Designers Versus AI

How do stronger AI technology and the potential for intelligent NPCs affect game design? It's clear that improvements in the field of AI have opened up possibilities for the design. In the near future, advancements will undoubtedly lead to other expansions in "design space."

However, intelligent NPCs are changing the methods of game design, too. These issues discussed in this section are causing tension within the development teams, and the subject is very topical. Clashes are a recurring theme—even among the very best developers.

Obstacles

Beyond any technical details, a major problem is making room for intelligent behavior in the design, and dealing with those behaviors:

• Intelligent game characters can behave autonomously.

• Designers need to control the behavior of NPCs.

An undeniable clash exists in these roles; do we actually need AI? If AI is present, do we need designers? The overlap lies between the designers' wishes and what the AI can provide. A reasonably intelligent NPC can challenge the authority of the designer!

Two Types of Games

Different attitudes toward control have led to two distinct varieties of video games. In the first variety, designers implement their vision in a top-down fashion, controlling every detail of the game. This is the explicit design approach. It's particularly common in games based on a single story line (Doom 3, Unreal 2).

When the design script is extremely detailed, such as in single-player scenarios, there is little need for AI techniques. It's even arguable whether NPCs are "intelligent" characters at all. (They do the same thing every time regardless of changes in the situation.) Standard programming or scripting can bring the design to life. The less detail there is in the story, the more AI technology is necessary.

The second type of game results from bottom-up design, whereby the AI and the environment combine and emerge into an interesting game. The key observation is that there is no master script. Working together, all the characters in the game create an interesting world that makes a game (that is, an emergent story line, as in Pizza Tycoon).

This is implicit design, because the story is not controlled directly by the designer, although each individual NPC behaves as told (low-level explicit control). If the NPCs are intelligent rather than scripted, the designer's control lies in the environment only!

Many games are built in the implicit fashion, but very few of these actually have a story line (for instance, SimCity, Transport Tycoon). The story line is whatever path the player decides to take. In these games, designers can't easily control how the AI comes together to affect the gameplay.

The combination of top-down and bottom-up approaches to game design is a topical problem in AI research. One practical solution that's already been used (in Grand Theft Auto III, for instance) is to alternate sequences of explicit control (to set up the story line), and implicit control (to allow the player to play it). In this case, the behavior of the NPC is overridden during the "cut scenes" and left to autonomous AI control otherwise.

What Conflict?

These distinct types of games reduce the overlap between the AI and the design. In general, the designer's job is to craft the game, from the low-level behaviors to the overall story line. Animating characters is a crucial part of this. It's the AI's responsibility to control the behavior of in-game characters, bringing the vision to life.

Conflict can arise when the designer wants explicit control of an intelligent NPC. In this case, the AI has to perform a particular action in a specific situation as told. This is the easiest problem to resolve—even with standard AI techniques—because the AI can be directly overridden (as in cut scenes).

Further conflict can arise when designers want the story line to emerge in a different fashion (for instance, get the characters to force the player into this situation instead). This cannot be resolved as simply because the control is implicit; there's often no easy way to tell the NPC directly what to do for a particular situation to emerge.

Designers of games with story lines seem to have particular issues with implicit control, not realizing the challenges presented to the AI programmer. Identifying the situation can help. As a rule of thumb, control is implicit when it requires more than one operation to achieve.

For example, making sure that there is another player—any player—behind the door when it opens is explicit control; it's just a matter of spawning a new entity into the game world. However, guaranteeing that teammate Marvin is behind the door when it opens is implicit control. Just moving Marvin would make the game world inconsistent. (He might have been visible at the time.) So he actually needs to go through the door, which takes more than one operation; it's implicit control.

The solution is for designers to agree with the AI programmer upfront about all types of implicit control required, and then to specify explicit control as necessary. The idea is that the AI system needs to be engineered specially to provide "handles" for implicit control, whereas explicit control is just a matter of overriding the system.

Setting up guidelines allows the AI developers to exploit the freedom they are given to build the AI system. Designers will make the most of the AI system with the agreed forms of control—harnessing their power to improve the gameplay.