Figure 1: Scattering parameters
The ScatteringDefines how fast light gets scattered when traveling through the medium. map is a Medium with single-scattering SSS as well as AbsorptionDefines how fast light is absorbed while passing through a medium.. The Scattering parameter defines how fast light scatters when traveling through the Medium, similar to how Absorption is defined. A very high value means light scatters very fast, and a value of 0.0 means no scattering when the Scattering parameter is set to a value. The Scattering parameter also accepts a Texture map or a color.
You should add participating media to Meshes that define a closed Volume rather than planes. Using planes to model plant leaves with SSS will not work. Using a single plane as ground plane should be okay (it's treated as an endless, deep MaterialThe representation of the surface or volume properties of an object.). The Mesh can have opaque Objects nested inside, but nested participating media are not supported.
Figure 1: Scattering parameters
Density - This parameter multiplies against Scattering.
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.
Absorption - Controlled by Absorption color, which defines how fast a medium absorbs light passing through it. A 0.0 or black value means no absorption. Higher values result in faster light absorption. The specified color in the Absorption parameter produces its complimentary color in the rendering (Figure 1). The Absorption texture is multiplied by the Density parameter. This allows setting a wide range of values.
Scattering - Determines how fast light scatters as it moves through a surface. High values mean that light scatters sooner as it enters a surface, and low values mean that light passes deeper into the surface before scattering. A 0 value disables Scattering.
Phase - Controls light direction as it scatters through the surface. A value of 0 results in light scattering equally in all directions; a positive value results in forward scattering, where photons continue the same approximate direction as when they enter the surface; and a negative value results in backwards scattering, where light moves through the surface in the same direction, but opposite to the angle that they entered the surface. This is known as backscattering.
Emission - Attaches an Emission node to the Emission input pin. When you connect an Emission node to a Medium node, it defines emission inside the volume instead of on the object's surface. In this case, Power controls how fast a ray's radiance increases while traveling through the volume; it doesn't represent total power. It's not multiplied with the Scale parameter. This effect works best with large, not-too-bright objects - small, bright objects create lots of noise.