max planck institut informatik
Physically based Simulation of Twilight Phenomena
Jörg Haber, Marcus Magnor, Hans-Peter Seidel
To appear in: ACM Trans. on Graphics, 2005.
[pdf]
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.
Importance of multiple scattering for twilight phenomena. Left to right: single scattering (i.e.\ direct illumination only), multiple scattering with five scattering events, and difference image (gamma-corrected with gamma=3.0, exaggerating the otherwise invisible concentric atmosphere model structure). For all simulations, a polluted continental climate and a local time of 6:30\,pm (DST, sun elevation +10 deg. above the horizon) has been selected.
Effect of varying humidity on the colors of the sky. Left to right: 50 %, 80 %, and 95 % humidity. All simulations have been performed for the same location, time (6:35,pm DST, sun elevation +9 deg.), and type of climate (continental, average pollution).
Different types of climate. Left to right: continental climate with average pollution, urban climate with heavy pollution, and tropical maritime climate with no pollution. Location, time (6:05,pm DST, sun elevation +15 deg.), and humidity (80 %) is identical for all simulations.
Sunset sequence for a continental climate with average pollution and 80 % humidity. Left to right: local time 5:30,pm DST (sun elevation +22 deg.), 6:15,pm DST (sun elevation +13 deg.), and 6:40,pm DST (sun elevation +8 deg.)
Left to right: Photograph of a late-autumn sunset after a cold front had passed through. For the same time, place and view as the photo, sunset appearance has been calculated for average continental conditions (middle) and clean continental air (right). For ease of comparison, the photograph's foreground has been added in the simulated views.
Demo Movie. (50 MB)