https://rmanwiki.pixar.com/display/REN/PxrVolume

• Constant fog (a constant homogeneous volume with single scattering);
• Smoke (a heterogeneous volume where single scattering suffices);
• Fire (a heterogeneous volume with emissive properties);
• Clouds (a heterogeneous volume where multiple scattering is usually required)

PxrVolume Attributes

• Diffuse Color
  Diffuseの色。
• Emit Color
  Volumeを出す色。
• Multiple Scattering
• Velocity
• VelocityMultiplier

Density

• Density Float PrimVar
• Density Float
  Volumeの濃度。
  0.1ぐらい低くする。
• Density Color PrimVar
• Density Color
  エッジの色
• Max Density
  濃度の最大値。
  1.0がデフォルト。

Anisotropy（異方性）

• Primary Anisotropy
• Secondary Anisotropy
• Lobe Blend Factor

Sampling

• Equiangular weight
• Samples
• Step Size

Multi Scatter Optimization

• MultiScatter Optimization
• Extinction Multiplier
• Contribution Multiplier