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Subsurface Scattering is a material that is primarily designed to render translucent materials like skin, marble, etc. The implementation is based on the concept of BSSRDF originally introduced by Jensen et al. (see the references below) and is a more or less physically accurate approximation of the sub-surface scattering effect, while still being fast enough to be used in practice.

Subsurface Scattering is a complete material with diffuse and specular components that can be used directly, without the need of a Blend material. Specifically, the material is composed of three layers: a specular 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.

The Subsurface Scattering material settings are organized in Basic and Advanced modes. You can switch the mode from the toggle button under the Preview Swatch or globally from the Configuration rollout of the Settings tab.

From the Add Attribute button, you can select additional attributes that can add up to the appearance of the material. For more information, see the Attributes section.

The context menu of the Color slot provides options to Copy and Paste, as well as to Reset the color.

A Reset option is provided in the context menu of each Number Slider. You can reset the slider value to the default one.

Some of the parameters are available only in Advanced mode.

Scale – Additionally scales the subsurface scattering radius. Normally, Subsurface Scattering takes the scene units into account when calculating the subsurface scattering effect. However, if the scene was not modeled to scale, this parameter can be used to adjust the effect. For more information, see the Scale example below.

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

Overall Color – Controls the overall coloration for the material. This color serves as a filter for both the diffuse and the sub-surface component.

Opacity – Specifies how opaque or transparent the material is.

Diffuse Color – Specifies the color of the diffuse portion of the material.

Diffuse Amount – The amount for the diffuse component of the material. Note that this value in fact blends between the diffuse and sub-surface layers. When set to 0.0, the material does not have a diffuse component. When set to 1.0, the material has only a diffuse component, without a sub-surface layer. The diffuse layer can be used to simulate dust etc. on the surface.

Some of the options are available only in Advanced mode.

Sub-Surface Color – Specifies the general color for the sub-surface portion of the material. 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 Radius (cm) – Determines the specular color for the material. For more information, see the Scatter Radius example below. 

Phase Function – 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 in the same direction as it comes from; negative values mean that light scatters mostly backward. Most water-based materials (e.g. skin, milk) exhibit strong forward scattering, while hard materials like marble exhibit backward scattering. This parameter affects most strongly the single scattering component of the material. Positive values reduce the visible effect of single scattering component, while negative values make the single scattering component generally more prominent. For more information, see the Phase Function example or the Phase Function: Light Source example below. 

The options in this rollout allow you to control the quality of the final result. This rollout is available only in Advanced mode.

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

None – No single scattering component is calculated.
Simple – The single scattering component is approximated from the surface lighting. This option is useful for relatively opaque materials like skin, where light penetration is normally limited. 
Raytraced (solid) –  The single scattering component is accurately calculated 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 option is useful for highly translucent materials like marble or milk, which at the same time are relatively opaque. 
Raytraced (refractive) – Similar to the Raytraced (solid) mode, but refraction rays are traced as well. This option is useful for transparent materials like water or glass. In this mode, the material also produces transparent shadows.  

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

Refraction Depth – Determines the depth of refraction rays when the single scatter parameter is set to Raytraced (solid). 

This rollout is available only in Advanced mode.

Mode – Specifies one of the following methods for adjusting textures.

Multiply – Multipliers can be specified to adjust colors and textures.
Blend Amount – Blend amounts can be specified to adjust colors and textures.

Opacity – Controls the intensity of the Opacity value, which determines how opaque or transparent the overall material is.

Overall Color – Controls the intensity of the material's Overall Color.

Diffuse Color – Controls the intensity of the material's diffuse color.

Diffuse Amount – Blends between a texture assigned (if such) and the a color. 

Sub-Surface Color – Controls the intensity of the material's sub-surface color.

Scatter Color – Controls the intensity of the internal scattering color.

Specular Color – Controls the intensity of the material's specular color.

Specular Glossiness – Controls the sharpness intensity of the material's specular highlights, which affects the highlight shape.