ABSTRACT
Participating media (e.g., clouds, fog, smoke, or dusty air) are media that participate in the light interaction.e light does not pass through the medium without being ašected.e media may absorb and scatter the light rays due to režections and refractions at microscopic particles (e.g., water drops). Computing the true
scattering ešects, with multiple režections or refractions as the light passes within the medium, is still too time consuming for real-time applications. However, limiting the scattering to a single režection simplies computations, which allows for real-time applications while achieving visually pleasing results (see Figure 9.1). In single light scattering, the dominant visual ešect is assumed to be when
light travels from the light source into the medium and then undergoes one inscattering along the view ray (see Figure 9.2). is assumption is true when the participating medium is optically thin (i.e., the transmittance is close to 100%); the light transmits through themedium fairly undisturbed, which is true for highly transparent materials such as air. en, the primary visual factor comes from the single in-scattering along the view ray, making it more reasonable to ignore the subordinate and complex multiple scattering ešects. Clouds and smoke have a very high albedo,1 making single scattering a crude approximation. e albedo for cumulus and stratus clouds are 0.7 to 0.9 [Nishita96]. Nevertheless, for air containing a relatively low amount of small particles such as dust or thin fog, single scattering produces plausible and eye-pleasing results. In particular, the singlescattering model convincingly captures the ešects of halos around light sources. Light that is scattered towards the viewer,making the participatingmediumvisible, is oŸen referred to as airlight. It is also common to consider the light attenuation due to absorption and out-
scattering during the light’s traversal through the medium, both along the ray path from the light source to the point of in-scattering and along the remaining way to the eye.ese extra calculations are both simple and oŸen have a dramatic impact on the visual result.
