I think it was Tuhopuu that had it as -1 to 1 when it was first put in there. When it was migrated over to bf-blender it was eventually changed to 0 - 2.
In the developer mailing list, they say about Tuhopuu, “Jorge implementation mix the 2 in one implementation, with a darkness parameter where darkness = 0 gives Lambert.” (i.e. 0 in Tuhopuu is the “middle”)
The shader this was modeled after had a parameter called “k” that was varied from 0 to 2. The developer mailing list says about this shader, “For k = 1, gives same result as standard Lambert.”
Therefore, bf-blender’s Blender 2.4’s default value of “1” would have the model appear just like a stock Lambert shader, 0 lighten’s edges, and 2 darkens edges. Blender 2.4’s dark 1 == Tuhopuu’s dark 0.
The April 10th IRC meeting decided that:
“* will be switched to a single slider with range 0-2, 0-1 is standard
behavior and 1-2 is the variation.”
The developer discussion can be read here:
P.S. Funny enough, j.merritt commented that having a “middle” point of 1 might cause confusion They need to hint at the behaviour in the dark slider’s tooltip!