Game AI

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Game AI Finite State Machine
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Introduction (1/2)

• Finite State Machine (FSM) is the most commonly
  used Game AI technology
• Simple
• Efficient
• Easily extensible
• Powerful enough to handle a wide variety of
  situations
• Theory (Simplified)
• A set states, S
• An input vocabulary, I
• Transition function, T(s, i)
• Map a state s and an input i to another state

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Introduction (2/2)

• Practical use
• State
• Behavior
• Transition
• Across states
• Conditions
• Its all about driving behavior
• Flow-chart Diagram
• UML (universe modeling language) state chart
• Arrow
• Transition
• Rectangle
• State

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An Example of FSM
seeEnemy
wander
Attack
Win
Dead
Rot
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FSM for Games

• Character AI
• Decision-Action Model
• Behavior
• Mental state
• Transition
• Players action
• The other characters actions
• Some features in the game world

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Implement FSM

• Code-based FSM
• Simple Code One Up
• Straightforward
• Most common
• Macro-assisted FSM language
• Data-Driven FSM
• FSM Script Language

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Coding an FSM Code Example 1
void RunLogic(int state) switch(state)
case 0 // Wander Wander()
if (SeeEnemy()) state 1 if
(Dead()) state 2 break case 1
// Attack Attack() if (Win())
state 0 if (Dead()) state 2
break case 2 // Dead
SlowlyRot() break
What is the problem with the above code ?
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Coding an FSM Code Example 2
void RunLogic(FSM fsm) // Do action based
on the state and determine next input input
STATE_NULL switch(fsm-gtGetStateID())
case STATE_WANDER // Wander
Wander() if (SeeEnemy()) input
STATE_SEE_ENEMY if (Dead()) input
STATE_DEAD break case
STATE_ATTACK // attack Attack()
if (Win()) input STATE_WANDER if
(Dead()) input STATE_DEAD break
case STATE_DEAD // Dead SlowlyRot()
break // DO state transition
based on computed input fsm-gtStateTransition(in
put)
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Mealy Moore Machines

• Mealy Machine
• A Mealy machine is an FSM whose actions are
  performed on transitions
• Moore Machine
• A Moore machines actions reside in states
• More intuitive for game developers

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FSM Language Use Macros

• Coding a state machine directly causes lack of
  structure
• Going complex when FSM at their largest
• Use Macro
• Beneficial Properties
• Structure
• Readability
• Debugging
• Simplicity

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FSM Language Use Macros An Example
define BeginStateMachine define State(a)
bool MyStateMachineStates(StateMachineEvent
event, int state)
BeginStateMachine State(STATE_WANDER)
OnUpdate Wander() if
(SeeEnemy()) SetState(STATE_ATTACK)
if (Dead()) SetState(STATE_DEAD)
State(STATE_ATTACK) OnUpdate
Attack() SetState(STATE_WANDER)
if (Dead()) SetState(STATE_DEAD)
State(STATE_DEAD) OnUpdate
RotSlowly() EndStateMachine
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Data-Driven FSM

• Scripting language
• Text-based script file
• Transformed into
• C
• Integrated into source code
• Bytecode
• Interpreted by the game
• Authoring
• Compiler
• AI editing tool
• Game
• FSM script engine
• FSM interface

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Data-Driven FSM Diagram
Authoring
Games
FSMs
bytecode
FSM Script Engine
Compiler
Artist, Designers, Developers
AI Editing Tool
FSM Interface
Condition Action Code
Game Engine
Condition Action Vocabulary
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AI Editing Tool for FSM

• Pure text
• Syntax ?
• Visual graph with text
• Used by designers, artists, or developers
• Non-programmers
• Conditions action vocabulary
• SeeEnemy
• CloseToEnemy
• Attack

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FSM Interface

• Facilitating the binding between vocabulary and
  the game world
• Gluing layers that Implement the condition
  action vocabulary in the game world
• Native conditions
• SeeEnemy(), CloseToEnemy()
• Action library
• Attack()
• Evade()
• Flee()
• Wander()

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FSM Script Language Benefits

• Accelerated productivity
• Contributions from artists designers
• Ease of Use
• Extensibility