Octane for Houdini

The Null material node is used for mesh objects that have an invisible surface but contain a medium. This is equivalent to setting up a specular material with IOR 1 and a reflection of 0 and a transmission of 1. However, that methodology no longer makes sense with nested dielectric. The Null material was created to make a medium with no surface work in all cases. 

Figure 1: The Null material with an Octane Scattering Medium connected to it's Medium input 

Null Material Parameters

Medium Pin - There are three primary types of mediums used with materials: Absorption, Random Walk, and Scattering. Details can be found in the Mediums and Volumes section. 

Opacity - Controls the transparency of the material via a greyscale texture.

Affect Alpha - If enabled, refractions will affect the alpha channel.

Displacement Pin - Allows adjustment for the height of points on a surface based on an image value. Displacement differs from Bump or Normal mapping by providing true displacement of an objects surface. Displacement mapping is covered in more detail under the Displacement Overview section.

Smooth - If disabled, normal interpolation will be disabled and triangle meshes will appear facetted. 

Smooth Shadow Terminator - If enabled, self-intersecting shadows are smoothed according to the polygon's curvature.

Round Edges Pin - This creates a shader effect at render time that rounds the sharp edges of objects without modifying and reloading the geometry. Higher values will round the edges more. This is useful to bevel hard edges during render time, like when using low-polygon models. See the Round Edges section for more information.  

Priority - Used to resolve the ambiguity in overlapping surfaces, the surface priority control allows artists to control the order of preference for surfaces. A higher number suggests a higher priority for the surface material, which means it is preferred over a lower priority surface material if a ray enters a higher priority surface and then intersects a lower priority surface while inside the higher priority surface medium.