Graphics Reference
In-Depth Information
(a)
(b)
Figure 4.1
(a) A BRDF models light reflection at a surface point. (b) A BSSRDF models light coming
from a surface including light that has been scattered inside the material below the surface.
(After [Jensen et al. 01b].)
direction. The function that represents this quantity is the bidirectional subsur-
face scattering reflectance distribution function (BSSRDF) ( Figure 4.1(b) ) . A
BSSRDF is a function of eight parameters: a pair of values is needed to represent
each of the surface points, and another pair is needed for each direction (there
are two spherical coordinates for each direction). This made the BSSRDF model
too cumbersome to use for most practical purposes, so the model was simplified
to assume that the entry point and exit point were the same. The result is the
ordinary surface BRDF described in the previous chapters ( Figure 4.1(a) ) .
Subsurface scattering was actively studied in the field of optics during the
1970s. Computer graphics researchers began applying these models in the late
1980s. The complexity of BSSRDFs prevented them from being used much in
computer graphics, so BRDF-based approximations were used instead. This trend
continued until the mid-1990s. Although BRDFs are defined strictly in terms
of surface reflection, they can and do include subsurface scattering, but only in
a limited way. Except for pure mirror reflection, a BRDF is really a kind of
locally averaged BSSRDF. It assumes that a small neighborhood of the surface
around a point is illuminated, and some of the “reflected” light at a point comes
from subsurface scattering in this neighborhood. The limitation of BRDFs in
representing subsurface scattering comes in the lack of control of the size of the
neighborhood.
4.1.3 Single Scattering and Multiple Scattering
Since the 1990s subsurface scattering has been described using the light transport
equation introduced in the previous chapter, by treating the material below the
 
 
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