Graphics Reference
In-Depth Information
4
Subsurface Scattering
4.1 Subsurface Scattering
4.1.1 What is Subsurface Scattering?
The general BRDF approach to modeling surface light reflection implicitly as-
sumes that light is reflected immediately at the intersection point. In reality, how-
ever, some of the incident light penetrates the object, bounces around below the
surface, then emerges at some different point. This behavior is called
subsurface
scattering
. While most materials in the real world exhibit some amount of subsur-
face scattering, it is a significant part of the appearance of translucent materials
such as marble, milk, and skin.
Various models for subsurface scattering have been proposed. These models
typically split the behavior into two parts: pure surface reflection and scattering
below the surface. However, the use of physically accurate subsurface scattering
models for practical rendering had long been avoided, due to the high computa-
tional cost. This situation has changed recently with the increasing demand for
more realistic rendering of natural materials, especially in the motion picture in-
dustry. Subsurface scattering has been identified as a key component of realism,
so emphasis has been placed on its practical implementation. More efficient mod-
els with greater flexibility have been actively studied.
4.1.2 BRDF and BSSRDF
In optics, light reflection at a surface was first modeled in terms of a surface en-
trance point and a surface exit point. The scattering is measured as the ratio of
the radiance exiting the surface at the exit point in a particular direction, to the
irradiance
caused at the entry by light at the entry point from a separate incoming
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