Index of /Kuliah2016-2017/KecerdasanBuatanUntukGame

Artificial Intelligence in
Game Design
Representing NPCs as
Finite State Machines

Finite State Machines
• Non-player character in one of several possible
states
– Must be some initial state

• That state controls actions taken by the NPC
• Transitions to other states caused by
internal/external stimuli
Current State

Stimuli

Another State

Stimuli

Another State

Actions in
current state

“Guardbot” Example
start

Chase

Patrol
Move back and
forth in front of door
Energy at 100%

Move towards player
Player visible Energy = Energy - 1
Player not
visible
At door

Energy
below 50%

Return
Move towards door
Energy = Energy - 1

Pac-Man Example

Finite State Machines
• Each state can be reflex agent
Example: “fight” state
Fight
Player swings  Raise shield
Player waits  Swing sword
Player moves  Move towards player

Run
Hit points < 5

Designing FSMs
• What different things does NPC do?
– Types of action taken
• Chase, wander, etc.

– Steps in some process
• Get food from fridge  Cook food  eat food
– Can include “terminal” state
• Object destroyed and deallocated

Return
Dead

Energy == 0

Move towards door
Energy = Energy - 1

Implementing FSMs

• Can treat each state like a C++/Java class
– Possibly derived from some base “State” class

• Typical methods in class:

– Enter()

Executed once when state entered

Example: entering “Fight” state causes NPC to select weapon

– Exit()

Executed before go to another state

Example: exiting “Guard door” state causes NPC to lock door

Implementing FSMs
• Typical methods in class:

– Update()

Executed by game engine each frame
while NPC in this state

Example: reflex actions taken by NPC in “Fight” state

– int CheckTransitions()

• Executed by game engine each frame
while NPC in this state
• Returns ID of the next state to enter based on
current stimuli (same as current state if no change)

Implementing FSMs
class Chase extends State {
int stateNumber = 1; // Patrol = 0, Return = 2
public:
void Enter() {say(“intruder alert”);}
void Exit() {say(“intruder has escaped”);}

void Update() {
moveTowards(player.getLocation);
if (rand() < 0.3) say (“exterminate”);
energyLevel--;
}
int checkTransitions() {
if (energyLevel 10) return 2;
else return 1;
}
}

Emotional FSMs
• States represent emotions for character
• Actions express emotion
• Stimuli change emotional state
Player HP < 10

Confident

Angry

Player hit > 5 HP

Heavy hit
by me

Player hit
> 10 HP

Frightened

My HP < 10

Emotional FSMs
• Can combine with action states appropriate to emotion
– Looks more realistic if orc displays fear before running
Player HP < 10

Confident

Angry

Player hit > 5 HP

Heavy hit
by me

Player hit
> 10 HP

Frightened
My HP < 5

Running

My HP < 10

Emotional FSMs and Personalities
• Can “tweak” parameters for different NPCs
• Differences must be large enough to be noticeable
Player HP < 5

Confident

Angry
Player hit > 1 HP

Heavy hit
by me

Player hit
> 20 HP

Frightened

My HP < 5

Orc with
anger
management
issues

Emotional FSMs
• NPC must clearly express emotional state
– Facial expression (difficult in low resolution)
– Body language
• Posture, motion, etc.

– Sound (speakers must be on)
• Spoken phrases
• Sounds (growl, etc.)

– Abilities
• Strong emotion might make character less accurate!

Emotional FSMs
Confident

Angry

• Smiles
• Shouts insult

• Stands ground

• Growls
• Frowns
• Faster attack
• Less accurate

Fearful
• Backs away
• Grimaces
• Slower attack

Timeouts in FSMs
• Problem: Abrupt transitions in FSMs
Player < 5 feet away

Player >= 5 feet away

– As player approaches, NPC jumps back and forth
between “Walk around” and “Run” states

Timeouts in FSMs
• Solution: State “timeouts”
– Continue high-emotion states for fixed amount of time
after stimulus gone
• Keep running for time even after at safe distance
• Supported by evidence from biology
Stay in running
state for at least
10 seconds even
in player not close

Timeouts in FSMs
class Run extends State {
int timeout;
void Update() {
Flee(player.getLocation());
if (distance(location, player.getLocation()) < 5)
timeout = 10;
// run for 10 frames even after escape)
}
int CheckTransitions() {
if (timeout > 0) {
timeout--;
return 1; // stay in run state
}
else return 0; // go to walk around state
}

Extended “Guardbot” Example
Player
in front

Turning

Player visible

Patrol
Move back and
forth in front of door
Energy at 100%

Player
to side

Forward

Dodge

Player
visible
Player not
visible

At door

Obstacle
in front

Player within
2 units

Fire

Escaped
Move towards door
Energy = Energy - 1

Energy
below 50%

Return
Move towards door
Energy = Energy - 1

FSM Issues
Problems with Finite State Machines:
• Complexity
– Potentially hundreds of states
• Design difficult
• Debugging impossible

• Duplication
– Many blocks of states similar
• Example: Return state also needs “Turn”, “Move”, and “Dodge”

– Many transitions similar
• Example: Need “low energy” transition from all states in “Chase”

Hierarchical State Machines
Single high-level state contains entire low-level FSM
– Need initial state that high-level passes control to
– Need final states that correspond to transitions from
high-level state

Chase
Player
to side

Start

Turning
Obstacle
not in front
Player
in front

Player
to side

Forward

Player not
within 10
units

Escaped

Dodge
Obstacle
in front
Player within
2 units

Fire

Hierarchical State Machines
• Usually implemented as stack
– Push low-level state on stack when enter
– Pop and move to next state when finished
Start

Turning

Chasing

Chasing

Chasing

Guarding
Door

Guarding
Door

Guarding
Door

Escaped
…

Chasing

Escaped

Guarding
Door

Guarding
Door

Exiting Low-level States
• “Interrupts” may cause exit before task is completed
– Example: Caution flag interrupts passing

• All states on stack may have interrupt conditions
– Check all at each frame
– Better than having same transition for all states
Player
in front

Turning
Chasing
Guarding
Door

Energy
< 50%

Forward
Return

Emergency
recall

Go to HQ

Exiting Low-level States
• Can store current low level state and return if
necessary once exception handled
Build Farm
Clear land

Dam break

Build barn

Plant crops

Return to
appropriate
state when
dam fixed

Fix Dam

Weaknesses of FSMs
• Abrupt transitions between emotional states
– Confident  Terrified not a realistic transition
– May need more intermediate states
Confident  Worried  Terrified

• Multiple next states may be indicated by stimuli
– Must make sure all are mutually exclusive
Within 1 unit

Attack

Chasing
Energy < 10

Return