This page provides some details on the settings available for the Fast SSS Material in V-Ray.
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
||Hypershade|| > Create panel > VRay section > VRay Fast SSS2
||Create Render Node window|| > VRay section > VRay Fast SSS2
||V-Ray Shelf|| > Right-click Create V-Ray Materials button > VRay Fast SSS2
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.[ 3 ]. 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.
Diffuse and Sub-Surface Scattering Layers Parameters
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.
Procedural textures plugged in the Diffuse amount must have Alpha is luminance enabled.
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.
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.
The Scatter color is set to beige.
Example: Scatter Color
The Sub-surface color is white.
Example: Scatter Coefficient
This example demonstrates the effect of and the relation between the Scatter Coefficient and the Fog color parameters when the Scatter Mode is set to Scatter Coefficient and Fog Color. The Fog color is set to white for all the images.
Example: Fog Color
This example demonstrates how Fog Color works together with the Scatter Coefficient color. The Scatter Coefficient color is white for all the images.
Example: Scatter Radius and Scale
This example shows the effect of the Scatter radius and Scale parameters. Note how increasing them allows the inside (Scatter coefficient) color to show up more and leads to a softer look.
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.
Phase function = -0.6 (Backward Scattering)
Phase function = 0.0 (Isotropic Scattering)
Phase function = 0.8 (Forward Scattering)
Phase function = -0.5 (Backward Scattering)
Phase function = 0 (Isotropic Scattering)
Phase function = 0.5 (Forward Scattering)
The red arrow represents a ray of light going through the volume; the black arrows represent possible scattering directions for the ray.
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 material uses Color mode: Scatter coefficient and fog color, large Scatter radius and Single scatter: Raytraced (Refractive).
Phase function = -0.9
Phase function = 0.0
Phase function = 0.7
Specular Layer Parameters
Specular color – Specifies the specular color for the material.
Specular amount – Specifies the strength of the specular component for the material. Note that there is an automatic Fresnel falloff applied to the specular component, based on the Index of refraction (IOR) of the material.
Specular glossiness – Determines the glossiness (highlights shape). A value of 1.0 produces sharp reflections, lower values produce more blurred reflections and highlights.
Specular subdivisions – Determines the number of samples used to calculate glossy reflections. Lower values render faster, but may produce noise in the glossy reflections. Higher values reduce the noise, but may be slower to calculate. Note that this parameter is available for changing only when Use local subdivs is enabled in the DMC Sampler Settings. This parameter is not available when the renderer is set to CUDA.
Cut-off threshold – Specifies a threshold below which reflections are not traced. V-Ray tries to estimate the contribution of reflections to the image, and if it is below this threshold, these effects are not computed. Do not set this to 0.0 as it may cause excessively long render times in some cases. This parameter is not available when the render engine is set to CUDA.
Trace reflections – Enables the calculations of glossy reflections. When disabled, only highlights are calculated.
Trace depth – Specifies the number of reflection bounces for the material.
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.
Front lighting – Enables the multiple scattering component for light that falls on the same side of the object as the camera.
Back lighting – Enables the multiple scattering component for light that falls on the opposite side of the object as the camera. If the material is relatively opaque, turning this off will speed up rendering.
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.
Subdivs – Controls the quality of the subsurface scattering effect. Lower values will render faster, but the result will be noisier. Higher values take longer but produce smoother results. Note that this parameter is available for changing only when Use local subdivs is enabled in the DMC Sampler Settings. This parameter is not available when the renderer is set to CUDA.
The Multiple Scattering options from previous versions of V-Ray are no longer supported in V-Ray 4.0. Rendering scenes saved with previous V-Ray versions where those options were available will automatically switch to fully raytraced volume scattering. For example, scenes saved with the Prepass-based or Object-based Illumination map mode will render as if the mode was set to Raytraced. Disabling Multiple Scattering in V-Ray 4.0 renders as if the mode was set to None in previous versions, or in other words this disables the subsurface scattering effect.
Example: Single Scatter Mode
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.
Bump and Normal Mapping
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.
Note: When a texture with color corrections is used as a normal map, V-Ray will display a warning for unexpected results .
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.
References and Links
Here is a list of links and references used when building the VRayFastSSS2 material.
-  H. C. Hege, T. Hollerer, and D. Stalling, Volume Rendering: Mathematical Models and Algorithmic aspects
An online version can be found at http://www.cs.virginia.edu/~jdl/bib/appearance/subsurface/donner05.pdf
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
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 practical 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 https://sites.cs.ucsb.edu/~holl/pubs/hege-1993-vrm.pdf
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.