Modeling and Animating Human Figures - Computer Animation: Algorithms and Techniques - page 308

9.4 Coverings
9.4.1 Clothing
It is the rare application or animation that calls for totally nude figures. Simulating clothing and the
corresponding interaction of the clothing with the surfaces of the figure can be one of the most com-
putationally intensive parts of representing virtual humans. The clothes that protect and decorate the
body contribute importantly to the appearance of a human figure. For most human figures in most sit-
uations, cloth covers the majority of the body. Cloth provides important visual qualities for a figure and
imparts certain attributes and characteristics to it. The way in which cloth drapes over and highlights or
hides aspects of the figure can make the figure more or less attractive, more or less threatening, and/or
more or less approachable. For a figure in motion, clothes can provide important visual cues that indi-
cate the type and speed of a character's motion. For example, the swirling of a skirt or the bouncing of a
shirttail indicates the pace or smoothness of a walk.
The simulation of clothing on a virtual human is a complicated task that has not been fully solved,
although significant advances are being made. Real-time application, such as computer games, still
often use virtual humans that sport rigid body armor that merely rotates along with whatever limb
it is attached to. Some applications can get away with simply texture mapping a pattern onto a
single-sheet-defined human figure to simulate tight-fitting spandex clothes. High-end off-line anima-
tion systems are starting to offer advanced clothing simulation modules that attempt to calculate the
effects of gravity as well as cloth-cloth and cloth-body collisions by using mass-spring networks or
energy functions. Streamlined versions of these procedures are finding their way into interactive or
even real-time applications.
The animation of cloth has been discussed in Section 7.5 , but clothing presents special consider-
ations, especially for real-time situations. Collisions and surfaces in extended contact are an almost
constant occurrence for a figure wearing clothes. Calculation of impulse forces that result from such
collisions is costly for an animated figure. Whenever a vertex is identified that violates a face in the
environment, some procedure, such as a transient spring force or enforcing positional constraints, must
be invoked so that the geometry is restored to an acceptable position.
To simulate clothing, the user must incorporate the dynamic aspect of cloth into the model to pro-
duce the wrinkling and bulging that naturally occur when cloth is worn by an articulated figure. In order
for the figure to affect the shape of the cloth, extensive collision detection and response must be cal-
culated as the cloth collides with the figure and with itself almost constantly. The level of detail at
which clothes must be modeled to create realistic wrinkles and bulges requires relatively small trian-
gles. Therefore, it takes a large number of geometric elements to clothe a human figure. As a result, one
must attend to the efficiency of the methods used to implement the dynamic simulation and collision
handling of the clothing.
9.4.4 Hair
One of the most significant hurdles for making virtual humans that are indistinguishable from a real
person is the accurate simulation of a full head of hair. Hair is extremely complex. A head of hair con-
sists of approximately 100,000 strands, each strand a flexible cylinder [ 45 ] . Because the strands are
attached to the scalp and are in close proximity to each other, the motion of a single strand is constantly