FUNDAMENTALS OF GAME DESIGN, SECOND EDITION
Designing Opponents
The easiest way to design a variety of opponents in a vehicle simulation is simply to provide different drivers' vehicles with different performance characteristics. One plane climbs slightly faster than another; one can turn more sharply. The player will experience different challenges in dealing with each opponent based on its design parameters. However, once the player figures it out, the opponent is easily beaten. As soon as the player discovers that a Supermarine Spitfire can consistently outrun a Messerschmitt Bf 109 in level flight, the situation offers an obvious strategy for Spitfire pilots: boom and zoom (hitting and running away).
To create further variety, modify the behavior of individual opponent drivers (or pilots). Design the AI for these opponents by starting with perfect performance and then creating variations from perfection. For example, it's possible to create a "perfect" AI driver in a racing simulation, one that always follows the most efficient line around the track, always shifts gears at precisely the correct moment, and knows the ideal speed at which to take each corner without spinning out. If such a driver has a better car than the player's, he will be unbeatable. The trick, then, is to modify the AI driver's judgment so that it isn't perfect—so that he doesn't always shift at exactly the right time or follow the most efficient line. This combination of factors, both vehicle characteristics and variable driver skill, provides the variety among opponents in vehicle simulators.
As you research flying or driving, you will discover other tricks to incorporate in the AI: drafting behind other cars, for example, and diving out of the sun to surprise the enemy in a dogfight.
You need to decide what to do about damage. Comical or arcadelike racing sims may not simulate damage at all; if the car hits something, it simply bounces off, although doing so usually slows the car down. This allows the driver to be much more careless, and it is a good solution for casual and children's games. They can afford to hit a few things and still win the race—at least in the earlier, easier stages of the game. Other games model damage as a single variable, like unit health points in a war game. When damage reaches a certain level, the vehicle simply stops running (which, in the case of an airplane, means that it crashes or explodes). If your target machine doesn't have much CPU power (as in a cell phone, for example), these approaches mean you don't have to model the physics very accurately.
To model damage accurately, you should divide the vehicle into separate areas that can suffer damage in a collision (or, in a military simulator, from enemy fire), and decide how that damage affects the performance of the vehicle. For instance, a race car with minor damage to the airfoils or body can continue, although with a performance penalty, but a blown tire forces it to halt. With airplanes, the consequences can be dramatically different depending on what part of the aircraft sustains damage. A plane can still fly without its tail, but it is unstable and extremely difficult to handle. These approaches give great verisimilitude but require sophisticated physics models to accomplish.
