Generate a multi-layered participating atmosphere for Radiance

With GCC installed on a Linux computer, this should build and run with:

git clone https://github.com/markstock/genmistsky.git
cd genmistsky
make
make test

The last command will generate the Earth and some mist volumes and render a view of the sunrise line from space in an rvu window. It assumes that Radiance is installed and the binaries are in your PATH.

Rayleigh scattering (from gas molecules) is the primary source of the sky's color; Mie scattering (from larger particles such as dust and water droplets) serves to spread the sun's rays into a halo, and refraction (due to density changes) causes very low-angle sun rays to curve. The combination of these three effects can be simulated in Radiance with mist and interface materials applied to Earth-sized sphere and bubble geometry.

This program assumes that the scene is centered around (0,0,0) with East along positive x, North toward positive y, up along positive z, and all spatial units in meters. The center of the Earth (and of all atmospheric shells) is (0,0,-6378100).

It it important to note that this code does not specify the sun's direction, nor does it provide the bubble that is necessary if the view point is inside of one of the mist volumes. The user is responsible for those.

• Consider adjusting the top-of-atmosphere color such that a grey object at the Earth's surface is really achromatic.
• Generate rendering samples: of a standard scene at Earth's surface but with varying Mie coefficients, the same but with different numbers of layers, the same but at different altitudes.
• Add option to load image of Earth lights at night, correlate them to viewer's location, and apply them as area light sources on the ground in order to allow distant cities to add horizon-glow realistically.

The original idea for modeling skies in Radiance came from Taoning Wang's post on the Radiance Discourse server. The standard atmosphere code is from R.J. Yager. Radiance is a lighting simulation package created by Greg Ward.

I don't get paid for writing or maintaining this, so if you find this tool useful or mention it in your writing, please please cite it by using the following BibTeX entry.

@Misc{Genmistsky2022,
  author =       {Mark J.~Stock},
  title =        {Genmistsky: Generate a multi-layered participating atmosphere for Radiance},
  howpublished = {\url{https://github.com/markstock/genmistsky}},
  year =         {2022}
}