General Parameters

Preset - allows you to choose one of several available preset materials. Most of the presets are based on measured data provided by Jensen et al. in [3].

Prepass rate - VRayFastSSS2 accelerates the calculation of multiple scattering by pre-computing 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 interpolated glossy reflections/refractions, for example. 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, VRayFastSSS2 will replace it with a simple diffuse term. This can happen, for example, in objects that are very far away from the, or if the subsurface scattering effect is very small. This simplification is controlled by the Prepass blur parameter.

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.

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

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 a nd 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 co lor - the general color for the sub-surface portion of the material.

Scatter color - the internal scattering color for the material. Brighter colors cause the material to scatter more light and thus appear more translucent; darker colors cause the material to look more diffuse-like.

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 convert it into scene units based on the currently selected system units.

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, moving in the same direction that 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 most strongly affects the single scattering component of the material. Positive values reduce the visible effect of the single scattering component, while negative values generally make the single scattering component more prominent.

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 take longer to calculate.

Specular reflections - enables the calculations of glossy reflections. When off, only highlights will be calculated.

Specular trace depth - the number of reflection bounces for the material.

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 refraction rays are traced as well. 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).

Refraction depth - this 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 the rendering.

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.

Prepass blur - controls whether 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.

Notes

• When using either Raytraced (solid) or Raytraced (refractive) mode for the Single scatter parameter, you need to use VRayShadows for standard lights in order to get correct results.

• 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 one.

• For the reason explained above, VRayFastSSS2 will render as a diffuse one 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.