Game Engine Architecture

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joints within the skeleton, plus weights governing how much infl uence

each joint should have on that vertex’s position.

• Animation clips. Many hundreds or even thousands of animation clips
  are created for a character’s skeleton. These may be full-body clips, par-
  tial-skeleton clips, or diff erence clips for use in additive blending.
  A UML diagram of these data structures is shown in Figure 11.45. Pay
  particular att ention to the cardinality and direction of the relationships between
  these classes. The cardinality is shown just beside the tip or tail of the relation-
  ship arrow between classes—a one represents a single instance of the class,
  while an asterisk indicates many instances. For any one type of character, there
  will be one skeleton, one or more meshes, and one or more animation clips.
  The skeleton is the central unifying element—the skins are att ached to the
  skeleton but don’t have any relationship with the animation clips. Likewise,
  the clips are targeted at a particular skeleton, but they have no “knowledge”
  of the skin meshes. Figure 11.46 illustrates these relationships.
  Game designers oft en try to reduce the number of unique skeletons in
  the game to one, or just a few, because each new skeleton generally requires
  a whole new set of animation clips. To provide the illusion of many diff erent

1

*

1*

1

*

Skeleton

-uniqueId : int

-jointCount : int

-joints : SkeletonJoint

SkeletonJoint

-name : string

-parentIndex : int

-invBindPose : Matrix44

1

*

1*

1

*

Mesh

-indices : int

-vertices : Vertex

-skeletonId : int

AnimationClip

-nameId : int

-duration : float

-poseSamples : AnimationPose

Vertex

-position : Vector 3

-normal : Vector3

-uv : Vector2

-jointIndices : int

-jointWeights : float

SQT

-scale : Vector 3

-rotation : Quaternion

-translation : Vector 3

AnimationPose

-jointPoses : SQT

Figure 11.45. UML diagram of shared animation resources.

11.10. The Animation Pipeline