Absorption, Scattering, And Volume Mediums


There are four types of medium nodes: AbsorptionDefines how fast light is absorbed while passing through a medium., ScatteringDefines how fast light gets scattered when traveling through the medium., Random Walk, and Volume. These settings are stored in Medium nodes, which are attached to the corresponding input pins of DiffuseAmount of diffusion, or the reflection of light photons at different angles from an uneven or granular surface. Used for dull, non-reflecting materials or mesh emitters. or SpecularAmount of specular reflection, or the mirror-like reflection of light photons at the same angle. Used for transparent materials such as glass and water. materials.

Scattering has parameters for absorption, light scattering through the medium, and emission inside the medium. Absorption is a simple version with only absorption. The Random Walk node is a newer variant of subsurface scattering that utilizes a stochastic or random process for the scattering of light through an object. Volumes are described in more detail in the Effects Overview section.

Rendering a medium requires the Path Tracing or PMC kernel, with a large Maxdepth setting. For Diffuse materials, you can use the Direct Light kernel with Diffuse GI mode as well.

You can find MediumsThe behavior of light inside a surface volume described by scatter, absorption, and transmission characteristics. in the Medium category when right-clicking in an Octane VOP network (Figure 1). Mediums can be connected to either a Diffuse, Universal. or Specular materialUsed for transparent materials such as glass and water.'s Medium input.


Figure 1: Accessing the Medium nodes in an Octane VOP network


You should add participating media to Meshes that define a closed volume, rather than planes. Using planes to model leaves of a plant with SSS will for instance not work. Using a single plane as ground plane should be OK (it will be treated as an infinitely deep material).


How To Make A Medium Node Work

A Specular material works best. Set TransmissionA surface characteristic that determines if light may pass through a surface volume. to a value other than 0. However, for the best result, set both Reflection and Transmission to a non-zero value (Figure 2).


Figure 2: Connecting a Scattering node to the Medium input of a Specular material


On Diffuse materials, subsurface scattering only works after you activate Transmission using a value, color, or Texture map.

Set the Reflection value to a low value because the part of the spectrum that is not reflected can enter the object for scattering. If you set the Reflection to 1.0, all light gets reflected regardless of the Transmission value. If you set Transmission to 0.0, all light gets transmitted, but this gives an unnatural appearance. Values of 0.1 - 0.2 are a good starting point.

Also, if the reflection is colored, the transmitted light has the complementary color For example, if the reflection is set to yellow, the transmitted light is bluish.