A Game AI Case Study

1
A Game AI Case Study

• Greg Alt
• Surreal Software
• galt_at_eskimo.com

2
The Suffering Prison is HellSurreal Software,
2004

• Hierarchical Behavior System
• Movement Graph and Pathfinding
• Movement and Animation
• Demo

3
Hierarchical Behavior System

• Each NPC has a tree of current behaviors
• Each behavior is a C class
• Behaviors share common interface

Hostility KillEnemies KillObject KillMelee Mel
eeCombatReposition MoveTo FollowPath SimpleMove
To MovementAnim ArriveSteer FlankingSteer
4
Behavior Variation

• Designer-specified constants
• Alternate behaviors for different NPC-types
• BehaviorChangers to change root behavior
  dynamically
• Mixture of scripted and autonomous behavior

5
Its All Fine Until You Get Shot

• External interruptions complicate things
• Hit-Reaction must restore high-level state
• Solution push whole tree onto stack
• Push member function handles invalidated behaviors

6
Movement Graph and Pathfinding

• Nodes 3D squares with scale and rotation
• Edge Regions convex hull of 2 nodes
• Pathfinding A between 2 edge regions

7
Paths Arent Just for Moving From A to B

• Fast pathfinding allowed multiple uses
• Accurate distance check
• Evasion paths
• Burrower flyby behavior
• NPCs leading player

X
X
X
8
Movement and Animation

• Intelligent Movement Animation for NPCs in AI
  Game Programming Wisdom 2
• Steering behaviors are children of SimpleMoveTo
  behavior
• Weighted average of steering accelerations
• Velocity determines translation and animation

9
Conclusion

• Hierarchical Behavior System
• Movement Graph and Pathfinding
• Movement and Animation

10
The Suffering a Game AI Case Study
Greg Alt Surreal Software galt_at_eskimo.com http//w
ww.eskimo.com/galt