Game Engine Architecture

540 11. Animation Systems

• Keep hip rotations to a minimum in the reference clip.


• The shoulder and elbow joints should usually be in neutral poses in the
  reference clip to minimize over-rotation of the arms when the diff erence
  clip is added to other targets.


• Animators should create a new diff erence animation for each core pose
  (e.g., standing upright, crouched down, lying prone, etc.). This allows
  the animator to account for the way in which a real human would move
  when in each of these stances.
  These rules of thumb can be a helpful starting point, but the only way to
  really learn how to create and apply diff erence clips is by trial and error or by
  apprenticing with animators or engineers who have experience creating and
  applying diff erence animations. If your team hasn’t used additive blending in
  the past, expect to spend a signifi cant amount of time learning the art of ad-
  ditive blending.

11.6.6. Applications of Additive Blending

11.6.6.1. Stance Variation

One particularly striking application of additive blending is stance variation.

For each desired stance, the animator creates a one-frame diff erence anima-

tion. When one of these single-frame clips is additively blended with a base

animation, it causes the entire stance of the character to change drastically

while he continues to perform the fundamental action he’s supposed to per-

form. This idea is illustrated in Figure 11.38.

Target +

Difference A

Target +

Difference B

Target Clip

(and Reference)

Figure 11.38. Two single-frame difference animations A and B can cause a target animation

clip to assume two totally different stances. (Character from Naughty Dog’s Uncharted:

Drake’s Fortune.)