Choreographed AIs

In its simplest form, an action AI system can be implemented as a preprogrammed action sequence that is executed repeatedly. This form of AI is used in industrial robots and can be applied to simple games as well. An elevator in Quake, for example, is just executing a very simple movement sequence. These systems are merely state machines with no optional transitions just a series of states the automata loops through endlessly.

But we can use the same technique for quite interesting AI entities, well beyond elevators and robot arms. A security guard in an adventure game, for example, can be programmed to walk up and down the alleys in a prison, maybe performing some specific actions. Ships in top-down arcade scrollers sometimes exhibit quite beautiful choreographies, often involving tens of ships. Gameplay in these games emerges from analyzing movement patterns and detecting the pitfall that allows us to avoid the guard, shoot down enemy ships, and so on.

Choreographed AI systems usually employ a very simple scripting engine to represent movement sequences. Figure 7.1 shows a diagram of such a system. Note that we are not talking about a full-blown AI system built with a scripting engine. We are starting with very simple and deterministic behaviors that are stored in files, as in the following example:

 set animation "walk" go to 0 10 0 set animation "rotate" rotate 180 set animation "walk" go to 0 0 0 set animation "rotate" rotate 180 

Figure 7.1. Sequencer for a choreographed AI system.

The preceding example could very well be an animation for a security guard in a prison. He just walks up and down an alley, executing different animation cycles. Most coin-operated machines from the 70s and 80s used variations of this approach, shared (among other classics) by the LOGO programming language and the BigTrak vehicle toy sold by Milton Bradley.

The main design decision you must make when coding any choreographed AI system is selecting the range of commands the AI will respond to, or in other words, the instruction set for your programming language. I recommend you take a second to carefully think about the level of abstraction you will use with your AI system. Should any movement be decomposed into a series of straight lines, or should more complex tasks like spirals and circles be directly built into the language? Clearly, setting the right abstraction level will allow us to create more interesting scripts with less work. Unfortunately, this is a context-sensitive decision. Designing the scripting language largely depends on the kind of game (and AI) you are trying to craft. Once the level is set, decide on the instructions and parameters you will be implementing. Again, try to think in terms of what gives you the most expressive potential for the game.

Implementation

The first choreographed AIs stored sequences that were hard-coded into the game's assembly source. But as time passed and sequencing languages became more and more powerful, small scripting languages were evolved, so content creators could work on more complex sequences easily.

Today, implementing a choreographed AI system is, fundamentally, implementing a very simple script language. You need a parser that reads the script into main memory. Then, a run-time interpreter processes the file and executes its instructions, affecting game engine internals such as enemy positions and orientations. We will discuss script languages in detail in Chapter 9, "Scripting," so I suggest you refer to it when trying to code a choreographed AI system.