Subsurface

asSubsurface

A raytraced, no precomputed required, subsurface-scattering material with a coupled specular term. This node capable of modelling a wide range of appearance thanks to built-in support for several diffusion profiles.


Parameters


Subsurface Parameters

Subsurface Profile

The diffusion profiles to use in the BSSDRF [1]. This parameter can take the following values

Reflectance
The surface reflectance.
SSS Amount
A scaling factor for the overall contribution of the subsurface scattering term.
Mean Free Path
Mean free path [2] controls how deep light can travel within the volume, scattering internally, until it’s fully absorbed or exits the medium. Lower values in the limit would have a response similar to a diffuse term, while high values would allow light to travel almost unhindered, producing a translucent like appearance.
MFP Scale
An overall scaling factor for the Mean Free Path.

SSS Advanced Parameters

SSS Ray Depth
The maximum ray depth allowed for a ray of type subsurface.

Fresnel Parameters

Index of Refraction
The index of refraction of the material.

Fresnel Advanced Parameters

Fresnel Weight
A value of 1.0, will scale the subsurface contribution by the Fresnel transmittance, while a value of 0.0 will disable any scaling. When using the provided specular BRDF, in order to keep energy conservation, this scaling factor should be set to 1.0. If you’re not using the provided specular term and/or want to compose one later in your workflow, you can disable this. You also have the freedom to use intermediary values.

Specular Parameters

Specular Weight
An overall scaling factor for the specular term. A value of 1.0 provides full intensity, 0.0 disabling it.

Note

This shader allows the user to texture map the specular weight, to control the specular term intensity, but it does not provide a way to tint or color the specular term. That is intentional sually only dielectrics [3] have subsurface scattering, and dielectrics have no tinted specular highlights.

Specular Roughness
The apparent surface roughness of the material. The distribution used is the GGX [Walter2007], and energy conservation to take into account multiple scattering [Heitz:2016:MMB:2897824.2925943] is applied automatically.

Anisotropy Parameters

Anisotropy Amount
The overall weight of the anisotropy, with a value of 0.0 producing isotropic specular highlights, and a value of 1.0 producing full anisotropic specular highlights.
Anisotropy Angle
A rotation angle in [0,1] range, that is mapped internally to a full 360 degrees rotation and applied on top of the anisotropy value provided by the explicit anisotropy vector or anisotropy vector map.
Anisotropy Mode

The anisotropy mode, which can either be a anisotropy vector map with the XY anisotropy encoded in the red and green channels of the image, or an explicit anisotropy vector, which can be provided via a asAnisotropyVectorField node. It can therefore take the values

  • Anisotropy Map
  • Explicit Vector
Anisotropy Map
The anisotropy vector map to use when Anisotropy Mode is set to Anisotropy Map.
Anisotropy Direction
An explicit anisotropy vector to use when the Anisotropy Mode parameter is set to Explicit Vector.

Bump

Bump Normal
The unit length world space normal of the bumped surface.

Matte Opacity

Enable Matte Opacity
Parameter that toggles matte holdouts.
Matte Opacity
Matte opacity scaling factor.
Matte Opacity Color
Holdout color.

Outputs

Output Color
The BSSRDF output with the optional added specular BRDF.
Output Matte Opacity
The matte holdout.

Screenshots


Footnotes

[1]See also bidirectional scattering distribution function.
[2]See mean free path wikipedia page for more details.
[3]Dielectric is a material which is an electric insulator, the opposite of conductors which as the name says, conducts electricity. See this page on dielectric materials for more details. In terms of look development an accepted simplification is that dielectrics have white or non-tinted specular highlights, while conductors have tinted or coloured specular highlights.

References

[Chr15]Per H. Christensen. An approximate reflectance profile for efficient subsurface scattering. In ACM SIGGRAPH 2015 Talks, SIGGRAPH ‘15, 25:1–25:1. New York, NY, USA, 2015. ACM. URL: http://doi.acm.org/10.1145/2775280.2792555, doi:10.1145/2775280.2792555.
[DEonI11]Eugene D’Eon and Geoffrey Irving. A quantized-diffusion model for rendering translucent materials. In ACM SIGGRAPH 2011 Papers, SIGGRAPH ‘11, 56:1–56:14. New York, NY, USA, 2011. ACM. URL: http://doi.acm.org/10.1145/1964921.1964951, doi:10.1145/1964921.1964951.
[dEonLE07]Eugene d’Eon, David Luebke, and Eric Enderton. Efficient rendering of human skin. In Proceedings of the 18th Eurographics Conference on Rendering Techniques, EGSR‘07, 147–157. Aire-la-Ville, Switzerland, Switzerland, 2007. Eurographics Association. URL: http://dx.doi.org/10.2312/EGWR/EGSR07/147-157, doi:10.2312/EGWR/EGSR07/147-157.
[DJ05]Craig Donner and Henrik Wann Jensen. Light diffusion in multi-layered translucent materials. In ACM SIGGRAPH 2005 Papers, SIGGRAPH ‘05, 1032–1039. New York, NY, USA, 2005. ACM. URL: http://doi.acm.org/10.1145/1186822.1073308, doi:10.1145/1186822.1073308.
[FHK14]Jeppe Revall Frisvad, Toshiya Hachisuka, and Thomas Kim Kjeldsen. Directional dipole model for subsurface scattering. ACM Trans. Graph., 34(1):5:1–5:12, December 2014. URL: http://doi.acm.org/10.1145/2682629, doi:10.1145/2682629.
[HHdEonD16]Eric Heitz, Johannes Hanika, Eugene d’Eon, and Carsten Dachsbacher. Multiple-scattering microfacet bsdfs with the smith model. ACM Trans. Graph., 35(4):58:1–58:14, jul 2016. URL: http://doi.acm.org/10.1145/2897824.2925943, doi:10.1145/2897824.2925943.
[JMLH01]Henrik Wann Jensen, Stephen R. Marschner, Marc Levoy, and Pat Hanrahan. A practical model for subsurface light transport. In Proceedings of the 28th Annual Conference on Computer Graphics and Interactive Techniques, SIGGRAPH ‘01, 511–518. New York, NY, USA, 2001. ACM. URL: http://doi.acm.org/10.1145/383259.383319, doi:10.1145/383259.383319.
[MHD16]Johannes Meng, Johannes Hanika, and Carsten Dachsbacher. Improving the dwivedi sampling scheme. Comput. Graph. Forum, 35(4):37–44, jul 2016. URL: https://doi.org/10.1111/cgf.12947, doi:10.1111/cgf.12947.
[WMLT07]Bruce Walter, Stephen R. Marschner, Hongsong Li, and Kenneth E. Torrance. Microfacet models for refraction through rough surfaces. In Proceedings of the 18th Eurographics Conference on Rendering Techniques, EGSR‘07, 195–206. Aire-la-Ville, Switzerland, Switzerland, 2007. Eurographics Association. URL: http://dx.doi.org/10.2312/EGWR/EGSR07/195-206, doi:10.2312/EGWR/EGSR07/195-206.