PBR Chapter11 Volume Scattering

Introduction

we have assumed that scenes are made up of collections of surfaces in a vacuum, which means that radiance is constant along rays between surfaces
introduces the mathematics to describe how light is affected as it passes through participating media—large numbers of very small particles distributed throughout a region of 3D space
volume scattering models are based on the assumption that there are so many particles that scattering is best modeled as a probabilistic process, rather than directly accounting for individual interactions with particles
tracking paths with hundreds or thousands of scattering interactions quickly becomes impractical, it is preferable to aggregate the overall effect of the underlying scattering process in a function that relates scattering between points where light enters and leaves the medium
contents
- basic physical processes
- medium
- BSSRDF

Absorption: the reduction in radiance due to the conversion of light to another form of energy, such as heat
Emission: radiance that is added to the environment from luminous particles
Scattering: radiance heading in one direction that is scattered to other directions due to collisions with particles
Out-Scattering and Attenuation
- albedo
- mean free path theta 单位是 1/m
- transmittance
- Beer's Law
In scattering

parameterized models (which can be used to fit a function with a small number of parameters to measured data)
analytic models that are based on deriving the scattered radiance distribution that results from particles with known shape and material
Henyey–Greenstein phase function
- meaning of g
- a weighted sum of phase functions
data structure

A key operation that Medium implementations must perform is to compute the beam transmittance
the boundary between two different types of scattering media is always represented by the surface of a GeometricPrimitive
sometimes we only need the shape for the boundary surface it provides to delimit a participating medium boundary and we don’t want to see the surface itself
- Material * that is nullptr , indicating that they do not affect light passing through them
- bsdf is null (not affect light transport) or not null
GridDensityMedium

there is thus an implicit assumption that the index of refraction is constant throughout the medium
separable BSSRDFs
- For high-albedo media, the scattered radiance distribution is generally fairly isotropic and the Fresnel transmittance is the most important factor for defining the final directional distribution
- Directional variation can be meaningful for low-albedo media; in that case, this approximation is less accurate
- Lo
  - diffusion profile
  - Second approximation, which assumes that the surface is not only locally planar but that it is the distance between the points rather than their actual locations that affects the value of the BSSRDF
  - tabulated BSSRDF
  - S_r
    - extinction coefficient = 1
      - fix the index of refraction and scattering anisotropy parameter