VRayFastSSS2 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. (see the references below). 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.

VRayFastSSS2 is a complete material with diffuse and reflection components that can be used directly without the need of a VRayBlendMtl material. 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.

^{Image courtesy of Antone Magdy}

Preset – Specifies one of several available preset materials. They can be used directly or as a starting point for building custom materials. Most of the presets are based on measured data provided by Jensen et al. in A Practical Model for Subsurface Light Transport. They come with a scatter radius value measured in cm so depending on the scale of your scene, the Scale value might need adjusting. 

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

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

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.

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.

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 values. A texture in the Diffuse amount can be used as a mask for the SSS layers to simulate dust or paint on the surface.

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

Sub-surface color and scatter radius – 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. It is suitable for relatively opaque materials and works best when Single scatter is set to Simple or Raytraced (Solid). The preset materials are designed to work in this mode: skin, marble, potato etc.
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. This mode allows for control of the SSS components similar to that in the VRayMtl. It is designed for translucent or refractive materials like juice or ice and works best when the Single scatter is set to Raytraced (Solid) or Raytraced (Refractive).

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. For more information, see the Sub-Surface Color example below. 

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. For more information, see the Scatter Color Example below.

Scatter Coefficient – Specifies the outside color for the sub-surface layer of the material and also affects its outer translucency. Brighter colors cause the material to look frosted and less transparent, while darker colors result in a clearer effect. Note that the Scatter coefficient color value is filtered/multiplied by the Overall color. Available when the Color Mode is Scatter coefficient and fog color. For more information, see the Scatter Coefficient example below.

Fog Color – Specifies the inside or backlit color of the object and affects its inner translucency. Brighter colors cause the material to scatter more light and appear more translucent; darker colors result in more diffuse-like look. The Fog color is filtered/multiplied by both the Scatter coefficient and the Overall color to achieve the final result.. Available when the Color Mode is Scatter coefficient and fog color. For more information, see the Fog Color example below.

Scatter radius – Controls the depth of scattering light inside the material for both color modes. Smaller values cause the material to have shallower layer of scattered light and to appear more diffuse-like. Higher values define a deeper layer where the material scatters light and make it look more translucent. Note that the Scatter radius value is always specified in centimeters (cm) regardless of Maya's current working unit and is multiplied by the Scale to calculate the effective depth of scattering. For more information, see the Scatter Radius and Scale example below.

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: boosts one or the other. Most water-based materials (e.g. skin, milk) exhibit strong forward scattering, while hard materials like marble exhibit backward scattering. For more information, see the Phase Function example and Phase Function: Light Source example below.

Phase function = -0.5 (Backward Scattering)

Phase function = 0 (Isotropic Scattering)

Phase function = 0.5 (Forward Scattering)

Phase function = -0.9

 Phase function = 0.0

Phase function = 0.7

Single scatter – Controls how the single scattering component is calculated. For more information, see the Single Scatter Mode example below.

None – No single scattering component is calculated.
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 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 will also produce transparent shadows.

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

Multiple Scattering – Enables the subsurface scattering effect that is achieved by true raytracing inside the volume of the geometry.

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

Consider All Objects – When enabled, the VRayFastSSS2 considers all intersecting objects with the same VRayFastSSS2 assigned when calculating the sub-surface scattering effect.

Map Type – Determines how the Map parameter is interpreted.

Map – Specifies a texture for the bump or normal map. Leaving this unconnected disables bump/normal mapping.

Bump Mult – A multiplier that controls the strength of the bump map effect.

Bump Shadows – When enabled, V-Ray will consider the bump maps when rendering shadows produced by objects with the bump material applied to them.