Chapter 10
Modular Reuse of
AI Behaviors for
Digital Games
Christopher Dragert, Jörg Kienzle,
and Clark Verbrugge
10.1 Introduction 216
10.2 Background and Related Work 218
10.2.1 Layered Statechart-Based AI 219 Sample AI for a Squirrel NPC 220
10.3 AI Module 222
10.3.1 AI Module Interaction 223 Event-Based Interaction 223 Synchronous Communication 223 Miscellaneous AI Module Properties 224
10.3.2 AI Module Interface 225
10.4 Reuse and Component Integration 226
10.4.1 Event Renaming 227
10.4.2 Associated-Class Connection 228
10.4.3 Functional Groups 229 Group-Private Events 230
10.5 Case Study: Squirrel to Trash Collector 231
10.5.1 Trash Collector Specication 231 Exploring 232 Collecting Trash 232 Using Trash Receptacles 233
216 Computer Games and Software Engineering
Complex and ubiquitous articial intelligence (AI) has become a staple of
modern computer games; players expect nonplayer characters (NPCs) to
intelligently react to player actions while exhibiting appropriate behaviors
depending on their role within the game context. Developing a good AI
for a real-time game environment, however, is a dicult task. Although a
variety of formalisms are employed, practical AI designs typically resort
to strongly customized approaches closely connected to the underlying
game architecture and the NPC type. is results in a relative lack of reuse
in game AI, increasing development costs and requiring the repetitive
development of oen quite similar AI behaviors.
e narrow game-by-game focus is a source of consternation for game
developers. At GDC 2011, Kevin Dill raised this issue, arguing that the
lack of behavioral modularity was stymieing the development of high-
quality AI [1]. Previous work has argued that the fundamental cause for
this lies within the formalisms employed [2]. e applicability of soware
engineering practices becomes limited due to the use of nonmodular cus-
tom approaches. As an alternative, the layered statechart-based approach
[3] provides inherent modularity with nesting capabilities.
Our approach to AI reuse builds from layered statechart-based AI.
Here, we provide a formalism and structure that encapsulates behaviors;
we are able to dene a development strategy that allows the extensive reuse
of dierent AI components, including both high- and low-level elements.
Partial behavioral composition is enabled by the introduction of functional
groups, providing an ecient way to reuse groups of related behaviors.
Clear identication of code dependencies further permits analysis and
the tool-based presentation required to ensure proper integration into the
actual game code. is yields a faster development process with the ability
10.5.2 Building the NPC 233
10.5.3 Case Study Summary 234
10.6 Scythe AI Tool 235
10.6.1 Workow and Key Features 235 Importing Modules 235 AI Construction 236 Outputting the AI 237
10.7 Conclusions and Future Work 237
References 238

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