Game Engine Architecture

516 11. Animation Systems

imation authored for one of these skeletons should work on any of them. The

only requirement is that the engine be capable of ignoring animation channels

for joints that cannot be found in the skeleton being animated.

Other more-advanced techniques exist for retargeting animations au-

thored for one skeleton so that they work on a diff erent skeleton. This is an

active area of research, and a full discussion of the topic is beyond the scope

of this book. For more information, see for example htt p://portal.acm.org/cita-

tion.cfm?id=1450621 and htt p://chrishecker.com/Real-time_Motion_Retarget-

ing_to_Highly_Varied_User-Created_Morphologies.

11.4.5. Continuous Channel Functions

The samples of an animation clip are really just defi nitions of continuous func-

tions over time. You can think of these as 10 scalar-valued functions of time

per joint, or as two vector-valued functions and one quaternion-valued func-

tion per joint. Theoretically, these channel functions are smooth and continu-

ous across the entire clip’s local time line, as shown in Figure 11.21 (with the

exception of explicitly authored discontinuities like camera cuts). In practice,

however, many game engines interpolate linearly between the samples, in

which case the functions actually used are piece-wise linear approximations to

the underlying continuous functions. This is depicted in Figure 11.22.

Figure 11.21. The animation samples in a clip defi ne continuous functions over time.

t

Qy 3

Samples

012345678910

Figure 11.22. Many game engines use a piece-wise linear approximation when interpolating

channel functions.