Modeling and Animating Human Figures - Computer Animation: Algorithms and Techniques - page 302

FIGURE 9.22
Rotation due to ankle-toe joints.
9.3.2 The kinematics of the walk
Animation of the leg can be performed by appropriate control of the joint angles. As previously men-
tioned, a leg's walk cycle is composed of a stance phase and a swing phase. The stance phase duration is
the time from heel strike to toe lift. The swing phase duration is the time between contact with the
ground—from toe lift to heel strike. The most basic approach to generating the walking motion is
for the animator to specify a list of joint angle values for each DOF involved in the walk. There are
various sources for empirical data describing the kinematics of various walks at various speeds.
Figures 9.23 through 9.27 , from Inman, Ralson, and Todd [ 32 ] , graph the angles over time for the var-
ious joints involved in the walk cycle, as well as giving values for the lateral displacement of the pelvis.
Specifying all the joint angles, either on a frame-by-frame basis or by interpolation of values
between key frames, is an onerous task for the animator. In addition, it takes a skilled artist to design
values that create unique walks that deviate in any way from precisely collected clinical data. When
creating new walks, the animator can specify kinematic values such as pelvic movement, foot place-
ment, and foot trajectories. Inverse kinematics can be used to determine the angles of the intermediate
joints [ 12 ]. By constructing the time-space curves traced by the pelvis and each foot, the user can deter-
mine the position of each for a given frame of the animation. Each leg can then be positioned by con-
sidering the pelvis fixed and the leg a linked appendage whose desired end-effector position is the
corresponding position on the foot trajectory curve ( Figure 9.28 ). Sensitivity to segment lengths
can cause even clinical data to produce configurations that fail to keep the feet in solid contact with
the floor during walking. Inverse kinematics is also useful for forcing clinical data to maintain proper
foot placement.
9.3.3 Using dynamics to help produce realistic motion
Dynamic simulation can be used to map specified actions and constraints to make the movement more
accurate physically. However, as Girard and Maciejewski [ 22 ] point out, an animator who wants a par-
ticular look for a behavior often wants more control over the motion than a total physical simulation
provides (Girard and Maciejewski discussed this in relation to walking, but it obviously applies in many
situations where physically reasonable, yet artistically controlled, motion is desired). Dynamics must
be intelligently applied so that it aids the animator and does not become an obstacle that the animator
must work around. In addition, to make the computations tractable, the animator almost always sim-
plifies the dynamics. There are several common types of simplifications: (1) some dynamic effects are
ignored, such as the effect of the swing leg on balance; (2) relatively small temporal variations are