Embracing Emergent Behavior with Goal-Based AI

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Embracing Emergent Behavior with Goal-Based AI

• Kevin Dill
• kad_at_bluefang.com

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Blue Fang Games

• To create compelling, emotionally engaging
  games, focused on the animal kingdom, that set
  the benchmark for broad-based family
  entertainment.

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The Plan

• Motivation
• Kohan 2 Demo
• Common AI Techniques
• Goal-based AI
• Boxy-dog Demo
• Questions

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The Plan

• What I Want
• Cunning Opponents
• Responsive Allies
• Replayability
• Organic Credibility
• Emotional Appeal
• Compelling Performances
• Or, more generally
• Situational Awareness
• Emergent Behavior

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Motivation, cont.

• Declarative AI (vs. Procedural AI)
• Wikipedia A program is declarative if it
  describes what something is like, rather than how
  to create it.
• Developers role not to tell the AI what to do,
  but to tell the AI how to decide for itself.

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Techniques Scripted AI

• AI Follows a script, much like a movie
• Script specifies exactly what will happen, when
  it will happen, where it will happen
• Advantages
• Gives designers explicit control
• Easy to write
• Easy to balance
• Disadvantages
• Repetitive and predictable

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Techniques State Machines

• A state is defined for every high-level action
  the AI can take
• Transitions are defined to move from one state to
  another state
• Each state has custom code to control the
  character while in that state

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First Person Shooter Example
Get Ammo
Search For Enemy
Got Ammo
Got Health
EnemyFound
EnemyLost
Get Health
Attack
Got Away,Low on Health
Got Away,Low on Ammo
Low on Ammo
Low on Health
Run Away
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Techniques State Machines

• Advantages
• Easy to conceptualize
• Easy to implement
• Middleware tools exist
• Disadvantages
• Complexity scales poorly as AI grows
• Rampant duplication of code
• Can only do one thing at a time

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Techniques Goal Based AI

• A goal is defined for every high-level action
  that the AI can take
• Each goal is assigned a priority
• Base priority (evaluate current situation)
• Allocate resources
• Optimize resources and calculate final priority
• Highest priority goal(s) are executed

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Goal Types

• Some common RTS goals
• Attack Construct Building
• Defend Repair Building
• Explore Upgrade Building
• Recruit Unit Sell Building
• Recover Health Give Resources To Ally
• Research

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Techniques Goal Based AI

• Advantages
• Flexible and powerful
• Complexity scales fairly well as AI grows
• Can execute more than one goal at a time
• Emergent behavior
• Disadvantages
• Priority calculations (Bucket of Floats)
• Emergent behavior

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Goal Selection Basics

• WWHD (What Would a Human Do?)
• Write down a list of the factors you would
  consider if you were playing the game
• Some common tricks
• Bonuses Repeat Penalties
• Multipliers One-Time Bonuses
• Exponents Actor Categories
• Fuzzy Factors Templates

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Example Attack Goal

• Base priority
• Actor value for the most desirable building or
  unit in the area (taking into account enemys
  economy if appropriate)
• Travel Distance
• Intervening targets (attack goals only)

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Attack Goal, cont.

• Priority Curve
• Target CV ratio
• Min CV to assign
• Low CV exponent
• Max advantage
• Max multiplier
• Other considerations
• Combined arms
• Wall strength
• Adjusting builder CV

CV Combat Value
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Loose Ends

• Goal Commitment
• IF you cant afford something now
• BUT you will be able to soon
• THEN dont perform lower priority goals that use
  the same resource
• Goal Inertia
• Bonus to keep doing what youre doing
• Locked Goals
• Disable all other goals until current goal is
  complete

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Jack the Terrier