Modeling and Animating Human Figures - Computer Animation: Algorithms and Techniques - page 309

affected by external forces. Collisions, friction, and even influences such as static electricity can be
significant. Surface properties of the hair strands, such as wetness, cleanliness, curliness, oiliness, split
ends, and cosmetic applications, affect its motion. In addition, hair can be classified into one of three
main types based on the geometry of a strand: Asian, African, and Caucasian. Asian generally has
smooth and regular strands with a circular cross section while African strands are irregular and rough
with elliptical cross sections. The qualities of Caucasian hair are between these two extremes.
Although the hair's motion is of primary interest for the present discussion, it should also be noted
that providing the user with hair style design tools and the complexities of the rendering of hair are also
difficult problems. All of these are the subject of ongoing research.
Depending on the requirements of the animation, the modeling and animation of hair can take place
on one or more levels of detail. The most common, visually poor but computationally inexpensive tech-
nique has been to merely construct a rigid piece of geometry in the rough shape of the volume of hair
and attach it to the top of the head, like a helmet, as in Figure 9.31 . Texture maps with transparency
information are sometimes used to improve the appearance, providing for a wispy boundary. FFDs can
be used to animate the global shape of rigidly defined hair.
To add more realism to the hair, a hairstyle feature, such as a ponytail or lock of hair, can be ani-
mated independent of the rest of the hair [ 59 ] . This provides some dynamics to the hair at minimal
computational cost. The animation of cartoon hair has even received some attention in the literature,
e.g., [ 62 ].
For more realistic hair, the strands, or at least groups of strands, must be able to move relative to the
rest of the hair [ 37 ]. Clusters of hair strands can be animated as a unit and the hair modeled as a
collection of clusters. Sheets of hair strands can be used ( Figure 9.32 , Color Plate 5) or a generalized
cylindrical collection of strands can be modeled, depending on the type of hair modeled. The strand
clusters can be animated as flexible sheets and held close together by connecting springs. Alternatively,
a few select master strands can be individually animated and then intermediate strands can be
interpolated in order to fill out the hair.
For the most realism, but the greatest computational cost, individual strands can be modeled and
animated separately (e.g., [ 55 ]). Strands are generated using small geometric tubes [ 3 ] [ 47 ] or particle
trails. Individual strands can be animated using the mass-spring-damper system or using the dynamics
of rigid body linkages. Often, strand-strand interaction is ignored in order to keep the computational
cost within reason.
FIGURE 9.31
Hair modeled as rigid geometry. (Image courtesy of Mike Altman.)