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The Two Faces of Video Game AI
Damian IslaAI Lead, Bungie Studios
Halo 3
• First person shooter• Epic Story• Epic Combat• Vehicles• Physics• Cooperative and Competitive Multiplayer• Saved Films• User-edited Levels
• Player-equivalent AI– in terms of abilities, not in terms of tactics!
The Two Faces of AI
• One outwards, towards the player• One inwards, toward the content creators• Two interfaces with completely different, at
times contradictory functions
This talk• General properties of these two interfaces• Two examples– Behavior selection– Combat dialogue
• Conclusions
Terminology
Game AI must be:
• Coherent• Transparent – for the player– for the
designer
• Workable
Player Interface
The Illusion of Intelligence
AI seems smart, focused, lifelike– timing seems right– decisions seem reasonable– attends to the things that demand attending
to– In Halo: the AI is a worthy opponent
How do we sell the illusion of life and intelligence?
The Fun: Transparent Decision-making
• Player can explain AI behavior– “He dove out of the way because I threw the grenade.”
• Player can predict AI behavior– “If I throw the grenade, I bet the AI will dive out of the
way.”
• Player forms an ongoing narrative in his/her head– “I threw the grenade, so he dove out of the way, and
then cursed at me, and threw a grenade back, but I shot it in the air and hurt him, so he went nuts and charged me …”
• The AI needs to facilitate / encourage that narrative
Designer Interface
Transparent Decision-making
• Designer can explain AI behavior– “He dove out of the way because his danger_dive
behavior is active.”• Designer can predict certain AI behavior
– “When I throw this grenade, there’s a 75% chance he’ll dive out of the way”
• Designer knows how to get different behavior– “Hmm … he reacted to that grenade too quickly
… I think I’ll increase his projectile-acknowledgement delay from 0.5 to 0.7 seconds.”
• Designer knows how to diagnose and fix MISbehavior– “WOAH … why the hell did he do THAT?!”
Scale
The Enemy of all Transparency
x 1,000
“I want X to do Y when Z”
Scale
The Enemy of all Transparency
The greater the variety of unique situations we need to handle well, the more complex the internal structure needed to disambiguate them, and the less expressible that internal structure is.
(We might also call that “competence”.)
Transparency vs. Competence
Case Study 1: Behavior Selection
Hide?Panic?Get in that vehicle?Throw a grenade?Hold my ground?
Behavior
A program that takes temporary control of an actor to achieve a specific goal.
• Fight• Hide• Enter vehicle• Throw grenade• etc.
How do I decide what to do when?
~120 of these in total
(i.e. ~ 7140 possible
transitions)
Behavior FSM
Behavior FSM
???
Behavior Tree
melee
shoot
grenade
uncover
pursue
sleep
fight
search
hide
idle
root
cover
root
fight
shoot
Behavior Tree
Three Simplifications:1.No sibling connections2.Prioritized-list decision-
scheme3.Binary relevance functions
Benefits:• No more n2 complexity• Scalability
Behavior Tree: Designer Tools
1. Behavior Parameters
2. Masks
bool hide_relevance() {
if (damage > MAX_DAMAGE) {
return true;}else {
return false;}
}
Players and Trees
Players understands TRIGGERS, not TRANSITIONS– i.e. when the brute’s armor depletes, he charges,
no matter WHAT he was doing at the time
Players understand priorities
Players understand discrete events
Players understand task-decomposition
The Tree is representative of the mapping we HOPE exists in the player’s head
Lessons from Halo 2 and Halo 3
• Figuring out why something happened is easy
• Figuring out why something DIDN’T happen is still fiendishly hard
Case Study 2: Combat Dialogue
Oh no, they killed
my friend!
Well done, friend!
Well done, Master Chief!
Ha! They killed
their own guy!
You idiot! You killed our own
guy!
Oh no, I killed my own guy!
I’m sorry!!!
Damn you,
Master Chief!
Aaargh!!!
Combat Dialogue
<A> kills <B> “Event”“Oh no, they killed our friend!”
“Vocalization”
Match ~100 events ~320 vocalizations(and find a speaker)
Combat Dialogue
Implementation: a rule-matching systemEvent: “Player killed a grunt”
killed(a,b)prs_plr_
killenemies(a, b),
player(a)friend(a)
event vocalization
relevance conditions
speaker condition
s
0.8
Combat Dialogue
killed(a,b)prs_plr_
killenemies(a, b),
player(a)friend(a) 0.8
Probabilistically choose among top contenders
killed(a,b)prs_ally
_killenemies(a, b) friend(a) 0.5
killed(a,b)crs_plr_
killenemies(a, b),
player(a)enemy(a
)0.5
killed(a,b)crs_plr_
btryfriends(a, b),
player(a)friend(a) 0.0
Implementation: a rule-matching systemEvent: “Player killed a grunt”
Combat Dialogue
killed(a,b)prs_plr_
killenemies(a, b),
player(a)friend(a) 0.8
Probabilistically choose among top contenders
killed(a,b)prs_ally
_killenemies(a, b) friend(a) 0.5
killed(a,b)crs_plr_
killenemies(a, b),
player(a)enemy(a
)0.5
killed(a,b)crs_plr_
btryfriends(a, b),
player(a)friend(a) 0.0
Implementation: a rule-matching systemEvent: “Player killed a grunt”
Lessons from Halo 1, 2 and 3
• Player is an egomaniac– Brute: “He’s gone into cover!”– Player: “uh … no I haven’t.”
• AIs need to speak consistently with each other– Marine1: “I see him!”– Marine 2: “I don’t see him!”
• Combat dialogue serves fundamentally different functions for enemies and allies– Allies are about aural backdrop– Enemies are about information
Combat Dialogue
• Use the appropriate tools– Excel has its place
• Expected results were extremely “soft”– target was a “narrative”– target was negotiated between design and
audio
• Data-mining was absolutely critical
But ... where’s all the real AI?!
Real AI
What about• Planning• Inference• Genetic
approaches• Learning• etc. …
• F.E.A.R (Monolith)– STRIPS Planning
• Black & White (Lionhead)– Decision-tree
learning– Gesture recognition
• Euphoria (NaturalMotion)– NN + GA
Conclusions
Technique Summary
• Behavior Tree• Rule-matching
Transparency in decision-making is important
Two parts to transparency:1.Transparent algorithm2.A tools infrastructure for editing and
debugging
Conclusions
• There are two important video game AI interfaces: one out towards the player, the other in, towards the content creators.
• Making them agree is hard.
• The Holy Grail:
A WYSIWIG AI Editor???
