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This page provides information on the V-Ray Fast SSS2 Material.

 

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Overview


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V-Ray Fast SSS2 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.

V-Ray Fast SSS2 is a complete material with diffuse and specular components that can be used directly, without the need of a V-Ray Blend Material material. More exactly, 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.

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UI Path: |Shading viewport| > Shader Tree > Add Layer button > V-Ray Materials > V-Ray Fast SSS2


 

General Parameters





Scale – Additionally scales the subsurface scattering radius. Normally, V-Ray Fast SSS2 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. 

Index of refraction – The index of refraction for the material. Most water-based materials like skin have IOR of about 1.3

Opacity – Allows the user to control the opacity of the material with a color or a texture

 

 


 

Example: Scale

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Scale = 1

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Scale = 10

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Scale = 100

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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 with GI to better show the sub-surface scattering. The Single scatter parameter was set to Simple. 

 

 


 

Diffuse and sub-surface scattering layers





Overall Color – Controls the overall coloration for the material. This color serves as a filter for both the diffuse and the sub-surface component.

Diffuse Color – 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 Color – The general color for the sub-surface portion of the material. For more information, see example:Sub Surface Color.

Scatter Color – 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 example: Scatter Color.

Scatter Radius (in cm) – 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 specified always in centimeters (cm); the material will automatically convert it into scene units based on the currently selected system units. For more information, see example: Scatter Radius.

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 example: Phase Function and example: Phase Function: Light Source .

Color Mode – This option allows you to determine which method is used to control the sub surface scattering effect. The available options are:

Diffuse surface reflectance + scatter radius – The subsurface effect is controlled with the help of the sub-surface color and scatter color parameters
Scatter coefficient + fog color – The subsurface effect is controlled with the help of the scatter coefficient and fog color parameters

Scatter Coefficient – This is the subsurface color, just beneath the surface of the material

Fog Color – This is the deep inside color of the object.

 


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Example: Sub Surface Color

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SubSurface Color Blue

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SubSurface Color Green

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SubSurface Color Red

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The following two examples demonstrate the effect of and the relation between the Scatter color and the Sub-surface color parameters. Note how changing the Subsurface 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 RGB [0.96; 0.75; 0.46].

 

 

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Example: Scatter Color

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Scatter Color Blue

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Scatter Color Green

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Scatter Color Red

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The Sub-surface color is RGB [0.74; 0.74; 0.74].

 

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Example: Scatter Radius

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Scatter Radius = 1 cm

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Scatter Radius = 2 cm

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Scatter Radius = 4 cm

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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.

 

 

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Example: Phase Function

 

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

Phase Function = -0.5 (Backward Scattering)

 

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Phase Function = 0 (Isotropic Scattering)
More light exits object.

Phase Function = 0 (Isotropic Scattering)

 

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Phase Function = 0.9 (Forward Scattering)
More light is absorbed object.

Phase Function = 0.5 (Forward Scattering)

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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.

 

 

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Example: Phase Function: Light Source

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Phase function -0.9

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Phase function 0.0

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Phase function 0.9

 

 

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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 default settings.


 

Specular Layer





Specular Color – Determines the specular color for the material.

Specular 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.

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

Specular Subdivs – 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: In order to use the Specular Subdivs parameter, you need to enable the Use Local Subdivs option in the DMC Sampler.

Cutoff Threshold – This is 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 off, only highlights will be calculated.

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

 

Multiple Scattering


The options in this roll out allow you to control the method used to calculate the sub surface effect and the quality of the final result.




Multiple scattering – This parameter controls 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
 – This method 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
 – This method is 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 – 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 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, V-Ray Fast SSS2 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. For more information, see example: Prepass Rate .

Prepass Id – This option allows several V-Ray Fast SSS2 materials to share the same illumination map. This could be useful if you have different V-Ray Fast SSS2 materials applied on the same object for example inside a VRayBlendMtl material. 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.

Prepass Mode – This option controls how the illumination map is used in animation.

New for each frame – A new illumination map is generated for each frame of the animation.
Save map for each frame
– A new illumination map is generated and then saved on the hard drive for each frame of the animation.
Load map for each frame
– In this mode V-Ray doesn't calculate the illumination map and instead loads it from the hard drive. You can use maps that you previously saved.
Save map only for the first frame
– An illumination map is calculated and saved only for the first frame of an animation.
Load map only for the first frame
– An illumination map is loaded only for the first frame of an animation.

 

These options are available when the Multiple Scattering is set to Object-based illumination map.

 

 

 

Auto Density – When this option is enabled V-Ray automatically assigns the number of samples to be used for each square unit of surface on the geometry. Enabling this check box 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 depends on the unit settings for the scene. Increasing this parameter means that more samples are going to be taken which produces higher quality results at the cost of increased render times.

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 that offset.

Preview Samples – When this option is 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 Irradiance Map.

Max Distance – Each sample is represented by a circle in the final image. This parameter allows the user to specify the radius of the sample.

Background Color – This is the color of the geometry where there are no samples present.

Sample Color – The color of the samples.

 

 

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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 V-Ray Fast SSS2 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, whereas at this rate, when Scatter radius is 1.0 cm, there are still visible artifacts.

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Prepass Rate = -3

Scatter Radius = 1

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Prepass Rate = -1

Scatter Radius = 1

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Prepass Rate = 0

Scatter Radius = 1

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Prepass Rate = 1

Scatter Radius = 1

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Prepass Rate = -3

Scatter Radius = 4

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Prepass Rate = -1

Scatter Radius = 4

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Prepass Rate = 0

Scatter Radius = 4

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Prepass Rate = 1

Scatter Radius = 4

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Options



 

 

Single Scatter– Controls how the single scattering component is calculated:

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 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 subdivs – 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).

Note: In order to use the Single scatter subdivs parameter, you need to enable the Use Local Subdivs option in the DMC Sampler. Otherwise, glossiness subdivs are controlled globally, which in most cases produces a good balance between render quality and performance.

Refraction Depth – This determines the depth of refraction rays when the Single scatter parameter is set to Raytraced (refractive) mode.

Scatter GI – Controls whether the material will accurately scatter global illumination. When off, the global illumination is calculated using a simple diffuse approximation on top of the sub-surface scattering. When on, 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.

 

 



Example: Single Scatter Mode

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Scatter GI is Disabled

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Scatter GI is Enabled 

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Single Scatter is Simple

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Single Scatter is Raytraced Solid

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Single Scatter is Raytraced Refractive

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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 2 cm.

 


 

V-Ray Mtl Common


The V-Ray Mtl Common tab includes rollouts like Layer and Bump and Displacement, which include settings that are general among many V-Ray (and Modo) Materials. For more details, please see the Common V-Ray Material Attributes page.

 

Notes


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  • V-Ray Fast SSS2 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 one.
  • For the reason explained above, V-Ray Fast SSS2 will render as a diffuse one with the progressive path tracing mode of the light cache.
  • You can layer several V-Ray Fast SSS2 materials using a V-Ray Blend Material 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.

 


Here is a list of links and references used when building the V-Ray Fast SSS2 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).


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[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 practial 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.