An edge finder with an unusual character, closer to an etching than an outline.
It measures how quickly the picture is changing in two directions: to the right, and downward. On a flat area both are zero. On an edge one or both are large.
What it does with the two measurements is the interesting part. A conventional edge detector would combine them into a single strength, treating a horizontal edge and a vertical one as equally strong. This one measures how much the two *disagree with each other*, which responds strongly to corners, texture and diagonal detail while treating clean straight edges more quietly. The result is less like an outline and more like a rubbing.
The answer multiplies the original colour rather than replacing it, so the edges come through in whatever colour they were found in. amount sets how far apart the compared pixels are, so it moves between fine texture and coarse structure.