Game Engine Architecture

564 11. Animation Systems

we must use a cascaded binary tree with the additive clips always applied to

the additive inputs, as shown in Figure 11.53.

11.10.4. Cross-Fading Architectures

As we saw in Section 11.6.2.2, cross-fading between animations is generally

accomplished by LERP blending from the previous animation to the next one.

Cross-fades can be implemented in one of two ways, depending on whether

your animation engine uses the fl at weighted average architecture or the ex-

pression tree architecture. In this section, we’ll take a look at both implemen-

tations.

11.10.4.1. Cross-Fades with a Flat Weighted Average

In an animation engine that employs the fl at weighted average architecture,

cross-fades are implemented by adjusting the weights of the clips themselves.

Recall that any clip whose weight wi = 0 will not contribute to the current pose

of the character, while those whose weights are non-zero are averaged togeth-

er to generate the fi nal pose. If we wish to transition smoothly from clip A to

clip B, we simply ramp up clip B’s weight, wB , while simultaneously ramping

down clip A’s weight, wA. This is illustrated in Figure 11.54.

Cross-fading in a weighted average architecture becomes a bit trickier

when we wish to transition from one complex blend to another. As an ex-

ample, let’s say we wish to transition the character from walking to jumping.

Clip A

β 1

Diff Clip B

+

β 2

Diff Clip C

+

Output Pose

β 3

Diff Clip D

+

Figure 11.53. In order to additively blend more than one difference pose onto a regular “base”

pose, a cascaded binary expression tree must be used.

t

w

1

0

tsta rt ten d

wA wB

Figure 11.54. A simple cross-fade from clip A to clip B, as implemented in a weighted average

animation architecture.