This page provides information about the Subsurface Scattering material in V-Ray for SketchUp.


Overview

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.




UI Paths

||V-Ray Asset Editor|| > Materials (right-click) > Subsurface Scattering

||V-Ray Asset Editor|| > Create Asset (left-click) > Materials > Subsurface Scattering



Parameters

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.





Example: Scale


This example shows the effect of the Scale parameter. Note how larger values make the object appear more translucent. The images are rendered without GI to better show the sub-surface scattering. The Single scatter parameter was set to Raytraced (solid).



Scale = 1

Scale = 3

Scale = 6




Diffuse Layer

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.




Sub-Surface Layer

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. 




Example: Sub-Surface Color


This example and the next demonstrate the effect of and the relation between the Scatter color and the Sub-surface color parameters. Note how changing the Sub-surface color changes the overall appearance of the material, whereas changing the Scatter color only modifies the internal scattering component. For all three renders, the Scatter color is set to green.


Sub Surface Color = Red

Sub Surface Color = Green

Sub Surface Color = Blue





Example: Scatter Color


The Sub-surface color is set to green for all the following renders.


Scatter Color = Red

Scatter Color = Green

Scatter Color = Blue





Example: Scatter Radius


This example shows the effect of the Scatter radius parameter.


Scatter Radius = 1.0cm

Scatter Radius = 2.0cm

Scatter Radius = 6.0cm



Example: Phase Function

This example shows the effect of the Phase function parameter. This parameter can be likened to the difference between diffuse reflection and glossy reflection on a surface. However, it controls the reflectance and transmittance of a volume. Its effect is quite subtle, and mainly related to the single scattering component of the material.

The red arrow represents a ray of light going through the volume; the black arrows represent possible scattering directions for the ray.


Phase Function = -0.9 (Backward Scattering)
More light comes out. 

Phase Function = -0.5 (Backward Scattering)

Phase Function = 0 (Isotropic Scattering)

More light exits object. 

Phase Function = 0 (Isotropic Scattering)

Phase Function = 0.0 (Forward Scattering)
More light is absorbed object. 


Phase Function = 0.5 (Forward Scattering)


  


Example: Phase Function: Light Source


This example demonstrates the effect of the Phase function parameter when there is a light source inside the volume. The images have large Scatter radius, Raytraced (refractive) mode for Single scattering, and IOR set to 1.0. Front lighting and Back lighting are disabled for these images; only Single scattering is visible. Note the volumetric shadows cast by the light inside the volume.



Phase Function = -0.9

Phase Function = 0

Phase Function = 0.9




Specular Layer  

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.




Scattering Options

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). 





Example: Single Scatter Presets

This example shows the effect of the Single scatter mode parameter.

For relatively opaque materials, the different Single scatter modes produce quite similar results (except for render times). In the following set of images, the Scatter radius is set to 1.0 cm.

In the second set of images, the Scatter radius is set to 50.0 cm. In this case, the material is quite transparent, and the difference between the different Single scatter modes is apparent. Note also the transparent shadows with the Raytraced (refractive) mode.


Preset = Simple

Preset = Ray Traced Solid

Preset = Ray Traced Refractive

Preset = Simple

Preset = Ray Traced Solid

Preset = Ray Traced Refractive




Multipliers

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

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.

Opacity – Enables refraction/opacity binding. Changing the SketchUp material opacity, however, does not change the V-Ray material. Instead, it disables the Opacity binding.

Texture Mode – Enables texture binding. Changing the V-Ray material texture changes the corresponding base application material texture and vice 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. 
Bake – In this mode complex texture networks are baked to a single image used as a viewport preview. Differences between the rendered result and the viewport preview may occur due to the fact that only the texture confined within the 0 to 1 UV square is baked. This affects most procedural patterns and textures with custom UV placement configuration. Raytraced and Tri-planar textures are not supported.

It is recommended to save all raw bitmap buffers (textures embedded in SketchUp like SketchUp library materials textures) to disk before activating the Bake mode to preserve them.

Texture – Selected textures are displayed in the viewport. Keep in mind that procedural textures are not displayed. Note that the viewport texture will not affect the way the material is rendered in V-Ray. It is mainly used for preview purposes.


SketchUp2023_VRay6.2.1_SubsurfaceScattering_Binding



Override Control

Can be Overridden – When enabled, the material can be overridden by the Material Override option in the Settings.



Attributes

For more information on Attributes, see the Attributes section of the Materials page. 


Bump


Mode/Map – Specifies the bump map type.

Bump Map – A height map should be used.
Bump Texture Channel – Some V-Ray textures have a special bump channel output that can be used here. It is most commonly used for Round Edges effect. Edges texture is used as a bump.

Normal Map – RGB normal map should be used with this option. Note that in most cases the normal map bitmap color space should be set to Linear to ensure correct results.

Amount – Multiplier for the bump/normal map.

Delta Scale – Specifies a scale for sampling the bitmap when using bump mapping. The exact value is calculated automatically by V-Ray, but can be called here.


 

Outline


The Outline attribute is available only when the engine is set to CPU. It is currently not supported for GPU.

Line Color – Specifies the color of the outlines.

Opacity – Specifies the opacity of the outlines.

Normal Threshold – Determines when lines will be created for parts of the same object with varying surface normals (e.g. at the inside edges of a box). A value of 0.0 means that only 90 degrees or larger angles generate internal lines. Higher values mean that smoother transitions between face normals can also generate a line. Setting this value to 1.0 fills curved objects completely.

Overlap Threshold – Determines when outlines will be created for overlapping parts of the one and the same object. Lower values reduce the internal overlapping lines, while higher values produce more overlap lines. Setting this value to 1.0 fills curved objects completely.

Width – Specifies the width of the outlines.

Inner Line Control – Enables a separate control for the inner edges.

Inner Line Color – Specifies the color of the inner lines

Inner Width – Specifies the width of the inner lines.

Some of the global parameters have an effect on all materials with Outline attribute. These parameters are Width Type, Trace Bias, No Inner Edges, Visible in Secondary, and Compensate EV.


 

Displacement


This is a legacy attribute that will be removed in the future. Consider using the geometry displacement modifier instead. It can be created as a geometry asset in the Outliner and can be applied to objects in the scene. Note that the displacement effect will no longer appear in the Preview Swatch.

Displacement – Enables or disables the displacement effect.

Mode/ Map – Specifies the mode in which the displacement is rendered. 

2D Displacement – Bases the displacement on a texture map that is known in advanced. The displaced surface is rendered as a warped height-field based on that texture map. The actual raytracing of the displaced surface is done in texture space and the result is mapped back into 3D space. The advantage of this method is that it preserves all details in the displacement map. However, it requires the object to have valid texture coordinates. You cannot use this method for 3d procedural textures or other textures that use object or world coordinates. The parameter can take any values. 
Normal Displacement – Takes the original surface geometry and subdivides its triangles into smaller sub-triangles, which then are displaced. 

Amount – The amount of displacement. A value of 0.0 means the object appears unchanged. Higher values produce a greater displacement effect. This parameter can also take a negative value, in which case the displacement pushes geometry inside the object. 

Shift – Specifies a constant, which is added to the displacement map values, effectively shifting the displaced surface up and down along the normals. This can be either positive or negative.

Keep Continuity – When enabled, tries to produce a connected surface, without splits, when there are faces from different smoothing groups and/or material IDs. Note that using material IDs is not a very good way to combine displacement maps since V-Ray cannot always guarantee the surface continuity. Use other methods (vertex colors, masks etc.) to blend different displacement maps.

Resolution – This option is available when the Mode/Map is 2D Displacement. It determines the resolution of the displacement texture used by V-Ray. If the texture is a bitmap, it is recommended to match this resolution to the size of the bitmap. For procedural 2D maps, the resolution is determined by the desired quality and detail in the displacement. Note that V-Ray also automatically generates a normal map based on the displacement map in order to compensate for details not captured by the actual displaced surface.

View Dependent – When enabled, Edge length determines the maximum length of a subtriangle edge in pixels. A value of 1.0 means that the longest edge of each subtriangle is about one pixel long when projected on the screen. When disabled, Edge length is the maximum sub-triangle edge length in world units.

Edge Length – Determines the quality of the displacement. Each triangle of the original mesh is subdivided into a number of subtriangles. More subtriangles mean more detail in the displacement, slower rendering times and more RAM usage. Less subtriangles mean less detail, faster rendering and less RAM. The meaning of Edge length depends on the View dependent parameter. The slider's minimum range is set to 0.4. Using lower values is still possible by manually typing them in the input box but it may cause significant render delay.

Max Subdivs – Controls the maximum sub-triangles generated from any triangle of the original mesh when the displacement type is Subdivision. The value is in fact the square root of the maximum number of subtriangles. For example, a value of 256 means that at most 256 x 256 = 65536 subtriangles will be generated for any given original triangle. It is not a good idea to keep this value very high. If you need to use higher values, it will be better to tessellate the original mesh itself into smaller triangles instead. The actual subdivisions for a triangle are rounded up to the nearest power of two (this makes it easier to avoid gaps because of different tessellation on neighboring triangles). 

Water Level – Clips the surface geometry in places where the displacement map value is below the specified threshold. This can be used for clip mapping a displacement map value below which geometry will be clipped. 

Level Height – Value below which the geometry is clipped. 

Materials need to be applied to objects (groups/components) to have working displacement. If various materials are applied to different faces of an object, the displacement from the top-level (group/component) material will be used on all of them. Normal Displacement will take into account the texture size of each different face material, while 2D Displacement will ignore them.



Raytrace Properties


Visible to Camera – When enabled, makes objects using this material visible to the camera.

Visible to Reflections – When enabled, this option makes objects using this material visible for to Reflection rays.

Visible to Refractions – When enabled, this option makes objects using this material visible for the Refraction rays.

Cast Shadows – When disabled, all objects with this material applied do not cast shadows.


Override


Shadows – The material that is used when a shadow ray hits the surface.

Reflection – The material that is used when a reflection ray hits the surface.

Refraction– The material that is used when a refraction ray hits the surface.

GI – The material that is used when a GI ray hits the surface.

Environment – The texture that will be used instead of the scene environment maps.


Material ID


ID Number – Isolates objects as an R/G/B mask in the MultiMatte render elements.

ID Color – Allows you to specify a color to represent this material in the Material ID VFB render element. 

Each material is assigned with an automatically generated ID Color.




Notes

  • The BRDFSSS2Complex material computes sub-surface scattering only during the final image rendering. During other GI calculations phases (e.g. light cache or photon mapping), the material is calculated as a diffuse one.

  • For the reason explained above, BRDFSSS2Complex will render as a diffuse one with the progressive path tracing mode of the light cache.
  • You can layer several BRDFSSS2Complex materials using a Blend material in order to recreate more complex sub-surface scattering effects. In this case, any raytraced single scattering will only be calculated for the base material, but multiple scattering, reflections etc will work correctly for any layer. It might be helpful to use the Prepass ID parameter to make the materials share the same illumination map so that some of the calculations are reused.
  • Materials created with previous version, which are migrated to a default value - Raytraced (geometry_based_sampling = 2)
  • The option Multiple Scatter is now legacy. Materials created with versions prior to V-Ray 5, have their Multiple Scatter migrated to Raytraced.
  • The 2D mapping (landscape)  method only supports one UV mapping channel.
  • Scene units in SketchUp are always calculated in inches.



References

Here is a list of references used when building the BRDFSSS2Complex material.


  • H. C. Hege, T. Hollerer, and D. Stalling, Volume Rendering: Mathematical Models and Algorithmic aspects
    An online version can be found at https://opus4.kobv.de/opus4-zib/frontdoor/index/index/docId/
    Defines the basic quantities involved in volumetric rendering and derives the volumetric and surface rendering equations.      
  • T. Farrell, M. Patterson, and B. Wilson, A Diffusion Theory Model of Spatially Resolved, Steady-state Diffuse Reflectance for the Noninvasive Determination of Tissue Optical Properties in vivo , Med. Phys. 19(4), Jul/Aug 1992 at https://pubmed.ncbi.nlm.nih.gov/1518476/
    Describes an application of the diffusion theory to the simulation of sub-surface scattering; derives the base formulas for the dipole approximation used by Jensen et al. (see below).
  • H. Jensen, S. Marschner, M. Levoy, and P. Hanrahan, A Practical Model for Subsurface Light Transport, SIGGRAPH'01: Computer Graphics Proceedings, pp. 511-518
    An online version of this paper can be found at http://www-graphics.stanford.edu/papers/bssrdf/
    Introduces the concept of BSSRDF and describes a practial method for calculating sub-surface scattering based on the dipole approximation derived by Farrell et al. (see above).
  • H. Jensen and J. Buhler, A Rapid Hierarchical Rendering Technique for Translucent Materials, SIGGRAPH'02: Computer Graphics Proceedings, pp. 576-581
    An online version of this paper can be found at http://graphics.ucsd.edu/~henrik/papers/fast_bssrdf/
    Introduces the idea of decoupling the calculations of surface illumination and the sub-surface scattering effect in a two-pass method; describes a fast hierarchical approach for evaluating subsurface scattering and proposes a reparametrization of the BSSRDF parameters for easier user adjustment.
  • C. Donner and H. Jensen, Light Diffusion in Multi-Layered Translucent Materials, SIGGRAPH'05: ACM SIGGRAPH 2005 Papers, pp. 1032-1039
    An online version of this paper can be found at http://graphics.ucsd.edu/~henrik/papers/layered/ (the link no longer exists)
    Provides a concise description of the original BSSRDF solution method presented by Jensen et al; extends the model to multi-layered materials and thin slabs using multipole approximation.