Subsurface scattering
Volume scattering (English subsurface scattering, abbreviation SSS ) refers to the scattering of light in translucent bodies.
Principle
Translucent body are partially transparent. Unlike opaque bodies they therefore reflect incident light not only directly on their surface, but sometimes only after it has penetrated into the matter.
In some articles, the reflection occurs at a layer below the solid surface, such as at the mirror glass which has an almost completely reflective silver layer to a transparent glass layer. Other are a plurality of partially light-transmissive layers are stacked, such as organic substances such as skin and cellulose. Body such as gas clouds are simply not dense enough to act as a solid surface; whether and when the light strikes a reflective particles, is determined by chance. In still other substances, mainly emulsions such as milk, various materials are mixed with different light permeabilities; the light is reflected it back and forth between matter accumulations.
Translucent body break through these effects apparently with the law of reflection, because if one considers the outer surface, so failure angle and angle of incidence are not always the same - even more: A beam of light can occur at one point at an angle with the body and on a completely different place under a completely different angle re-emerge. This effect makes the behavior of light virtually unpredictable. The probability distribution of the exit points and angles of the light rays corresponding to the incoming light beam can be modeled with a BSSRDF.
Importance in computer graphics
Of importance is the volume scattering in the 3D computer graphics, in which one tries to simulate the appearance of materials like skin, marble, milk or clouds as naturally as possible.
To the rendering methods that may be used for simulating scattering volume, one photon mapping. This so called volume photon map is used; this method was introduced in 1998. Also, Metropolis Light Transport, a rendering method for global illumination, can be used for the simulation of volume scattering.
2001 approximation method has been developed that the time required to these methods reduced enormously negligible for many applications differences. This so-called dipole approximation was published by Henrik When Jensen and others in a SIGGRAPH publication and further developed in 2002. 2005, the procedure has been further developed in order to simulate thin objects and multiple, different scattering layers.
A newer, more efficient method for the simulation of volume scattering and other effects are Multidimensional Lightcuts.
Examples of applications of volume scattering, the films Matrix 2, Matrix 3, Shrek and the character of Gollum in the films of The Lord of The Rings trilogy.