The last few weeks, on my way to work, I noticed the Moon in the morning sky
and it reminded me of a problem I had months ago doing a sci-fi scene. When
you look up at the crescent Moon in daylight, you don't see the unlit part
of the Moon. You just see the lit crescent in a blue sky. This effect is a
common sci-fi art theme, an alien landscape with two moons and a Saturn-like
planet in the sky.

What works great for a night-time or a space scene, an ambient of 0 does not
work for a daylight scene. When I try to duplicate the real world effect in
POV, I get both the lit and unlit portions of the Moon visible against the
sky. If I drop the ambient down to 0, the unlit portion is black. If I match
the ambient to the color of the sky sphere (or some fraction of it like <.2,
.6, .8>*.10), the unlit portion is visible. I have tried things like
"negative" spotlights aimed at the unlit portion. Something like:

light_source {
 <0, 10, 0>
 color rgb <-.8, -.4, -.2>
 spotlight
 radius 15
 falloff 20
 tightness 10
 point_at <0, 0, 0>
}

With simple white test spheres, this works. However, I spent a lot of time
tinkering with the "negative" light color and the ambient statement. And
then I found with any complex texture, it doesn't work. My best success has
been with a constant fog with a distance set far enough away not to
interfere the majority of the scene. The unlit portion of the Moon is
obscured, but not completely invisible. To make the unlit portion invisible
or mostly invisible, the lit portion becomes too "washed out". I haven't
tried CSG, and I have a feeling this may the best solution. Anyway, this
hasn't bothered me enough to make me to break out my old math book and
calculate the correct angle of the CSG difference to the light source(s). It
bugs me just enough to ask this group if anyone out there has any hints or
tricks for a raytracing a moon or planet in daylight?

---
jb

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