So I've got this planet.

http://twitpic.com/xiy1y (the planet from space)
http://twitpic.com/xixp0 (a low level view)

Actually it's the moon. The land is an isosurface using topographic data
from the Kaguya space probe (a 280MB .png file!). The sea is a simple
sphere. The atmosphere is a media with a modified version of Bruno
Cabasson's TerraPOV Rayleigh scattering code. The clouds are another
media with two density functions, one for the puffy clouds and another
to change the distribution.

It looks pretty good, even though I say so myself. However...

- I want to play with the atmospheric density function (to allow for the
moon's very low gravity gradient). This means I need to adjust Bruno
Cabasson's original code so that it doesn't depend on the height of the
atmosphere. Unfortunately there's something about how to scale media
that I'm just not getting --- the bigger I make the object containing
the atmosphere, the less dense the atmosphere gets, regardless how I
modify the density function. (Code below.)

- I want the sea to have waves, but I can't get a waves normal effect to
work. Dents work fine:

        normal {
            dents 0.1
            scale 0.001 * km
        }

Replacing dents with waves appears to do nothing. At the scale I'm
using, km == 1. I shouldn't be hitting any numerical domain limits,
surely? I can make planet-sized waves, but no matter what I do, when I
scale it down the magnitude appears to diminish to zero. And what is the
mysterious undocumented parameter to the dents and waves keyword?

- Right now the land texture depends solely on the altitude. I'd rather
like to use the slope pattern so that I can also make it depend on the
gradient, to avoid embarrassments like fields going up the side of
cliffs. Unfortunately, slope appears to work soley in cartesian space...
and my planet is spherical. Is there any way to persuade slope to work
in polar space instead?

- There's an occasional rendering artifact in the clouds --- which,
annoyingly, you can't quite see in the low-level scene. Thin
longitudinal lines trace the shape of the cloud. I assume this is a
sampling issue, but I don't understand how the adjustments work. Does
anyone have any advice?

Incidentally, the clouds take an absolute age to render. The
high-altitude picture took 20 minutes, with antialiasing on. An 800x600
version takes about six hours. Given that my scene contains precisely
four objects, not including the sun, I'd like to put in a bid for the
Least Efficient Scene prize...

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