exclusively battling human opponents. InMarathon 2, however, players are battling a
variety of alien species mixed with some robots. The enemies inMarathon 2are able to
get away with appearing stupid since they are non-human creatures. InDamage Incor-
porated, conversely, since the enemies are all humans they must look much smarter.
For another example, inDamage Incorporated, according to the game’s story and the
appearance of the levels in the game, the action is supposed to be transpiring in a
real-world environment. On the other hand,Centipede 3Dtakes place in a whimsical
fantasy world that bears only a tangential relationship to the real-world. Therefore,
while the guards inDamage Incorporatedneed to appear to be tracking players like real
human soldiers would, inCentipede 3Dit is less absurd that the centipedes are unable to
make a beeline for the player character and instead have to wind back and forth
between mushrooms. AI stupidity is acceptable relative to the type of world the com-
puter game is supposed to represent.
Be Unpredictable.............................
Humans are unpredictable. That is part of what makes them good opponents in a game
and one of the primary reasons that people enjoy playing multi-player games; a skilled
person will be challenging to fight in a way that a computer never will. A large part of
that is the unpredictability of a human opponent. The same should be true of the AI
opponents in a computer game. When the game gets to the point where players feel
with certainty that they know exactly what the enemy forces are going to do at any
given second, the fun of playing the game quickly wanes. Players want the AI to sur-
prise them, to try to defeat them in ways they had not anticipated. Certainly
multi-player games still have the advantage of including a social component, which is a
major factor in their success, and the AI in your game will never be able to be a friend to
players in the same way another human can. Since you cannot provide the social com-
ponent of multi-player games, you can at least strive to make the AI agents provide
much of the same challenge and unpredictability that is provided by a human opponent.
In all art, viewers want to see something they have not been able to anticipate,
something that challenges their expectations. When, within the first ten minutes, you
know the exact ending of a movie, book, or play, a big part of the thrill of experiencing
that work is removed. The same is true for computer games. Of course, games can sur-
prise players with their predetermined story, or what sort of environment the next
level will take place in, or what the big boss robot will look like. But if the AI can also
contribute to this unpredictably, the game gains replayability. Players will keep playing
a game until it no longer provides them with a challenge, until they no longer experi-
ence anything new from playing the game. An AI that can keep surprising them, and
thereby challenging them, will help keep their interest high.
Successful unpredictability can take many different forms in games. It can be as
simple as the random number that determines what piece will drop next inTetris.
Surely this is a very simple case, and optimally we would hope many games could pro-
vide deeper unpredictability than that. But at the same time, one must realize that for
Tetris, it is the perfect amount of unpredictability. If players knew what piece was com-
ing next, the game would lose a lot of its challenge. Indeed, with the “next” feature on
(which displays the next piece to drop on the side of the screen) the game becomes
somewhat easier. Pure randomness is often a really good way to keep players
Chapter 9: Artificial Intelligence 157