one fing leads . . .

Edge Detect by Hue and Brightness
Select a gap between brightnesses, and a gap between hues in the 360° colour circle. For each pixel, the band that its brightness falls in is established, as is the band of the pixel to the right of it and that immediately below it on the image, the higher of the differences between these two, i.e. the one to the right and the one below, and the brightness of the pixel being looked at, provides a value between 0 and 255, this value is used to determine the opacity of the output pixel, the higher the value the more opaque, with its colour being the selected line colour.
Or, to put it another way, please experiment!
For comments on edge detection in general, see my page Edge Detection Scripts.
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Line colour: pixel hue selected colour
Line colour
hue difference
auto = by colour name
and or
brightness difference
opacity brightness breaks every
anti-alias threshold
Fill lines every line breaks
Brightness calculation formula:

more experiments for the sake of my dreams