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  • Book Excerpt: AI for Game Developers

    - Glenn Seeman and David Bourg


    Feature!Flocking Example

    The example we're going to look at involves simulating several units in a continuous environment. Here, we'll use the same rigid-body simulation algorithms we used in the chasing and pattern movement examples we discussed earlier. This example is named AIDemo4, and it's available for download from the book's Web site (

    Basically, we're going to simulate about 20 units that will move around in flocks and interact with the environment and with a player. For this simple demonstration, interaction with the environment consists of avoiding circular objects. The flocking units interact with the player by chasing him.

    Steering Model

    For this example, we'll implement a steering model that is more or less identical to the one we used in the physics-based demo in Chapter 2. You can refer to Figure 2-8 and the surrounding discussion to refresh your memory on the steering model. Basically, we're going to treat each unit as a rigid body and apply a net steering force at the front end of the unit. This net steering force will point in either the starboard or port direction relative to the unit and will be the accumulation of steering forces determined by application of each flocking rule. This approach enables us to implement any number or combination of flocking rules—each rule makes a small contribution to the total steering force and the net result is applied to the unit once all the rules are considered.

    We should caution you that this approach does require some tuning to make sure no single rule dominates. That is, you don't want the steering force contribution from a given rule to be so strong that it always overpowers the contributions from other rules. For example, if we make the steering force contribution from the cohesion rule overpower the others, and say we implement an obstacle avoidance rule so that units try to steer away from objects, if the cohesion rule dominates, the units might stay together. Therefore, they will be unable to steer around objects and might run into or through them. To mitigate this sort of unbalance, we're going to do two things: first, we're going to modulate the steering force contribution from each rule; and second, we're going to tune the steering model to make sure everything is balanced, at least most of the time.

    Tuning will require trial and error. Modulating the steering forces will require that we write the steering force contribution from each rule in the form of an equation or response curve so that the contribution is not constant. Instead, we want the steering force to be a function of some key parameter important to the given rule.

    Consider the avoidance rule for a moment. In this case, we're trying to prevent the units from running into each other, while at the same time enabling the units to get close to each other based on the alignment and cohesion rules. We want the avoidance rule steering force contribution to be small when the units are far away from each other, but we want the avoidance rule steering force contribution to be relatively large when the units are dangerously close to each other. This way, when the units are far apart, the cohesion rule can work to get them together and form a flock without having to fight the avoidance rule. Further, once the units are in a flock, we want the avoidance rule to be strong enough to prevent the units from colliding in spite of their tendency to want to stay together due to the cohesion and alignment rules. It's clear in this example that separation distance between units is an important parameter. Therefore, we want to write the avoidance steering force as a function of separation distance. You can use an infinite number of functions to accomplish this task; however, in our experience, a simple inverse function works fine. In this case, the avoidance steering force is inversely proportional to the separation distance. Therefore, large separation distances yield small avoidance steering forces, while small separation distances yield larger avoidance steering forces.

    We'll use a similar approach for the other rules. For example, for alignment we'll consider the angle between a given unit's current heading relative to the average heading of its neighbors. If that angle is small, we want to make only a small adjustment to its heading, whereas if the angle is large, a larger adjustment is required. To achieve such behavior, we'll make the alignment steering force contribution directly proportional to the angle between the unit's current heading and the average heading of its neighbors. In the following sections, we'll look at and discuss some code that implements this steering model.


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