Hiding and Taking Cover

Sometimes we will need our AIs to run for cover or remain unnoticed by the player. For example, maybe they want to catch the player by surprise, or maybe the AI is controlling a soldier under heavy player fire. Whatever the case, knowing how (and where) to seek cover is always an interesting twist for the AI. Many game reviews praise this behavior as highly sophisticated, and players enjoy seeing it in action because it offers an extra challenge.

Taking cover is usually a matter of two actions performed sequentially. First, a good hiding spot must be located in the game level. Second, we must move there quickly. This second phase is nothing but a chase routine, very similar to those explained earlier in this chapter; so we will focus on detecting good hiding spots. To do so, we will need three data items:

• The position and orientation of the player

• Our position and orientation

• The geometry of the level

The geometry of the level must be stored in such a way that it allows proximity queries on objects that can actually become hiding spots. A good structure, for example, is having the world separated into terrain data with objects laid on top. Then, the objects must be laid out using quadtrees, spatial indices, or any other technique, so we can quickly compute which objects lie closest to certain map locations.

The actual algorithm involves finding the closest object to the AI's location and computing a hiding position for that object. The algorithm works as follows. We start by using the scene graph to select the closest object to the AI. This step largely depends on how the world is laid out. Once we have selected the object, we shoot one ray from the player's position to the barycenter of the object. We propagate the ray beyond that point, computing a point along the ray that's actually behind the object (from the player's standpoint). As shown in Figure 7.6, that's a good hiding spot. Keep in mind that hiding in places can be sped up if we can guarantee convex hiding spots.

Figure 7.6. Geometry of playing hide-and-seek with the player.

AIs that implement hiding behaviors can easily be enhanced to work with the player's view cone. This is very useful for shooters. As shooters, we remain hidden while the view cone is focused on us. As soon as the player is looking somewhere else, we know we can leave our hiding spot and attack him. Games such as Medal of Honor have made great use of this technique, with German soldiers staying under cover until the right opportunity arises.