Random Walk


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. Random Walk takes in an albedo texture for specifying the expected SSS color, as opposed to the traditional absorption/scattering color spectrum in other medium nodes.This provides the most realistic result when rendering scatter volumes.


Figure 1: The Clipping MaterialThe representation of the surface or volume properties of an object. applied to a cube primitive and used to clip away part of the Octane logo shape


Random Walk Parameters

Density - This parameter multiplies against ScatteringDefines how fast light gets scattered when traveling through the medium..

Volume Step Length - Depending on the surface, you may need to adjust this parameter. The default value is 4, but if the volume is smaller than this, you need to decrease the value. Decreasing this value decreases render speed, and increasing the value causes the ray marching algorithm to take longer steps. If Volume Step Length exceeds the volume's dimensions, then the ray marching algorithm takes a single step through the whole volume. To get the most accurate results, keep Volume Step Length as small as possible.

Volume Shadow Ray Step Length - The step length that is used by the shadow ray for marching through volumes.

Use Volume Step Length for Volume Shadow Ray Step Length - If active, uses Volume Step Length as Volume Shadow Ray Step as well.

Single Scatter Amount - Determines how often direct light is calculated in volumes, as a ratio of scatter events.

Sample Position DisplacementThe process of utilizing a 2D texture map to generate 3D surface relief. As opposed to bump and normal mapping, Displacement mapping does not only provide the illusion of depth but it effectively displaces the actual geometric position of points over the textured surface. - Allows a texture to control a volume's sample position displacement.

Albedo - The scattering albedo where the coefficients are determined for this parameter using the mean free path.

Radius - Determines the depth that the light scatters in the medium.

Bias - The bias of the subsurface scattering. Higher values use biased sampling, which usually yields better results with lower depth settings.


Note: When using Random Walk with specular materials, the Fake Shadows option needs to be enabled. This will allow Random Walk to perform direct light sampling, otherwise, that step will be skipped, and indirect sampling will only occur. This has the potential to increase noise, especially if your scene is illuminated with an Octane Sun (Octane Sky Object), as the indirect rays have a very low probability of reaching the sun, most noticeable around the highlight.