The ScatteringDefines how fast light gets scattered when traveling through the medium. medium creates the look of subsurface scattering. This is the phenomena where light rays enter a surface, scatter within the surface's material, and then exits the material. It is the key to creating realistic-looking human skin and other organic surfaces.
Density
Increases the absorption density (Figure 1).
Figure 1: Density value comparison
Volume Step Length
Applicable when rendering Volume mediums. The default value is 4 meters. If the volume is smaller than this, decrease the Step Length. Decreasing this value also reduces the render speed. Increasing this value causes the ray marching algorithm to take longer steps. If the 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 the 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.
AbsorptionDefines how fast light is absorbed while passing through a medium.
Assigns a grayscale or color texture to the material to determine its absorption value. When using greyscale values, 0 (black) means there is no absorption. Values greater than 0 determine how fast the medium absorbs white light (Figure 2).
Figure 2: Using a grayscale value to control absorption
You can also use color values to control Absorption. The observed color is the complementary color (opposite color) of the specified color value (Figure 3).
Figure 3: Using color values to determine absorption
Invert Absorption
Inverts the Absorption value and makes the specified absorbed color the actual visible color.
Scattering
Determines how fast light scatters as it moves through the surface. High values scatter light faster as it enters the surface, and low values pass light deeper into the surface before it scatters. A value of 0 disables scattering. When using a float or grayscale texture, values greater than 0 determine how fast the light scatters (Figure 4).
Figure 4: Various float values and their effects on the Scattering parameter
PhaseInput
Controls the direction of the light as it scatters in the surface. A value of 0 results in light scattering equally in all directions. Positive values result in forward scattering, where the photons continue in the same direction they were going when they entered the surface. Negative values result in backwards scattering, where the light moves through the surface in the direction opposite to the angle at which the light entered the surface. To achieve this level of control, a Phase Schlick node should be added to the Phase input on the Scattering medium node.
EmissionInput
This parameter allows the volume to emit or generate its own illumination.