Artificial Intelligence In Video Games: A Survey

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17 Ιουλ 2012 (πριν από 4 χρόνια και 11 μήνες)

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In Video Games : A Survey

Thomas Klemensen and Wesley Iliff

University of Northern Iowa


Artificial Intelligence research has seen many uses and
many dead ends. Video games are one of the newest
venues for artificial intelligence research, and many
have found it an excellent way to model human
AI, machine learning, and scripted behavior. In this
survey of the work that ha
s been done in the game
, we examine the pa
st, present, and future of
AI research
. We examine the primary uses for AI in
video games, the research‟s beginnings in board game
systems, as well as modern techniques used to model
computer opponents. Included in these techniques are
ax/Alpha Beta algorithms, decision trees,
scripted behavior, genetic algorithms and learning
agents. We also condense the future outlook of the
industry according to researchers and industry heads,
outlining the impact of new technologies such as multi
re processors and online gaming.

I. Introduction


ames have become big business; s
o large, in fact,
that in yearly earnings the video game industry has overtaken
the movie industry (Laird &

Lent 2001). As the industry
grows, more resources become allocated to the development
of video games. In the growing field of video game design,
artificial intelligence has taken a lead role

topic of choice

for researchers
. End users gauge a game‟s value by the
lity of the AI in the game. If the enemies and allies act
more human and more complex, then the game is more
entertaining. This requires walking a line that is rarely
addressed in other fields of AI, but often seen in popular
culture. AI agents in video

games must be smart enough to
seem competent and even challenging opponents, but must
also operate within the confines of human senses and
reaction times, to prevent the notion that the AI opponent is
somehow “cheating.”

ame AI has several applications

and can be broken
down into several roles an agent can fulfill (Laird & Lent
2001). Enemies need to act at an individual tactical level,

more complex

than approaching
and attacking the player. Groups of enemies need to work
each other to overcome common goals, flanking the
player or working together to trick them. Partners and
support characters must work with the player, anticipating
their needs and supplying relevant tips and assistance. In
sports games commentators must
react to uncertain actions,
and in role playing
games Non
Player Characters (
must drive the story forward based on player input. All of
these problems and more may be addressed b
y us
ing a host
f AI techniques. T
he resources that video game
evelopment has available allows for AI research that may
have implications outside the target field.

Figure 1: Grand Theft Auto IV, featuring a town that

on its own AI routine, had a budget of over $100

In this survey we will explore the history of AI research
in games, its current uses, and its possible future application.
By examining the most prevalent tactics for game AI, we can
see the growing trends in creating the ultimate game AI goal,

l AI. This will also demonstrate why video
games can be a driving force in AI research not only for
video games, but for the field at large.

II. Background

At the start of the interest in AI research, there were no
real video games
so there w
as a strong push to test
a computer‟s “intelligence

This was done

using the popular
games that had been played for years and in some cases, even
centuries. Trying to make a machine that could play chess,
checkers, bridge, poker and many other games that would
seem to
take intelligence to play was
too tempting to pass up.
The work on computer games has been one of the most
successful and

visible results of AI research

and has resulted
in advances

in numerous areas of computing


One of the first
games that grabbed the attention of AI
researchers was chess, when
, in 1950,

Claude Shannon
published his work

on how to program a computer to

(Schaeffer 2001)

It was another

50 years before the
hardware and AI programming techniques ha
d advanced far
enough for

a computer
to be

able to win against a
grandmaster in the game. In 1997, Grandmaster Gary
Kasparov lost a 6 game match to the chess computer DEEP
P BLUE was estimated to analyze

chess positions per second

while a human pl
ayer only

per second

(Schaeffer 2001)

As great a victory
for the AI community as this was, it also proved how great
the human mind is, and how far AI research has to go to even
come close to true intelligence. The score of th
e 6
match was DEEP BLUE: 2 wins, 3 draws and 1 loss,
showing the efficiency of the human cognitive power that
could lose by such a small margin while the margin of moves
analyzed per second was astronomically huge.

Arthur Samuel worked on a checkers
program. B
unlike DEEP BLUE

which used tables for opening moves,
closing moves and an Alpha
Beta search pattern, it focused
on the use of learning algorithms that learned checkers by
playing. It won a single game in an exhibition match against
checkers champion
Robert Ne
aley in 1963. In 1970, a Duke
University research team wr
ote a program that beat Nealey

a short match

(Schaeffer 2001)
. In 1989, a University of
Alberta research team, lead by Jonathan Schaeffer, wrote
playing program called
CHINOOK. In 1990

to World Champ
ion M
arion Tinsley

games to 2.

ut in the 1994 rematch, CHINOOK played all
6 games to a draw. CHINOOK uses alpha beta pruning with
iterative deepening, a transposition table, move ordering,
search ex
tensions, and search reductions

fer 2001)

CHINOOK has the honor of being the first program to win a
human world championship for any game. In 2007,
CHINOOK was declared by the scientists at the Web site of
Journal Science to be unbeatable. The best that an opponent
can ever hope to a
chieve would be a draw. Checkers is now
just a larger version of tic
Players desiring to

CHINOOK can do so at:

r games where computers have come to dominate
the human competition are Othello, Scrabble, Nine Men‟s
Morris, Connect
4, Qubic, Go Moku, and 8x8

witnessing the Scrabble program MAVEN

beat World
Champion Adam Logan, Brian Sheppard wrote

should be moved from the championship caliber
class to the
abandon hope class
” (Schaeffer 2001)

Even though there
are many disadvantages for a computer simulating a game,
some of the games have distinct and even u
nfair advantages.
For instance

the Scrabble program has the entire Scrabble
dictionary programmed into it as data.

Although great strides have been taken for artificial
intelligence for every area of game design, there are some
games that still are not able to play at the master level
, let
alone the grandmaster level. A couple of the reasons for this
are that some of the games have a very high branching factor
or they have too much unknown information. A few of the
titles that still defy computer AI ma
stery are Bridge, Go, and

specially Texas Hold‟em) (Schaefer 2001)

A notable phenomenon that arises when a game joins the
solved category is that researchers seem to lose interest in it.
A been
that attitude appears, and attention is
redirected to the next challenge
. This attitude
keeps driving

AI research ahead


keeps the field fresh and new. As new
games are created, they too benefit from all the research that
has been done and

in many cases, set the bar even


Video game AI is
in its infancy, but great

already being made, and a g
reat majority of the recent
advances in AI have their roots in game design.

Samuel, c
reator o
f the original checkers program,

“Programming computers

to play games is but one stage in
the development of understanding … it seems reasonable to
assume that these newer techniques will be applied to real
life situations with increasing frequency…” (Schaeffer 2001)

III. Learning Agents

Learning Agents

e a focal point of creating human
level AI in video games. If an intelligent agent can observe
and modify behavior based on what it sees the player doing,
the world becomes more immersive, more challenging, and
hopefully more entertaining for the player.

The idea of
evolving NPC AI has been broached for several topics, but
none more prevalently than evolving enemy AI.

The idea of evolving enemies has persisted for decades,
owing to the fact that many games increase difficulty simply
by making enemies mor
e resilient or increasing their number.
This makes the idea of tactically increasing difficulty
immediately more appealing. The foremost room for study
in this kind of learning is in First Person Shooters
(Overholtzer & Levi 2005). As shooting games can

real life combat tactics in use today, there are many places

we can get a baseline of behavior. However, scripting this
directly can only go so far. While the enemies may react like
trained war veterans, the player may act more erratically and
rupt established strategies. This is where an element of
learning comes in.

Overholtzer and Levi demonstrated that real
learning and generational variations
can mark a steady
increase in agent capabilities and challenge
. This was

by implementing a relatively simple genetic
learning algorithm into
open source first person shooter


. While the data set used is fairly rudimentary,
the advancement of gaming
hardware over time shows that
this kind of learning is promising and can be applied. In fact,
these ideas are used in AI today under the blanket term of
ALife (Baker 2002).

ALife is a blanket term used to describe AI


biology. This means

agents may choose from
multiple actions instead of a set script, an idea called „fuzzy
logic.‟ This may also include multiple agents coordinating
together, or genetic algorithms wherein the agents evolve
based upon a heuristic showing which agents have p
best. These genetic algorithms are an important part of the
evolution of video game AI

Learning for learning‟s sake does come with a price

While the end goal of a learning agent within a game may
to create a more believable character or a stiffer challenge,
many games focus so much on the learning aspect that they
become little more than virtual fish tanks.
Attempts to
incorporate this level of learning into an entertaining,
playable game have
been slow, but

NERO is an open source strategy game where learning
takes a key role in
game play

(Stanley, Bryant, Karpov, &
Miikkulainen 2006).
In NERO, players must spend a period
of time training their soldier agents using a given
before sending them against each other. The idea is that the
better army will win over the inferior one. Th
e s
ystem uses a
kind of
algorithm called a Neural Network (rtNEAT,
specifically) to
determine which routines are completing the
ining optimally and propagate those traits as best possible.
This displays the possibility of a real time learning agent, but
NERO does not show the graphical fidelity nor the depth of a
modern blockbuster game. Until processers can handle both
a wealth
of content and real time learning, alternative forms
of learning are being investigated.

Figure 2
: NERO shows impressive technology, but low
graphical quality

Some researchers are finding that behavior may be
learned dynamically, then transformed into

a script and
implemented statically at runtime (Kelly, Botea & Koenig
2008). In an

experiment w
ith the open

role playing
The Elder Scrolls 4: Oblivion
, researchers were able to
use large scale planners outside of the resource
implementation required to plan as the game is playing.
Instead, basic information is fed into a planning algorithm
and the work of creating a script is automated. Creating

unique set of behavior in this way takes, at its longest, thirty
seconds. A human doing the scripting manually, however,
ould take several hours or more

depending on the
complexity of the behavior desired.
This could be widely
applied to games curre
ntly using scripted behavior and allow
for scripts more complex than those we currently have.
While this does not allow for learning as the
game is played
it allows for agents that have already learned before the game

creating a more entertaining
experience for the user.
This may be preferable to many of the existing techniques
used for gaming AI today.

. Simple AI Technologies

Video game AI has not always been a complicated
ordeal. For years, programmers have relied on simple
mechanisms to
allow enemies to react to a player. For a long
time, “artificial intelligence” for an enemy meant very simple
behavior, such as approaching an enemy in a straight or
straight pattern. The classic arcade game

featured constructs that could only

the player in a short, repeated pattern, and

featured a
paddle that would simply detect the trajectory of a ball and
attempt to intercept (Wexler 2002).

Until the past decade or so, game AI made few

to progress past the charging enemy. Other forms of games,
however, had been setting the stage for more complex
Perhaps the most famous example is D

. By calculating hundreds of millions of moves per
second to a human‟s two,

managed to outwit
the champion by sheer brute force (Schaeffer 2001).

The most common form of this algorithm seen in game
playing is minimax. In a mini
max algorithm, an AI agent
recursively calls an optimal opponent to see what their best
move would be, whom calls the original agent to check their
move, until a decision is reached. This creates a tree
structure that allows the agent to decide the best m
available in its current state. This is recalculated each move,
allowing the agent to make the best possible move at all
times. This works well in a situation where the agent can
observe the entire world and all possible actions, such as in a
board g
ame. While this approach has been used in other
video games before, the size of the tree created can grow
exponentially and the computational time can swiftly get out
of hand.

As most video games must perform in real time,
this algorithm is increasingly
out of place in a gaming
landscape populated by fast
paced action games and swiftly
tactical first person shooters.

As technology has only recently found itself up to the
task of dynamically computing any behavior in real
time, the
most common form of vid
eo game AI for the past two
decades has been scripted behavior (Baker 2002). Scripted
behavior involves giving a computer opponent a set of
possible player inputs and a set of reactions to that input, so
that the opponent may know how to react to any give
n player
action. This allows the oppon
ent to react swiftly to stimuli
as it only needs to look up its action in some sort of table.

Figure 3: An example of scripted behavior

Scripted behavior is not without its detriments
. As

within games get more complex, it becomes
more and more difficult to maintain a reaction for every
possible player action. As a player finds failings in a script,
the player can exploit these reactions and make the game far
too easy. In addition, these
failings often ruin immersion in a
game world. A player who finds an opponent running
headfirst into a wall due to an overlooked scripting loophole
becomes acutely aware of the fact they are playing a game
with non
human opponents.

This issue can be
addressed reliably by incorporating
decision trees into existing scripts (Wexler 2002). While
scripts are necessary when true learning AI is too taxing on a
system, the behavior of an agent can be supplemented by
building a tree of behaviors based on the
success of previous
actions in similar situations. In this way, an opponent can
run on scripted behavior, but deem parts of the script a poor
idea after it fails to deliver the desired results too often. This
allows the heavy computing to be done with a
manual script,
but incorporates the leeway of a learning system to cover
loopholes in logic and inhuman behavior. While these
decision trees may be incorporated statically, as with a script,
the growth potential of using one in concordance with an
g script has been found to yield greater benefits.

Figure 4: A simple decision table for the game
Black &

Scripting and decision tables have carried the game
industry for the better part of two decades. With limited
player inputs and relatively isolated gaming experiences,
developers could make easy assumptions

about what the
player would do

and create oppon
ent behavior accordingly.
However, games are getting more complex. A player in
today‟s gaming landscape can interact with a game world
almost as intimately as they can with the real one. In
addition, online play has opened up communication between
rs, allowing AI exploits to be discovered significantly
more easily than in years past. As players find ways to break
the intended systems, the need for agents that can learn has
grown. One of the approaches at the forefront of the
movement for video gam
e learning agents is the concept of
genetic algorithms.

V. Genetic Algorithms

Genetic a

intend to copy the mechanism by
which nature finds the species most likely to survive, creating
opponents that grow and learn organically. This allows a
mputer opponent to replicate the learning undertaken by
human players.

The concept for a genetic algorithm is simple, and to
demonstrate it we will examine the typical use of a genetic
algorithm in a video game. Genetic algorithms involve
making several
agents compete

and allowing traits of the best
s to carry o
n to the next match. This weeds

out the
worst agents and keeping the best. The algorithm starts out

with a sample of agents

little instruction as to
what it needs to do other
than a heuristic on which to judge
its performance. In gaming, this may mean several agents
with little other than finite state machines mapping their
possible actions (Geisler 2005). These agents are given
random twe
aks to their poss
ible behaviors

, how
often an agent jumps when presented with a wall
agents are then set off to perform their tasks; within video
games this often means the agents play competitive games
against each other. While there is no set method for keeping
the best

performers, there is a notion of the top several
staying and some of the worst performers being replaced by a
breed of the top performers. This “child” of the best
performers of the last round is often given a random and
minor change, to allow for
continuing evolution of the
best performer.

Figure 5: A graph showing the evolution of a single
behavior for a genetic agent over fifty generations.

Several studies have been conducted showing that, over
time, a consensus is found on single behaviors after a point.
This usually shows roughly the optimal choice for a behavior.
In figure 5, a simple agent in a first person shooter is tasked
with determ
ining whether to jump over a wall when
approaching an enemy (Overholtzer & Levy 2005). After
roughly 40 generations, the algorithm finds that about
thirteen in every fifteen situations, jumping is preferable.
Even if this behavior is not integrated direc
tly into the
behavior of an agent, this algorithm shows what decision is
optimal in a given situation and can be incorporated statically
into a script in a finished product.

VI. Neural Networks

artificial intelligence research and programming, we
constantly run into things that are either incredibly difficult
or near impossible to represent on a computer. Oddly
enough, as programmers and software designers, we
take this
for granted and are
surprised when it comes up.

The thing
that makes this odd is that the problems that arise the most
often are usually things that we do naturally every day
without thinking about it. We recognize hundreds or even
thousands of faces, read writing in vario
us scripts, even if
part of the message is missing or destroyed. We can talk and
understand communication in verbal and even non
modes. The question that still needs an adequate answer is,
“What is the difference between computers and humans?”


neural network is modeled after the neurons in the
human brain. It can be thought of as a mathematical function
approximator where the input represents independent
variables and the output represents the
dependent variable or

(Bourg & Seemann 2
004). In

the human brain,
there are
10 billion neurons with 60 trillion connections

the mat
hematical function model does not

do justice to the
vastness of the human brain, but it is adequate for the current
capabilities of computers.

In the realm of

video games, neural networks are one of
the most exciting aspects on the horizon. From a single
player adventure game or first person shooter to a massively
multiplayer online role
playing game, the application
s for
artificial intelligence are

almost endless.

techniques can increase both the longevity of video games as
well as

decrease their production cost
(Stanley, Bryant &
Mikkulainen 2005).

Neural networks allow a game designer to set up scripts
and game agents that can act in a r
ealistic manner and even
challenge the most skilled
of gamers. In first person shooters,
neural networks can allow the computer controlled opponents
to learn as they go and get better. They will learn from
mistakes that they make to avoid repeating them,

as well as
learning strategies that are more effective against a particular

One problem that can arise from having an agent
actually learn while the game is being played is that the game
can become unpredictable. Another problem is that if the
me agent
s are allowed to learn too well

or too much, it is
likely that the game will become too hard or even
“unbeatable” and no one will want to play it. This is why the
more common approach to neural networks is to train the
agents outside the game and

then transfer the now static

learned network into the game.

The general algorithms for neural networks are normally
not well suited for a real
time game environment, because



arge state/action space

king it difficult
to check the many different possible actions
and checking the value of every possible
action on every game tick.


Diverse behaviors

every agent should not
settle on the same action as this would make
for a very boring game if every agent

did the
same thing.


Consistent individual behaviors

a player
doesn’t want to see agents in the game doing
something totally random for no reason,
because it destroys the illusion that the game
is a real word.


Fast adaptation

the game agents must be
le to adapt quickly as players would quickly
become bored

and/or frustrated if it took hours
before an agent adapted.


Memory of past states

agents should be able
to use past events to help to choose a better
action in the present.

ley, Bryant & Mikkulainen 2005)


a real
time game e
nvironment, the typical neural
network algorithms must be tweaked to update at a regular
interval, usually corresponding to a set number of game ticks.
One implementation of neural networks for real time video
games is rtNEAT (real
time NeuroEvolution of

NERO (NeuroEvolving Robotic Operatives) is

video game that was specifically wri
tten to explore the
potential of


The object


train a group of agents

military combat
by exposing them to experiences in a
controlled environment.
The agents begin with no skills at
all and
after the agents are trained to the satisfaction of the
player the

can be pitted against other groups of agents that
have been similarly trained. The

training is performed in an
interface that allows the player to reward or punish different

(See figure 6).

Figure 6
control pane used to set rewards
and punishments for all the actions that an agent can

The actions that can
be trained are

actions like

approaching enemies, hitting targets,
getting hit, following
friends, and
. So

could train
his or her

to charge the enemy and fire, avoid the
my and shoot
from a distance. One could

even train half the agents for one

and half for the other.

The control panel also allows the placement of objects
like walls, turrets (a stationary gun that shoots at the agents),
and static enemies.

One of the experiments that

done with NERO shows
the vast potential of Neural Networks. In this experiment,
the agents who spawn at one end were rewarded for going to
the other end of the map.
During e
ach training session, a
wall was put in the path of the agents so they
had to go

it. Each time a wall was added

the path to go around
the wal
ls became more and more complex

until a virtual
maze was create
d. The agents were able to tra
verse the maze
and make it to the other side wi
thout the aid of path

(Stanley, Bryan
t & Mikkulainen 2005).

. Future Outlook

While artificial intelligence in video games has started as
a business venture, some look to video games as an ideal
topic for generalized AI research. As video games strive to
make opponents that react like h
umans, it is a natural fit for
developing the kind of human
level AI dreamed of in science
fiction (Laird & Lent 2001).

There are several reasons some argue that video games
are an optimal venue for future AI research. First, funding is
abundant. Video
games are not just research reliant on

they are a self sustaining and highly profitable industry. If
only one idea every year of research gets put into practice,
this may still be enough for game companies to continue
giving money to AI research departments. Hardware is also
an increasing fa
ctor. Real
time, learning AI requires a large

amount of processing power. Luckily, today‟s video game
consoles and personal computers have multiple core systems,
and have taken to using separate hardware to take care of
graphical work. This means that
entire processers may be
dedicated solely to the artificial intellige
nce al
nning in games. Some speculate

that future consoles will
include dedicated AI logic units that can take care of basic AI
functions for the programmer (Snider


The fact that these agents are worki
ng out with a large
sample set

many games today sell mill
ions of copies in
America alone

allow for interactions that teach us more
about how AI agents work. Damian Isla of developer studio
Bungie speaks of hi
s experiences with complex AI systems:

The interaction of all those rules is

unpredictable. There is simply
no way that I as a programmer can predict
what is going to happen next. What we get
is „emergence,‟ one of the holy grails of AI:
me really complex, interesting results out
of very, very simple rules.

(Snider 2007)

Emergence is one of the cornerstones of human level AI.
A system that can act in ways that the programmer did not
intend shows some degree of independent thought, wheth
we think of this as conscious or simply reactions. Players
react positively to
an unpredictable opponent, provided that
there is some logic that can be gleaned from that
unpredictable behavior. Man
y „god games,

wherein the

takes an omniscient

role over subjects that the player
has no direct control over, strive for these kinds of
reactionary yet independent actions from their agents (Laird
& Lent 2001).

One of the signs of
possible research slowdown within
the industry, however, is the advent

of online gaming. In
today‟s gaming landscape, it is easy for players to use a
connected personal computer or video game console
to play games with or against players at distance, at any
Schaeffer 2001)
. The easy access of real humans

to t
player lessens the desire to play a multiplayer match against
human opponents, no matter how realistically they react.
While this may slow research somewhat, single player
campaigns remain a staple of any large
budget game. In
addition, modes wher
ein players can cooperate online against
AI opponents have seen a swell of popularity with recent
games such as Epic Games introduction of a cooperative
Horde mode in
Gears of War 2
, wherein human players take
on increasin
gly difficult swarms of enemies

nd the fully
cooperative online survival game
Left 4 Dead
by Valve.
These changes in the online landscape may offset the
possible loss of AI resources as research needs to be made
for these cooperative games.

The real future of video game artificial inte
lligence is in
continual growth. While some disciplines need a large
breakthrough to continue research, video games give a
suitable environment for incremental growth of AI
sophistication (Laird & Lent 2001). Game companies may
not be able to dedicate la
rge amount of resources per game to
AI research and developmen
t, but the number of games
released each year is steadily increasing, as are the budgets of
those games. The resources going into game development
should see
some growth in the AI rese
arch and development
portion of game development, making it an increasingly
attractive field for AI researchers.

. Conclusion

For decades of popular culture, humans have

interested in
artificial humans, or inanimate objects
that are seemingly i
Our m
ythology, writing,

movies and even
thoughts are full of references to
magically constructed automatons, talking mirrors, and

When comput
ers came along, it was only a matter of
time before

tried to project our own humanity into it.
There are so many areas that could be used as a testing
ground for artificial intelligence research, but

games have

quickly become

the chosen m
, because new
AI algorithms can be tested without
risking safety or vital
systems when tested in a game space

is much safer
better than testing it in areas where a
malformed algorithm could be high in cost of life and
, as well as financial loss

A great number

the major advances in artificial
intelligence were born in game design before they were used
in real world applications.

Alpha beta algorithms, minimax,
and advances in human level AI are just a few of the

important AI advances in the past several decades, and all
were borne of game design.

As the industry and budgets
increase, the chances of new breakthroughs increase
exponentially alongside them.

Many researchers are even calling for more research
the field. Michael Buro argues that real time strategy games
(RTS) have a wealth of opportunities for research that are yet
. C
urrent levels of RTS intelligence are low, but
the number of agents and the number of actions is high

. Laird and Lent insist that video games are the
premiere showcase for human level AI (Laird & Lent 2001).
And nearly every algorithm shown in this paper is intended
only to be
an early stage of

their kind of research,
and source
code is made available for researchers wishing to expand
upon it.

What could lie just over the horizon? What new
algorithm is waiting to change the world as we know it? Will
the rapid advances in medicine, hardware engineering and
software technology open doors that we can only dream of?
While nobody

knows for sure, the possibilities
invigorating researchers and the future will certainly be
exciting to see.


Baker, Tracy. “Game Intelligence: AI Plays Along.”

Jan. 2002.

Bourg, David M. and Glenn Seemann. “
Four Cool Ways to
Use Neural Networks in Games.”
On Lamp.



Buro, Michael. “Call for AI Research in RTS Games.”
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on for Artificial Intelligence

Geisler, Bob. “Integrated Machine Learning For Behavior
Modeling in Video Games.” Radical Entertainment (2005):.

Kelly, John
Paul, Adi Botea and Sven Koen
Planning with Hierarchical Task
Networks in Video Games.”
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