V-Ray Fast SSS2 is a material primarily designed for rendering translucent materials like skin, wax, marble, etc. The implementation is based on the concept of BSSRDF, originally introduced by Jensen et al. It is an approximation of the sub-surface scattering effect observed in the physical world while still being fast enough to be used in practice.

V-Ray Fast SSS2 is a complete material with diffuse and reflection components that can be used directly without the need for a V-Ray Blend Mtl. To be more specific, the material is composed of three layers: a reflection layer, a diffuse layer, and a sub-surface scattering layer. The sub-surface scattering layer is comprised of single and multiple scattering components. Single scattering occurs when light bounces once inside the material. Multiple scattering results from light bouncing two or more times before leaving the material.

Overall Color – Specifies the overall coloration for the material. This color serves as a filter for both the diffuse and the sub-surface components. The effect is a color tint, where pure white means neutral and doesn’t introduce tinting. A node can be connected to the input socket of this parameter to control it.

Diffuse Color – Specifies the color of the diffuse portion of the material. The Diffuse amount needs to be greater than 0 for it to have any effect. A node can be connected to the input socket of this parameter to control it.

Diffuse Amount – Controls the strength of the diffuse component of the material by blending between the diffuse and the sub-surface layers. When set to 0, the material does not use the diffuse component. When set to 1.0, the material shows no sub-surface scattering. Values in between can be used to “harden” the surface while retaining the SSS effect inside when using larger Scatter Radius (cm) values. A texture connected to the Diffuse Amount can be used as a mask for the SSS layers to simulate dust or paint on the surface. A node can be connected to the input socket of this parameter to control it.

Color Mode – Allows the user to determine which method is used to control the sub-surface scattering effect.

Sub-surface color and Scatter Radius (cm) – This mode uses a general Sub-surface Color and an inside Scatter Color, that becomes visible in backlit parts of the objects that are thinner than the scaled Scatter Radius (cm). This mode is suitable for relatively opaque materials.
Scatter coefficient and fog color – This mode uses a Scatter coefficient to define the outside scatter layer color and translucency and a Fog color for the respective inside values. The translucency for both components is multiplied by the scaled Scatter Radius (cm). This mode allows for control of the SSS components similar to that in the V-Ray Material. It is designed for translucent or refractive materials like juice or ice.

Sub-surface Color – Specifies the general color for the sub-surface layer of the material. Note that the Sub-surface Color value is filtered/multiplied by the Overall color and both filter the Scatter color. A node can be connected to the input socket of this parameter.

Scatter Color – Specifies the internal scattering color for the material. Brighter colors cause the material to scatter more light and to appear more translucent; darker colors cause the material to look more diffuse-like. A node can be connected to the input socket of this parameter to control it. 

Scatter Radius (cm) – Controls the depth of scattering light inside the material for both color modes. Smaller values cause the material to have a shallower layer of scattered light and to appear more diffuse-like. Higher values define a deeper layer, where the material scatters light and makes it look more translucent. Note that the Scatter Radius (cm) value is always specified in centimeters (cm). A node can be connected to the input socket of this parameter to control it.

Phase Function – Specifies a value between -1.0 and 1.0 that determines the general way light scatters inside the material. Its effect can be somewhat likened to the difference between diffuse and glossy reflections from a surface. However, the phase function controls the reflection and transmittance of a volume. A value of 0.0 means that light scatters uniformly in all directions (isotropic scattering). Positive values mean that light scatters predominantly forward. Negative values mean that light scatters mostly backward. This, depending on the direction of illumination, leads to changes in the blending between the two SSS colors, boosting one or the other. Most water-based materials (e.g. skin, milk) exhibit strong forward scattering, while hard materials like marble exhibit backward scattering.

Scale – Controls the depth of scattering by multiplying the Scatter Radius (cm). This can be useful when your scene is not modeled to scale. The default value of 1 means that the Scatter Radius (cm) is used as it is. For example, to render a 1:10 scale model, set the scale to 0.10. 

Index of Refraction – Specifies the index of refraction for the material. Most water-based materials, like skin, have an index of refraction of about 1.3.

Specular Color – Specifies the specular color for the material. A node can be connected to the input socket of this parameter to control it.

Specular Amount – Specifies the strength of the specular component for the material. Note that there is an automatic Fresnel falloff applied to the specular component, based on the Index of refraction of the material. A node can be connected to the input socket of this parameter to control it.

Specular Glossiness –  Determines the glossiness (highlights shape). A value of 1.0 produces sharp reflections, while lower values produce more blurred reflections and highlights. A node can be connected to the input socket of this parameter to control it.

Reflection Depth – Specifies the number of reflection bounces for the material.

Trace Reflections – Enables the calculations of glossy reflections. When disabled, only highlights are calculated.

Single Scatter – Controls how the single scattering component is calculated. 

None – No single scattering is applied.
Simple – Approximates the single scattering component from the surface lighting. This option is fast and useful for relatively opaque materials like skin, where light penetration is normally limited.
Raytraced (solid) – Accurately calculates the single scattering component by sampling the volume inside the object. Only the volume is raytraced; no refraction rays on the other side of the object are traced. This is useful for materials with a more pronounced sub-surface scattering effect, like marble or milk, which at the same time are relatively opaque.
Raytraced (refractive) – Similar to the Raytraced (solid) mode, but in addition refraction rays are traced. This option is useful for transparent materials like water or glass. In this mode, the material also produces transparent shadows.

Cutoff Threshold – Specifies a threshold below which reflections are not traced. V-Ray tries to estimate the contribution of reflections to the image, and if it is below this threshold, these effects are not computed. Do not set this to 0.0, as it may cause excessively long render times in some cases. This parameter is not available when the render engine is set to CUDA.

Refraction Depth – Determines the depth of refraction rays when the Single Scatter parameter is set to Raytraced (refractive) mode.

Scatter GI – Determines whether the material accurately scatters global illumination. When disabled, global illumination is calculated using a simple diffuse approximation on top of the sub-surface scattering. When enabled, global illumination is included as part of the surface illumination map for multiple scattering. Enabling this option creates more accurate renders, especially for highly translucent materials, but it may slow down the rendering quite a bit.