SSS2 Layer 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. 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.

General Parameters

Scale – additionally scales the subsurface scattering radius. Normally, SSS2 Layer will take the scene units into account when calculating the subsurface scattering effect. However, if the scene was not modelled 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 IOR of about 1.3.

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.

Auto Density – When using 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 is going to be taken for each square unit of the geometry surface. The size of one unit is controlled by C4D scene units set up. 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.

Prepass rate – SSS2 Layer 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 VRAYforC4D 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, SSS2 Layer 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.

Prepass ID –  this option allows several materials with SSS2 Layer to share the same illumination map. This could be useful if you have different materials applied on the same object inside a Vray Blend 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.

  • Single 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.

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 artefacts much like the show samples parameter of the Irradiance Map.

Max Distance – each preview sample is reprsented 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 preview samples present.

Sample Color – the color of the preview samples.

Scatter Parameters

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

Texture – here an image texture or shader can be defined.

 

Sub Surface Color – the general color for the sub-surface portion of the material.

Texture – here an image texture or shader can be defined.

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

  • Sub-surface color + scatter radius – the subsurface effect is controlled with the help of thesub-surface color and scatter color parameters
  • Scatter coefficient + fog color – the subsurface effect is controlled with the help of thescatter coefficient and fog color parameters

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.

Texture Map – here an image texture or shader can be defined.

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.

Texture – here an image texture or shader can be defined.

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.

 

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).

Refraction Depth – Specifies the ray depth for the refractions.

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 LOD – determines level of details that used for prepass.

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.

Example: Scale

Scale = 1

Scale = 10

Scale = 100

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). The Marble (white) preset was used for all images.

Click edit button to change this text.

Example: Sub Surface Color

Sub Surface Color = Red

Sub Surface Color = Green

Sub Surface Color = Blue

Note: The “Happy Buddha” model is from the Stanford scanning repository (http://graphics.stanford.edu/data/3Dscanrep/).

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.

Click edit button to change this text.

Example: Scatter Color

Scatter Color = Red

Scatter Color = Green

Scatter Color = Blue

Note: The “Happy Buddha” model is from the Stanford scanning repository (http://graphics.stanford.edu/data/3Dscanrep/).

The Sub-surface color is kept to green.

Example: Scatter Radius

Scatter Radius = 1.0cm

Scatter Radius = 2.0cm

Scatter Radius = 4.0cm

Note: The “Happy Buddha” model is from the Stanford scanning repository (http://graphics.stanford.edu/data/3Dscanrep/).

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.

The cube in the lower left corner has a size of 1cm.

Example: Phase Function

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

Phase Function = -0.5 (Backward Scattering)

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

Phase Function = 0 (Isotropic Scattering)

Phase Function = 0.0 (Forward Scattering)
More light is absorbed object. 

Phase Function = 0.5 (Forward Scattering)

Note: The “Happy Buddha” model is from the Stanford scanning repository (http://graphics.stanford.edu/data/3Dscanrep/).

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.

Example: Phase Function: Light Source

Phase Function = -0.9

Phase Function = 0

Phase Function = 0.0

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 Raytraced (refractive) mode for single scattering with IOR set to 1.0Front 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.

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 the at this rate, when Scatter radius is 1.0 cm, there are still visible artifacts.

Prepass = -3
Scatter Radius = 1cm

Prepass = -1
Scatter Radius = 1cm

Prepass = 0
Scatter Radius = 1cm

Prepass = 1
Scatter Radius = 1cm

Prepass = -3
Scatter Radius = 4cm

Prepass = -1
Scatter Radius = 4cm

Prepass = 0
Scatter Radius = 4cm

Prepass = 1
Scatter Radius = 4cm

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 the at this rate, when Scatter radius is 1.0 cm, there are still visible artifacts.

Example: Single Scatter Mode

Single Scatter = Simple

Single Scatter = Ray Traced Solid

Single Scatter = Ray Traced Refractive

Single Scatter = Simple

Single Scatter = Ray Traced Solid

Single Scatter = Ray Traced Refractive

Note: The “Happy Buddha” model is from the Stanford scanning repository (http://graphics.stanford.edu/data/3Dscanrep/).

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 0.5 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.