Department of Information and Computing Sciences, Utrecht University
The sky plays a prominent role in everyday life, and much research has been conducted to simulate the sky and all the phenomena in it. This report describes an experimentation project carried out under supervision of dr. Robby T. Tan. Two analytic skylight models are investigated, implemented, and analyzed. One of these is the well-known Preetham model, devised in 1999 and based on the Perez luminance model to perform fast sky simulations under clear daylight. The other is the Hosek-Wilkie model, which was presented earlier this year (2012) as a significant improvement of the Preetham model. Taking into account some additional phenomena that determine the color of the sky, this method still manages real-time performance. This report presents the theory behind these models, how to implement them, and an evaluation. The conclusion of this evaluation is that the Hosek-Wilkie model is indeed an improvement upon the Preetham model, although there are some drawbacks as well. By design - a more extensive formula is used for simulation - the computational performance is 30% lower than the Preetham model. Furthermore, the Hosek-Wilkie method generates an increase in brightness at lower solar elevations, which goes against the natural course of things - nature dictates a decreasing brightness as the sun gets lower. Finally, under certain conditions, the Hosek-Wilkie model gives wrong results, which is attributed to the fact that it is an analytic approach. In previous research, the same problem was encountered in the Preetham model as well.
Analytical sky simulation [PDF]