Participating Media
Digital Image Synthesis Yung-Yu Chuang
12/27/2005
with slides by Pat Hanrahan and Torsten Moller
Participating media
• We have by far assumed that the scene is in a vacuum. Hence, radiance is constant along the ray. However, some real-world situations such as fog and smoke attenuate and scatter light.
• Natural phenomena
– Fog, smoke, fire – Atmosphere haze – Beam of light through
clouds
– Subsurface scattering
• VolumeRegion, similar role of BSDF
Volume scattering processes
• Absorption
• Emission
• Scattering
Absorption
The reduction of energy due to conversion of light to another form of energy (e.g. heat)
Transmittance Absorption
Emission Emission
Out-scattering
Light heading in one direction is scattered to other directions due to collisions with particles
Extinction
Extinction In-scattering
Increased radiance due to scattering from other directions
Source term Phase function
Henyey-Greenstein phase function Importance sampling
Schlick approximation Homogenous volume
HomogenousVolume
Varying-density volumes
• Density is varying in the medium and the volume scattering properties at a point is the product of the density at that point and some baseline value.
• 3D grid, VolumeGrid
• Exponential density, ExponentialDensity
3D grid
• Standard form of given data
• Tri-linear interpolation of data to give continuous volume
• Often used in volume rendering
Exponential density Light transport
Infinite length, no surface With surface
Emission only Emission only
Emission only
exponential density
