Mediums - Absorption & Scattering

OctaneRender supports participating media inside objects (absorption and subsurface scattering). These settings are stored in medium nodes, which are attached to the corresponding input pin of Diffuse or Specular material nodes.

There are two types of medium nodes, absorption and scattering. Scattering has parameters for absorption, scattering of light passing through the medium, and emission inside the medium. Absorption is a simple version with only absorption.

Rendering a medium requires the Path Tracing or PMC kernel, with a sufficiently large maxdepth setting. For Diffuse materials, the Direct Light kernel with Diffuse (4) GI mode can be used as well.

Meshes

Participating media should only be added 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, by default it is set up properly. Transmission must be nonzero; for the best effect, reflection and transmission should be set to 1.0.

On Diffuse materials subsurface scattering only works after Transmission is activated using a value, color, or texture map.

The reflection value needs to be set to a sufficiently low value because only the part of the spectrum that is not reflected can enter the object for scattering. If the reflection is set to 1.0, all light gets reflected regardless of the transmission value. If set to 0.0, all light gets transmitted, however, 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 will have the complementary color (e.g. if the reflection is set to yellow, the transmitted light is bluish).

The Absorption Medium & Scattering Medium Parameters

The Absorption Medium

Absorption is controlled with the absorption map, which defines how fast light is absorbed while passing through a medium. A setting of 0.0 means no absorption. The higher the value the faster light is absorbed by the medium.

The absorption texture is multiplied with the Scale parameter. This allows setting a wide range of values more easily.

Scattering Medium

Subsurface scattering is controlled by the scattering parameter (multiplied by the Scale value), and the Phase function. The scattering texture defines how fast light gets scattered when traveling through the medium, in a similar way to how absorption is defined. A very high value means light gets scattered faster and a value of 0.0 means no scattering.