Atmospheric light scattering

A surprisingly large fraction of whatever we get to see from an airplane is light scattered somewhere in the atmosphere. This includes the obvious phenomena like the blue color of the sky and the golden-red sunrise and sunset light, but also any form of haze and fog, for instance the effect that faraway objects loose their colors and fade into blue-white. In a typical situation, around 70% of the color values of the scene outside the cockpit are not determined by the color of the scenery textures but by sunlight and haze colors. Having a detailed model of atmospheric light scattering is therefore important for a realistic visual experience in a flight simulation.

However, atmospheric light scattering physics cannot actually be solved in real time. Imagine looking into the sky. The light you see could have been scattered into that ray at any distance along the ray, but part of the light which has been scattered in has already been scattered out again if the in-scattering point is too far away. Even for a single ray, the problem thus requires two nested integrals to determine the observed light as the correct balance between averaged in-scattering vs. out-scattering given the density of scattering centers in the atmosphere along the ray. Allowing for multiple scattering leads to even more nested integrals. Any integral however is numerically tough to solve, and much more difficult to solve in real time.

The aim of this project is to create a set of shaders which approximate the problem in a suitable way by using for instance analytical solutions for the light scattering physics under certain assumptions or parametrized versions of the true solution such that all essential physics determining the visual appearance of the scene is captured. This effort is by its nature closely linked to the weather system which determines how the atmospheric conditions are while the light scattering code determines how this translates into a visual impression.