Subsurface Scattering

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. It can also be used to modify the effect of the presets, which reset the Scatter radius parameter when loaded, but leave the Scale parameter unchanged. 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. 

Some of the options are available only in Advanced mode.

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

Color – Determines the specular color for the material.

Amount – Determines the specular amount for the material. Note that there is an automatic Fresnel falloff applied to the specular component, based on the IOR of the material.

Glossiness – Determines the glossiness (highlights shape). A value of 1.0 produces sharp reflections, lower values produce more blurred reflections and highlights.

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

The options in this rollout allow you to control the method used to calculate the subsurface effect and the quality of the final result. This rollout is available only in Advanced mode.

Multiple Scatter – Specifies the method used to calculate the subsurface scattering effect.

Prepass-based illum map – Uses an approach similar to the irradiance map to approximate the sub-surface scattering effect. It requires a prepass and the quality of the final result depends on the Prepass rate parameter.
Object-based illum map – Similar to the Prepass-based illumination map in that it also creates an illumination map used to approximate the final result. The only difference is the method used for sample placement. Rather than using the resolution of the image as a guide the samples are placed based on the surface area of the geometry. When this mode is used the final quality depends on the Samples per Unit Area parameter.
Raytraced – Uses true raytracing inside the volume of the geometry to get the subsurface scattering effect. This method is physically accurate and produces the best results.
None (diffuse approx.) – Does not calculate the multiple scattering effect and uses a diffuse approximation instead.

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 scatterin. 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 option is available when the Multiple Scatter parameter is set to Prepass-based illumination map, Object-based illumination map or None. 

Prepass Mode – This parameter is similar to the Mode parameter of the irradiance map and controls the way V-Ray handles the illumination map for the subsurface scattering.

New Map Every Frame – Calculates a new map for every frame of the animation. 
Save Every Frame – Calculates a new map and saves it on the hard drive for every frame of the animation. 
Load Every Frame – Looks for and loads a previously saved illumination map for each frame of the animation. 
Save Map for First Frame – Calculates a new map for the first frame of the animation.
Load Map for First Frame – Loads a previously saved illumination map for the first frame of the animation.

Prepass File Name – Specifies a file name for the illumination map to be saved or loaded from.

Prepass Rate – Accelerates the calculation of multiple scattering by precomputing the lighting at different points on the surface of the object and storing them in a structure called an illumination map, which is similar to the irradiance map used to approximate global illumination, and uses the same prepass mechanism built into V-Ray that is also used for e.g. interpolated glossy reflections/refractions. This parameter determines the resolution at which surface lighting is computed during the prepass phase. A value of 0 means that the prepass is at the final image resolution; a value of -1 means half the image resolution, and so on. For high quality renders it is recommended to set this to 0 or higher, as lower values may cause artifacts or flickering in animations. If the chosen prepass rate is not sufficient to approximate the multiple scattering effect adequately, BRDFSSS2Complex will replace it with a simple diffuse term. This can happen, for example, for objects that are very far away from the camera, or if the subsurface scattering effect is very small. This simplification is controlled by the Prepass blur parameter. For more information see the Prepass Rate example. 

Prepass ID – Allows several BRDFSSS2Complex materials to share the same illumination map. This could be useful if different BRDFSSS2Complex materials applied on the same object - either through a Multi/Sub-Object material, or inside a VRayBlendMtl material. If the Prepass ID is 0, then the material computes its own local illumination map. If this is greater than 0, then all materials with the specified ID share the same map.

Auto Calculate Density – When enabled, V-Ray automatically assigns the number of samples to be used for each square unit of surface on the geometry. Enabling this option disables the Samples per unit area parameter.

Samples Per Unit Area – This parameter has effect only when the Auto calculate density check box is disabled. It allows you to control the number of samples that are going to be taken for each square unit of the geometry surface. The size of one unit is controlled by the scene units set up. Increasing this value means that more samples are going to be taken which produces higher quality results at the cost of increased render times.

Samples Offset – To prevent artifacts, each sample is taken a tiny distance away from the actual surface in the direction of the normal. This parameter controls that offset.

Prepass Blur – Controls if the material uses a simplified diffuse version of the multiple scattering when the prepass rate for the direct lighting map is too low to adequately approximate it. A value of  0.0  causes the material to always use the illumination map. However, for objects that are far away from the camera, this may lead to artifacts or flickering in animations. Larger values control the minimum required samples from the illumination map in order to use it for approximating multiple scattering.

Interpolation Accuracy – Controls the quality of the approximation of the multiple scattering effect when the type is Prepass-based illumination map or Object-based illumination map. Larger values produce more accurate results but are slower to render. Lower values render faster, but too low values may produce blocky artifacts on the surface.

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.

Binding – Enables connection/binding between V-Ray and the corresponding base application material.

Color – Enables color binding. Changing the V-Ray material color changes the corresponding base application material color and vice versa.

Texture Mode – Enables texture binding. Changing the V-Ray material texture changes the corresponding base application material texture and vise versa. 

Auto – By default binds the Diffuse texture to the base app material. 
Texture Helper – Allows the use of a helper texture as a base application material map. The same helper is used if the binded texture is a procedural map. This is useful if every time you have to set texture placement for a map that can't be displayed accurately in the base app. 
Custom – Allows the use of a custom texture as base application material map.

Disabling this parameter allows changing the base app material texture without affecting the V-Ray material.