Sub-surface Scattering is an advanced material property found only in the Advanced Material Editor on the Transparency Page. Using this effect, light is permitted to penetrate the surface of your object and scatter in any direction. Many translucent materials can be modeled using this effect. Certain surfaces, such as stone or skin can be realistically “softened” by allowing the light to penetrate a short distance.
The lighting in nXtRender normally occurs only at the surface of objects. Subsurface scattering is essentially a volumetric technique, where interactions also occur within the medium. It's required to accurately model materials which we think of as "translucent". It's also helpful with materials where the light penetrates the surface a little bit-- such as the marble dragons-- and then scatters on its way out. The feel of the images above would be very difficult to achieve without this technology.
- Sub-surface Scattering requires the Path Tracer to fully realize this effect. It is only partially implemented in the standard engine.
- Scattering controls the probability of the light encountering a particle per unit length. Units can be centimeters (default), meters, or millimters. Higher numbers produce more scattering.
- Scattering Direction controls how the light is scattered. Values > 0 produce forward scattering. Values < 0 produce backward scattering, similar to reflection. The default of 0.0 produces an isotropic scattering pattern which.
- Absorption is the probability of light being absorbed per unit length. Higher numbers produce darker materials.
- The material must have some transparency in order for sub-surface scattering to take place.
- This is a volumetric effect. The objects with this material attached must be solid or “space enclosing” for this to work properly. “Thin” objects should be tagged using Object Properties. Objects tagged as thin will use standard translucency as defined by the Scattering setting (unitless in this case.)