Graphics Reference
In-Depth Information
R
TRT
Tilted
cuticle
scales
> 2α
α
2α
Interior:
refrac. index
h
absorption
s
α
2α
< 2α
Surface roughness
b
Elliptical
cross section
axis ratio
α
:1
root
tip
TT
Figure 8.52
Basic geometry of the Marschner hair reflectance model.
(From [Marschner et al. 03]
c
2003 ACM, Inc. Included here by permission.)
Marschner became interested in the scattering properties of hair fibers while
working at Microsoft Research. His colleagues there had been working on real-
time rendering of fur based on the Kajiya-Kay model. A certain lack of realism
in the results got Marschner thinking of ways to improve it.
Later he had the opportunity to collaborate with Pat Hanrahan and Henrik
Wann Jensen at Stanford University, where he became convinced of the impor-
tance of a physically-based hair reflectance model. In their background research,
he and his collaborators came across some relevant papers in the cosmetics liter-
ature that inspired the model published in the 2003 paper.
The Marschner model incorporates several ideas for rendering hair that were
new at the time, but two aspects of the model particularly influenced subsequent
research. The first is the notion of modeling hair fibers as colored dielectric
cylinders having elliptical cross sections. The second is the representation of
the scattering components. Light scattering in a dielectric cylinder has three
strong components or modes, illustrated in Figure 8.52. The modes are named
with a sequence of “R”s and “T”s; “R” stands for “reflection,” “T,” for “transmis-
sion.” The “R” component is thus the first reflection of light off the surface of
the hair fiber. The “TT” component represents the transmission of light through
the hair fiber, and the “TRT” is the result of a single internal scattering event on
the opposite side of the fiber. The authors note that the specular peak is shifted
slightly toward the root of the fiber, which is likely a result of the scaled surface
of hair fibers. The outer surface, or
cuticle
, of a hair fiber makes the fiber look
