I'm going to build this shader with the assumption that we're dealing with a UV mapped surface, and that we want to produce a 2D function that uses the UV distance from the shaded point to a line segment to drive the whole thing.

How do you find the distance between a given point and a line segment? Rather than explain this myself, a perfectly good explanation is here:

http://local.wasp.uwa.edu.au/~pbourke/geometry/pointline/

Study that for a bit.

---

Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)