The rest is just regular compositing and flood fills etc.
These filters draw a 1 px wide (zooming independently) contour line around the source's luminosity levels -areas with the same tone.
Can be handy if you are tracing a photograph and adding gradients -especially, for node placement on gradient meshes.
svg source
Another variant on the concept, with black and white fills covering the same tone values is available here.
Also, the test image set was made for trying out such filter packs in the future, by remixing some top-notch cliparts at openclipart. Here is the link for that.
