Analytic Rendering of Multiple Scattering in Participating Media

We consider the addition of physically-based weather effects like haze, fog and mist to images. Most computer graphics images are rendered under clear day or night conditions and little attention has been devoted to efficiently adding realistic weather effects. Multiple light scattering is dominant in a variety of atmospheric conditions (in general, most participating media) and is hard to model accurately using simple approximations such as single scattering and diffusion. In the past, accurate multiple scattering simulation has therefore required very expensive volumetric Monte Carlo methods. This paper focuses on multiple scattering from light sources immersed in participating media such as bad weather and fluids. We derive a new analytic formula for multiple scattering from a point light source in a medium. We extensively validate our model using monte carlo simulations as well as using controlled experiments with a light source immersed in milk. The model accurately predicts complex scattering effects produced by a wide range of concentrations of the medium. For rendering area sources of complex shapes and radiance distributions, we show that the analytic formula reduces to a simple depth-dependent convolution of the image. This leads to an efficient implementation of adding physically-based glows around sources. We demonstrate fast and accurate addition of weather effects to real photographs. Although the paper focuses on atmospheric effects, the results can be used to render sources through virtually any participating medium such as smoke, tissue and blood.