The Halo 2 AI In 10 Minutes or Less

Damián IslaBungie Studios

Halo

• First person shooter for XBox• Interesting sci-fi world• Lots to do (combat, exploration, vehicles, weapons)• Lots of decisions to make• Slower-paced More strategic in nature?

The AI

Allies• Not terribly effective• “Kid brothers”• ~10 seconds or ~30 minutes

Enemies• Large variety• Clear behavioral patterns for the player

to learn and exploit• 4 - 30 second lifespans

Single-player campaign only (no multiplayer)

Character-based

The “I” in “AI”• Perception and world-modeling

– Each AI maintains mental world model

– Internal mental object-state updated according to perceivability via the “senses” (e.g., line of sight, sound events, etc.)

– Perception sharing (via explicit communication)

• Behavior– Take cover when taking damage

– Uncover hidden enemies

– Use vehicles when appropriate

– Joint behaviors

• Skills– Use weapons appropriately

– Drive vehicles

– Navigate the environment

Strongly agent-based approach

“Strategy” controlled through scripting

Sense

Plan

Act

Behave

BehaviorDecision-making: Behavior DAG

(HFSM)

Behavior: a program that takes temporary control of the actor in order to get something done.

• Good– Intuitive– Modular– Scalable (more IS better)

• Bad– Memory is hard– Debugging is hard– Never really that simple

Root Self-preservation

Engage Search

Charge

Fight

Guard

Grenade

Cover

Presearch

Uncover

Guard

Grenade

Vehicle

Investigate

Suppressing fire

Grenade

Postcombat Shoot corpse

Check corpse

Idle Guard

Moving, aiming, shooting, animation

Perception and World Modeling

Object states, events

Orders

An ethological interlude…

Root Self-preservation

Engage Search

Charge

Fight

Guard

Grenade

Cover

Presearch

Uncover

Guard

Grenade

Vehicle

Investigate

Suppressing fire

Grenade

Postcombat Shoot corpse

Check corpse

Idle Guard

Baerends, The Functional Organization of Behavior, 1976

What about Reasoning and Planning?

• Spatial Reasoning– Firing point selection

• Path-planning– A*– Dynamic obstacle interaction

… but no concept-reasoning or action-planning

So in the end…

• Second-to-second decision-making• Reactive in nature• Closer to A-Life than classical AI• Closer to animal than to human intelligence