Let's talk about digital games (computer games), their relation to play, their relation to you and to artificial intelligence. One way they could be divided up is the following: there are games you play, games you play with, games that play you and games that play with you. The first category is very common, the second rather common, the third a current research topic and the fourth an unknown territory awaiting (or perhaps provoking) new advances in artificial intelligence.
Games you play
These are the games that most people come to think of when you mention computer games, and that usually dominate sales charts. Games like Doom, Asteroids, Tetris, StarCraft, Angry Birds and Call of Duty. Games of progression (Juul) where there are goals and there is a clear sense of what it means to perform the game better or worse. Games where "instrumental play" (Taylor), i.e. playing the game like you were performing a sport, is possible (but not necessary). In these games, the rules are set and enforced by the game and it's up to you as a player to do your best according to what the game thinks you should do and within the possibility space the game affords.
It is very common that these games feature some sort of artificial intelligence, typically in the role of controlling the opponents and allies in order to provide a challenge. But there are also examples of AI-based dynamic difficulty adjustment and other forms of experience management, such as the AI Director in Left 4 Dead. Additionally, AI techniques can be used during the design and development phases of such games, especially for procedural content generation.
Games you play with
Some games have less explicit goals, less clear sense of progression and perhaps less structure overall. These are games which are sometimes describes as more of toys than games, and which when replayed from scratch may play out completely differently. Games of emergence, like Sim City, Minecraft and The Sims. As these games try to hard to make you feel like you can do whatever the hell you want, you are encouraged to express your own desires, dreams and whims using the mechanics of the game. Of course, no games have so far been made where you can actually do whatever you want; therefore, you are likely to run up against the limits of the simulation and game mechanics sooner or later. (Sooner, if you are of the "non serviam" player type.) Of course, most of the games that are meant to be played can be played with with varying degrees of success. For example, you can decide to build sculptures instead of clearing lines in Tetris, and you can enact little dances in Doom. The very fuzzy boundary beetwen games you play and games you play with can be further illustrated with how hard it is to classify Civilization or Skyrim as one or the other of these.
There are more potential roles for artificial intelligence in games you play with than there are in games you play. Because of the impossibility of scripting a free-form game, they are often crucially relying on a complicated simulation of the game world, many parts of which could be called AI. A good example is the use of "smart objects" that match characters' needs with provided services in The Sims. Given the risk of such free-form simulations getting out of hand due to emergent unbalance, there is also a role for experience managers here (e.g. causing an earthquake if your city is doing too well).
Games that play you
You can become a challenge for the game. That is, if the game sees as its objective to make you happy, to keep you challenged, to make you sad, to keep you playing or perhaps to make you stop playing. You might not be inclined to do as the game wants you to do, and then the game needs to manipulate you. Luckily (for the game) the game knows quite a lot about you, as you keep feeding it with information all the time, via the keyboard, mouse or gamepad interface you use to interact with the game. (The game might also have other nefarious ways to glean information about what you feel and think, such as watching what you do via a Kinect device or webcam, or measuring your stress level via galvanic skin response.) Thus, the game can take actions by introducing new elements, new levels, changing music and art, distributing rewards, changing difficulty level and behaviour of in-game characters etc, and measure the success of these actions by your behaviour. This is isomorphic to how you take actions such as jumping, shooting, looting and choosing dialogue options when you play a game, and measure the success of these actions through score, progress meters, achievements etc.
Games that play you is at the center of our AI research program here at IT University of Copenhagen. Georgios and me recently wrote a survey/position paper that outlines our view of how to achieve such games. We call our approach "Experience-driven Procedural Content Generation". This is a very AI-heavy approach, where various AI techniques are used both for modelling what you think and feel while playing the game, predicting what you will do under various circumstances, and creating new game content that will bring the game closer to its goal according to the derived model. An example of this approach is our work with a version Super Mario Bros with adaptive levels. This game wants to keep you entertained. Therefore it has let hundreds of players play pairs of different levels, and asked the players which of the levels were most entertaining. When you play it, it will match your play style with its model of player experience, and deliver a freshly created level which it thinks will keep you optimally entertained.
Games that play with you
Remember the games you used to play as a kid? Not the digital games or board games, but the ones where you never quite knew in advance what the game was about or what the rules were, because the basic premise was "to play". The ones where you went "let's pretend I'm an astronaut, and this is our spaceship" and your friend went "ok, but I'm a space knight and I ride next to your spaceship on my space horse!". Now that you're a grown-up you don't do this very often, except if you engage in improvisational theatre. In particular, you don't do this while playing games on your computer. The basic idea of a digital game is that you play against or with a stable set of rules that is more or less predictable and therefore "fair"; if you fail (or things play out differently than expected) it was your own fault or simply bad luck. The game enforces these rules with such strictness that anything that works is correct. Any change in the rules during the game session (such as removing your jump ability or making your friends your enemies) is carefully scripted.
There are some attempts to undermine the status of rules in computer games, for example the game B.U.T.T.O.N (see Wilson's article) where the rules are underspecified. Players are required to invent new rules, which are not observable or enforceable by the game, as they play. However, what this game does is simply to reduce the role of the computer in the game, and mixing in more of classic non-digital play. The game does not express any desire on its own, and is even more of a mindless automaton than most of the classic games you play. This is of course unappealing from the perspective of an AI aficionado interested in technological progress, as is the fact that the game needs to be played in physical space with other humans.
Imagine that we could replace these people with AI, and integrate this system into the game itself. Imagine a game that is actually playful. A game that wants to play with you, not because it wants to fulfil some sort of goal (keeping you playing, or entertained), but because it wants to keep itself entertained, and explore the possibility space of the play session together with you. A game that treats you as a means, not as an end. That could refuse to play with you if you are too boring or hard to play with. That gives you the feeling of really playing with someone, not just against a ruleset.
Unfortunately, we don't really know how yet how to create a game that plays with you. How can we implement a desire in the game for having fun and a lust for exploring the possibilities of interaction with a player? How can we implement the concrete mechanisms where the game co-creates the rules with you and plays along, both constructively and obstructively? Actually, this is not unfortunate at all. It means that we have a new research problem for AI and games, and yet another angle of attack on the elusive problem of artificial intelligence. For we could surely not have any real artificial intelligence that was not capable of playing and enjoying itself.
One idea for how to approach this problem is Schmidhuber's theory of artificial curiosity, where agents choose what to learn so that they optimise their predicted learning rate. There is also related work in evolutionary and developmental robotics. Another strand of work which could potentially inform the development of games that play with you is work in mixed-initiative procedural content generation, such as Tanagra and Sketchaworld. In these systems, the designer and the software take turns to work on the game content, typically so that the software acts as an resourceful and inventive assistant to the designer. Last year, I published a paper outlining a simple system that tries to do mixed-initiative rule generation while you're playing the game. Or perhaps you're playing the system, and the game emerges from the play session. The system tries to understand what rules you're playing according to, and then enforcing these rules, which might force you to play differently (especially if it has misunderstood you) and see the flaws in your implicit design concept. What is missing from this system is intentions and desires on part of the system; the game is still trying to please you, rather than itself. I should get to work right away on changing this.
This blog post was inspired in part by Sicart's article Against Procedurality, and the opposition it sets out between rule-centric play/interpretation and player-centric play/interpretation. I have nothing against any of these, and think that both are interesting. However, I think that it's important to point that just because something is procedural, it's not necessarily rigid, predictable and intentionally designed; it could be adaptive, emergent and have a will of its own.