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terms of elementary functions [Jensen et al. 01b]. The paper presents a BSSRDF
model that combines this multiple scattering model with an improved version of
the single scattering model developed by Hanrahan [Hanrahan and Krueger 93].
The resulting model, which is effective yet simple, helped launch subsurface scat-
tering into mainstream movie production and game systems.
4.2 Modeling Subsurface Scattering
4.2.1 Modeling Single Scattering
A multilayer approach for subsurface scattering was used in the field of optics
in the 1970s. In a multilayer model, the material is divided into several ho-
mogeneous layers, each parallel to the material surface. This kind of model
is appropriate for materials that vary in consistency primarily by depth. Hu-
man skin is an example; Figure 4.4 illustrates light transport between multiple
skin layers. Multilayer models were adopted in computer graphics in the late
1980s. However, at the time these models did not properly consider scattering
within the layers because the light transport was described with BRDFs at the
layer interfaces.
The first multilayer subsurface scattering model to properly account for scat-
tering within the media was presented in the 1993 paper “Reflection from Layered
Surfaces due to Subsurface Scattering” [Hanrahan and Krueger 93]. In this paper,
the layers themselves are regarded as participating media; scattering in the layers
and the behavior of light at the interfaces between layers are computed according
to the light transport equation.
L
L
L
i
r , s
r , v
θ i
θ r
θ r
Surface
Epidermis
Dermis
θ t
L
z
t , v
Figure 4.4
A multilayer model for human skin. (After [Hanrahan and Krueger 93].)
 
 
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