Diffuse and sub-surface layers parameters
Specular layer parameters
References and links
Search keywords: sub-surface scattering, sss, skin, wax, marble
VRayFastSSS2Mtl is a material that is primarily designed for rendering of 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.
The VRayFastSSS2Mtl is a complete material with diffuse and reflection components that can be used directly, without the need of a 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.
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 .
Prepass rate - VRayFastSSS2Mtl 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, 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.
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 reset 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 an index of refraction 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 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.
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.
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 specified always in centimeters (cm); the material will automatically take care to 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 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.
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.
Trace reflections - enables the calculations of glossy reflections. When off, only hilights will be calculated.
Reflection depth - the number of reflection bounces for the material.
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.
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.
Cut-off 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.
Prepass blur - controls 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.
The Material ID group is described here
Here is a list of links and references used when building the VRayFastSSS2 material.