Game Engine Architecture

510 11. Animation Systems

As we saw above, playing a looping animation is like laying down an

infi nite number of back-to-front copies of the clip onto the global time line.

We can also imagine looping an animation a fi nite number of times, which

corresponds to laying down a fi nite number of copies of the clip. This is il-

lustrated in Figure 11.15.

Time-scaling a clip makes it appear to play back more quickly or more

slowly than originally animated. To accomplish this, we simply scale the im-

age of the clip when it is laid down onto the global time line. Time-scaling is

most naturally expressed as a playback rate , which we’ll denote R. For example,

if an animation is to play back at twice the speed (R = 2), then we would scale

the clip’s local time line to one-half (1/R = 0.5) of its normal length when map-

ping it onto the global time line. This is shown in Figure 11.16.

Playing a clip in reverse corresponds to using a time scale of –1, as shown

in Figure 11.17.

Clip A

110 sec

τsta rt = 102 sec

Clip A ...

τ = 105 sec

Figure 11.15. Playing a looping animation corresponds to laying down multiple back-to-back

copies of the clip.

Clip A

τsta rt = 102 sec

τ = 105 sec

Clip A

R= 2

(scale t by 1/R= 0.5)

t= 0 sec t= 5 sec

t= 0 sec 5 sec

Figure 11.16. Playing an animation at twice the speed corresponds to scaling its local time line

by a factor of ½.

t= 5 sec 0 sec

τsta rt = 102 sec

τ = 105 sec 110 sec

A pilC

Clip A

t= 0 sec 5 sec

R= –1

(flip t)

Figure 11.17. Playing a clip in reverse corresponds to a time scale of –1.