This page provides some details on the settings available for the Fast SSS Material in V-Ray.

 

Page Contents

 

Overview


VRayFastSSS2 is a material primarily designed for rendering 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). It is more or less a physically accurate approximation of the sub-surface scattering effect while still being fast enough to be used in practice.  

The VRayFastSSS2 is a complete material with diffuse and reflection components that can be used directly without the need of a Blend Material | VRayBlendMtl material. More exactly, 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

 

 


 

UI Paths:

||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

 

 

 

General Parameters


 

 

 

Preset – Specifies one of several available preset materials. Most of the presets are based on measured data provided by Jensen et al.[ 3 ]. For more information, see the VRayFastSSS2 Presets example below. 

Scale – Additionally scales the subsurface scattering radius. Normally, VRayFastSSS2 will take 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 resets the Scatter radius parameter when loaded, but leave the Scale parameter unchanged. For more information, see the Scale 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.

 


 

Example: VRayFastSSS2 Presets


Skin (brown)

 

Skin (pink)

 

Skin (yellow)

 

Milk (skim)

 

Milk (whole)

 

Marble (white)

 


Ketchup

 

Cream

 

Potato

 

Spectralon

 

 

 

 

Example: Scale

 

This example shows the effect of the Scale parameter. Note how larger values make the object appear more translucent. In its effect, this parameter does essentially the same thing as the Scatter radius parameter, but it can be adjusted independently of the chosen preset. 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 = 5

 

 







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 component.

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

Diffuse amount – Controls the strength of the diffuse component of the material. Note that this value 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 effects such as dust on the surface.

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 – Controls the amount of light scattering in the material. Smaller values cause the material to scatter less light and to appear more diffuse-like; higher values make the material more translucent. Note that this value is always specified in centimeters (cm); the material will automatically take care to convert it into scene units based on the currently selected system units. For more information, see the Scatter Radius 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 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 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 green.

 

 


Sub-surface color
= Red

 


Sub-surface color = Green


Sub-surface color = Blue

 

 

 

 


 

Example: Scatter Color

 

The Sub-surface color is kept to green.

 

 


Scatter color = Red

 


Scatter color = Green

 


Scatter color = Blue

 

 

 

 


 

Example: Scatter Radius


This example shows the effect of the Scatter radius parameter. Note that the effect is the same as increasing the Scale parameter, but the difference is that the Scatter radius is modified directly by the different presets.

This set of images is based on the Milk (skimmed) preset.

 

 


Scatter radius = 1.0cm

 


Scatter radius
= 2.0cm

 


Scatter radius = 4.0cm

 

 

 

 


 

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 (Isotropic Scattering)
More light exits the object.



Phase function = 0.5 (Forward Scattering)
More light is absorbed by the object.


 

 

Phase function = -0.5 (Backward Scattering)

 

Phase function = 0 (Isotropic Scattering)

 


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 are based on the Skin (pink) preset with large Scatter radius and Single scatter set to Raytraced (refractive) with Index of refraction (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 = 0.5

 

 

 


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 that will be 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.
Cut-off threshold – Specifies a threshold below which reflections will not be 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.
Trace reflections – Enables the calculations of glossy reflections. When disabled, only highlights will be calculated.
Trace depth – Specifies the number of reflection bounces for the material.

 

Multiple Scattering Options


 

 

 

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

Raytraced – True raytracing inside the volume of the geometry is used to get the subsurface scattering effect. This method is physically accurate and produces the best results.
Prepass-based illumination 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 illumination 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.   

Prepass rate – VRayFastSSS2 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. An illumination map 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 interpolated glossy reflections/refractions. The prepass rate parameter determines the resolution at which surface lighting is computed during the prepass phase. A value of 0 means that the prepass will be 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, VRayFastSSS2 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 below.

Prepass ID – Allows several VRayFastSSS2 materials to share the same illumination map. This could be useful if you have different VRayFastSSS2 materials applied on the same object. If the Prepass ID is 0, then the material will compute its own local illumination map. If this is greater than 0, then all materials with the specified ID will share the same map.

Auto Calculate Density – When enabled, V-Ray will automatically choose an appropriate value for the Samples per unit Area parameter.

Samples per Unit Area – Controls the resolution of the illumination map by setting a number of samples for each square unit of surface. Higher values make V-Ray take more samples and produce a better result. Available when the Multiple Scattering is set to Object-based illumination map.

Surface 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 the offset from the surface.

Prepass Mode – Similar to the Mode parameter of the V-Ray 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 and then discards it after rendering.
save every frame – Calculates a new map and saves on the hard drive for every frame of the animation.
load map every frame – Looks for and loads a previously saved illumination map for each frame of the animation.
save/load fly through map – Creates/loads a single illumination map for all frames of the animation when only the camera is moving.

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

Preview Samples – When enabled, V-Ray renders an image that displays the samples distribution along the surface of the geometry. It can be used for debugging artifacts much like the Show Samples parameter of the V-Ray Irradiance Map.

Max distance – Represents each preview sample as a circle in the final image. This parameter allows the user to specify the radius of the sample.

Background color – Specifies the color of the geometry where there are no preview samples present.

Samples color – Specifies the color of the preview samples.

 

 


 

Example: Prepass Rate


This example shows the effect of the Prepass rate parameter. To better show the effect, the Prepass blur parameter is set to 0.0 for these images, so that VRayFastSSS2 does not replace the sub-surface component with diffuse shading when there are not enough samples. Note how low values of the Prepass rate reduce render times but produce blocky artifacts in the image. Also note that more translucent objects can do with lower Prepass rate values, since the lighting is blurred anyways. In the examples below, when Scatter radius is 4.0 cm, the image looks fine even with Prepass rate of -1. However, when Scatter radius is 1.0 cm with a Prepass rate of -1, there are still visible artifacts.

 


Scatter radius = 1cm
Prepass rate = -3

 


Scatter radius = 1cm
Prepass rate = -1

 


Scatter radius = 1cm
Prepass rate = 0

 


Scatter radius = 1cm
Prepass rate = 1

 


Scatter radius = 4cm
Prepass rate = -3

 


Scatter radius = 4cm
Prepass rate = -1

 


Scatter radius = 4cm
Prepass rate = 0

 


Scatter radius = 4cm
Prepass rate = 1

 

 

 


 

Options


 

 

 

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 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 highly translucent materials 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.

Single scatter subdivisions – Determines the number of samples to make when evaluating the single scattering component when the Single scatter mode is set to Raytraced (solid) or Raytraced (refractive).

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.

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.

Prepass blur – Determines if the material will use 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 will cause 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 approximation quality 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 values that are too low may produce blocky artifacts on the surface.

 


 

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.

 

 


Single scatter = Simple

 


Single scatter = Raytraced (solid)

 


Single scatter = Raytraced (refractive)

 

 

 

Single scatter = Simple

 

Single scatter = Raytraced (solid)

 

Single scatter = Raytraced (refractive)

 

 




 

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.  

 

Notes


  • VRayFastSSS2 uses the V-Ray prepass system to simulate and interpolate the sub-surface scattering. During other GI calculations (e.g. light cache or photon mapping), the material is calculated as a diffuse.
  • For the reason explained above, VRayFastSSS2 will render as a diffuse with the progressive path tracing mode of the light cache.

 

References and Links


Here is a list of links and references used when building the VRayFastSSS2 material.

  • [1] 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.ucsb.edu/~holl/publications.html
    Defines the basic quantities involved in volumetric rendering and derives the volumetric and surface rendering equations.
     
  • [2] 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).
  • [3] 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).
  • [4] 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.
  • [5] 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/papers/layered/
    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.