ever achieve human-like results. It would be interesting to see a comparison of their algorithm against human-labeled images of e.g. totally abstract art (which the human has never been exposed to and which does not carry any recognizable meaning in it). I believe that such a comparison will have a significantly smaller gap between human and machine since the human can't take as much advantage of his inherent knowledge of what constitutes an object.