Gradient Norm
My attempt to make the lines more balanced in brightness and thickness.
Remarks
I would like the lines to be more even and defined, sort of what you get from colouring book outlines; actually that might be a good filter to work on (or maybe something like that already exists…).
At the moment, there are inherent weaknesses to the gradient norm method. The two I mentioned before: uneven brightness and thickness. And a few others: thin objects such as the whiskers “double”, and tips and corners either disappear (darkened) or become less defined (rounded).
Also, if you toggle between the original sample tiger and my results, you would see that the outline is not smack dab in the middle of the edge. It ends up either on the bright side or the dark.