I am working on a planet earth model, and I found this cool map of the
cities at night (Thanks Jon Berndt! for the reference). I am using it as a
mostly transparent shell 0.000001 bigger than my earth image map sphere.

Now, the question. I of course want to only see the city lights on the unlit
side of earth. The easy unelegant way is to cut the sphere in half and make
sure that the city lights are applied only opposite the sun, but that
requires a knowledge of where the sun is, and more calculations than I care
to do.

Then I came up with a briliant (I thought) Idea: Set the ambient of the
shell to 0.5, then the diffuse to -0.5. My thinking is that a negative
diffuse setting means that the more light you hit it with, the darker it
gets. So a full light hitting normal to this shell should result in a total
effect of 0.5 Ambient+(-0.5 Diffuse)=0 light.

This kinda works, but not as well as I had hoped. It seems as though diffuse
is constrained to be >0. Does anyone know if this is true? Does anyone have
any better ideas?

Chris Jeppesen

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