Graphics Reference
In-Depth Information
(a) Single scattering
(b) Multiple scattering
Figure 4.2
(a) Single scattering involves light paths that have one scattering event below the surface;
(b) multiple scattering involves paths having more than one scattering event.
surface as a participating medium. Subsurface scattering is often modeled by
separating it into two components:
single scattering
, which describes the light
leaving the surface after only a single scattering event, and
multiple scattering
,
which involves several scattering events. This separation was not a new idea when
it was first applied to subsurface scattering. As described in the previous chapter,
similar approaches had already been proposed by Jim Blinn and Jim Kajiya in
the early 1980s in their work that introduced the LTE into CG rendering. By the
time it was applied to subsurface scattering in the 1990s, the volume rendering
equation was formulated and a number of versatile techniques were available for
solving it.
Single scattering (
Figure 4.2(a)
)
can be described as a solution to the volume
rendering equation because each light path is fixed. The solution is obtained by
integrating both sides of the LTE along the light path from the viewpoint to the
light source. For example, Blinn used this scheme to handle single scattering
inside a participating medium having a small scattering albedo [Blinn 82]. Han-
rahan and Krueger proposed a multilayer subsurface scattering model based on
single scattering in 1994 [Hanrahan and Krueger 93]. Multiple scattering (
Fig-
ure 4.2(b)
)
is much more difficult, because light can change direction many times
along a single path. Consequently, simpler alternative methods were employed to
approximate the full volume rendering equation.
The single scattering model was the dominant method for subsurface scat-
tering until the mid-1990s, although its limitations were well understood. As de-
mand for more accurate subsurface rendering grew, interest shifted to the daunting
problem of multiple scattering in subsurface light transport. It remains an active
area of research in computer graphics.
