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7.5. BRDF Theory 225
Figure 7.16. Light interacting with a surface. On the left, we see the subsurface interac-
tions causing light to be re-emitted away from the entry point. The red and green circles
represent the region covered by a pixel at two different scales of observation. On the
right, all subsurface scattered light is shown emitting from the entry point—the details
of what happens under the surface are ignored.
The BRDF abstracts how light interacts with an object. Section 5.3
discussed the various phenomena that occur: Some light is scattered into
the surface (refraction or transmission), and some light is scattered away
(reflection). In addition, the light transmitted into the object may undergo
absorption and additional scattering, eventually causing some of it to exit
the surface—a phenomena called subsurface scattering. The left side of
Figure 7.16 shows these various types of light-matter interactions.
Figure 7.16 also contains two colored circles that show the areas cov-
ered by a pixel in two cases. The red circle represents a case where the
area covered by a pixel is small compared to the distance between the
entry and exit locations of the subsurface scattered light. In this case, a
BRDF cannot be used to describe the subsurface scattering; instead a more
general equation must be used. This equation is the bidirectional surface
scattering reflectance distribution function (BSSRDF) [932]. The general
BSSRDF encompasses large-scale subsurface scattering by adding incoming
and outgoing locations as inputs to the function. The BSSRDF describes
the relative amount of light that travels along the incoming direction, then
from one point to the other of the surface, then along the outgoing direc-
tion. Techniques for rendering objects that exhibit large-scale subsurface
scattering will be discussed in Section 9.7.
The green circle in the left side of Figure 7.16 represents a case where
each pixel covers a larger area (perhaps the camera is farther away). In
this case, the pixel coverage is large, compared to the scale at which the
subsurface scattering occurs. At this scale reflection, refraction and sub-
surface scattering can be approximated as happening at a single point, as
seen in the right side of the figure.
This approximation enables modeling all the light interaction—including
subsurface scattering—with a BRDF. Whether a BRDF can be used de-
pends both on the surface material (for example, subsurface scattering
occurs over larger distances in wax than in marble) and on the scale of ob-