So, to correct my math for backward-facing polygons, I introduce one more node.

This is the Math:Max node. I Max my calculated value with 0.

Suppose the calculation produces .3, Then the maximum of .3 and 0 is .3. No change.

Suppose the calculation produces -.3. Then the maximum of -.3 and 0 is 0. The negative value is ignored, in favor of 0.

This is what the Diffuse node normally does. It takes L dot N and does max with 0.

Now I see all black, because there is no meaningful difference between the two calculations.

---

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)