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Figure 8.75 Setup for measuring reflectance from a “wisp” of hair fibers. (After [Zinke et al. 09].)
was to improve the realism of the color of rendered hair. The authors attempted
to recover the parameters of a specific hair-scattering model adapted from the
Marschner model by measuring the reflectance of actual macroscopic hair sam-
ples. The approach is similar to methods for recovering BRDF model parameters
from measured data (see Section 8.1.5).
However, practical measurement of reflectance from individual hair strands
is difficult. Moreover, it has been established that multiple scattering in the hair
volume has more of an impact on the appearance of hair than scattering from
individual hair fibers. The approach taken by Zinke and his collaborators was
an extension of the approach of the “dual scattering” paper, in which the local
scattering was approximated by a local cluster of hair. Instead of measuring the
reflectance of individual hair strands, the authors adapted the approach to mea-
sure the reflectance of hair clusters as a BRDF. The parameters of the hair fiber
scattering function can be obtained by fitting the hair cluster BRDF to measured
data.
To obtain measurements, a “wisp” of hair is tightly coiled around a cylin-
der, and an image is captured of the cylinder so that its axis is parallel to the
horizontal image axis ( Figure 8.75 ) . The light source is a flash unit attached
to the camera. Because of the curvature of the cylinder, fibers in the coil are
seen from a variety of different longitudinal angles. That is, the variation in
incoming and outgoing angles is primarily in the plane of the axis of the hair
fiber. Consequently, different points in the image capture the reflected radiance
from hair fibers at different longitudinal angles with respect to the light. A sin-
gle image thus simultaneously captures a set of longitudinal reflectance measure-
ments. The fibers are also spread out horizontally over the surface of the cylin-
der, so different azimuthal angles are captured as well. However, the variation
is not as large because the cylinder is flat in that direction, so this variation is
ignored.
The appearance of hair largely depends on the appearance of the specular
highlights. One of the most important features of the Marschner model is the
separation of reflectance into longitudinal and azimuthal components. The color
 
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