Artificial Intelligence in Game Design

Artificial Intelligence in Game Design Behavior Trees

Reactive Planning • Plan: Set of steps to accomplish some goal • Move to door  Open door  Enter room • Can easily be done with state machine • Reactive planning: Altering plan if steps not currently possible • What if player locks door? • What if player barricades door? • Key idea in real-world AI

Reactive Planning • Decision-making process like tree • Possible courses of action may require many steps • Decision trees just defined for single step Door opens Enter room Door opens Enter room Get key Try unlocking door Try breaking down door Door doesn’t opens Door doesn’t opens

Behavior Trees • Designed for appearance of reactive planning • Plans, contingencies actually scripted • Combine features of several representations • Decision trees (unambiguous) • Finite State Machines (hierarchical) • Often used to design games (Halo 2, etc.) • Easy to translate into code • Easy for non-programmers to design, understand

Tasks • Leaves of tree = tasks performed by NPC • Conditions: Is something currently true or not? • Internal to character or external in world • Ideally, can be quickly checked against game state Door open? Hit points < 5?

Tasks • Actions: Outputs/instructions to game engine • Internal to character state • External, often in form of firing animation • Can fire other actions(hear player  start path planning routine) Open door Move(room) hit points --

Composites • Define how tasks combined • Sequence node: Execute tasks in given order • Often involve tests at beginning to insure sequence can be completed • Sequence halts and fails if any tasks fail  Door open? Move(room) Close door

Composites • Selector node: Try tasks in order until one succeeds • Multiple tasks with same goal • Like if / else if/else if …/ else • Best if last task always succeeds (like else) ? Open door Break down door Bang on door

Decision Tree Structure • Example: Entering a room • Problem: Player may have closed the door to prevent guard from entering ?   Door open? Move(room) Move(door) Open door Move(room)

Hierarchical Structure • Can nest to any level • Usually alternate levels of selection, sequence ?   ? Door open? Move(room) Move(door) Move(room) Open door Break down door Door open?

Translation into Code • Each task/node is subroutine that returns boolean • Sequence node:for each task T { success = execute T if (!success) return false }return true • Selector node:for each task T { success = execute T if (success) return true }return false

Decorators • Used to modify structure of nodes below in tree • Common case: looping • For limited number of times • Until fail … ? Limit = 3 Open door Bang on door Break down door

Nondeterminism • Characters should not always take actions in same order (appears scripted) • Door locked • Set fire to door: Get matches, get gasoline Barge door Don’t always try the same one first! Set fire to door Don’t always do in this order!

Partial-Order Planning • Understand what steps must happen in a certain order • Other steps can be done in any order • Choose order randomly in those cases Barge door Try in either order Try unlocking door Try opening door Set fire to door Get gasoline Pour gasoline on door Ignite gasoline Get matches Do in any order

Nondeterministic Behavior Tree ? Nondeterministic selector node ~?   Door open? Move(room) Barge door Nondeterministic sequence node ~> Douse door Ignite door Get matches Get gasoline

Parallelism • Controls behavior for multiple characters working towards common goal • Each character has behavior subtree that runs in parallel (usually implemented as concurrent threads) • All characters exit tree when goal succeeds/fails   Charcter 1’s task Charcter 2’s task Charcter 3’s task

Parallelism   • “Sequence” parallelism: Exit (and fail) if anythread fails • “Selection” parallelism: Exit (and succeed) if anythread succeeds Quaterback avoids rushers Reciever gets open ? ?? Rifleman shoots at boss opponent Grenadier throws grenade at boss opponent

Artificial Intelligence in Game Design