I'm experimenting with clouds. The attached image demonstrates. (The
view under the clouds is looking north along the western shore of the
Sea of Smyth, on the eastern edge of the moon --- the big crater is
probably Napier. The camera is 200km up.)

The clouds are implemented as a textured sphere 35km above the surface.
The cirrus texture on the sphere is adequate, but I can fix that. The
thing I'm worried about is the dark edge on the horizon.

I know what this is: my sphere is defined as:

	sphere
	{
		<0, 0, 0>, 1
		hollow
		double_illuminate

		texture {
			pigment {
				color_map {
					[0 rgbt <1, 1, 1, 1>]
					[1 rgbt <1, 1, 1, 0>]
				}
			}

			finish {
				diffuse 1
			}
		}

		scale r
	}

Where the color_map is selecting a partially transparent color, then
we're always going to be blocking some of the light coming through,
therefore it's going to be darker.

What's a good way round this? Should I be using reflection for this,
rather than just a simple pigment colour? That would give me access to
the huge zoo of reflection options, including the energy conservation
feature. Is there an easier way?

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