To create it, I've written a spectral rendering system, modeled the atmosphere, simulated light transport in the atmosphere, and more. To make the output as realistic as possible even when the sun is below the horizon, I simulate multiple scattering, I include preferential absorption by ozone (which gives the twilight sky its blue color), and I use unbiased distance sampling. To make the renderer as efficient as possible, I sample the sun directly.
For lots of details, check out my project blog at skyrenderer.blogspot.com.
|A fisheye shot of twilight, rendered in my new sky renderer. The dark blue area at the bottom is the shadow of the Earth which is cast into the atmosphere when the sun is below the horizon.|