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Abstract:
We present a physically based approach to compute the colors of the sky during
the twilight period before sunrise and after sunset. The simulation is based
on the theory of light scattering by small particles. A realistic atmosphere
model is assumed, consisting of air molecules, aerosols, and water. Air
density, aerosols, and relative humidity vary with altitude. In addition, the
aerosol component varies in composition and particle size distribution. This
allows us to realistically simulate twilight phenomena for a wide range of
different climate conditions. Besides considering multiple Rayleigh and Mie
scattering, we take into account wavelength-dependent refraction of direct
sunlight as well as the shadow of the Earth. Incorporating several
optimizations into the radiative transfer simulation, a photo-realistic
hemispherical twilight sky is computed in less than two hours on a conventional
PC. The resulting radiometric data is useful, for instance, for high-dynamic
range environment mapping, outdoor global illumination calculations, mesopic
vision research and optical aerosol load probing.
