Artificial Intelligence in
Game Design
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?
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
doesn’t
opens
Try breaking down door Get key
Try unlocking door
Door opens Enter room
Door
doesn’t
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
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?
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)
--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
Hierarchical Structure
•
Can nest to any level
– Usually alternate levels of selection, sequence
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 }
Decorators
•
Used to modify structure of nodes below in tree
•
Common case: looping
– For limited number of times
– Until fail …
Open door
Break down
door
Bang on
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
Set fire to door
Don’t always
try the same
one first!
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
Try
opening door
Try
unlocking door
Barge door
Set fire to door
Try in either order
Get matches
Get gasoline Pour gasoline on door
Ignite 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 any
thread fails
• “Selection” parallelism:
Exit (and succeed) if any
thread succeeds
Quaterback avoids rushers Reciever gets open Rifleman shoots at boss opponent
Grenadier throws grenade at boss
opponent
