818 Chapter 12 Designing Game AI The AI characters in Splinter Cell also use a cone-of-sight for vision checks, and there is a simple sound model where sound travels in the current room up to a certain radius depending on the volume of the sound. Very similar techniques are used in the Metal Gear Solid [Konami Corporation, 1998] series of games. 12.2.4 Pathfinding and Tactical AI In Soldier of Fortune 2: Double Helix [Raven Software, 2002], links in the pathfinding graph were marked with the type of action needed to traverse them. When a character reached the corresponding link in the path, it could then change behavior to appear to have knowledge of the terrain. The link might represent an obstacle to vault over, a door to open, a barrier to break through, or a wall to rappel down. The AI team responsible, Christopher Reed and Ben Geisler, call this approach "embedded navigation." It is becoming almost universal to incorporate some kind of waypoint tactics in shooters. In the original Half-Life [Valve, 1998], the AI uses waypoints to work out how to surround the player. A group of AI characters will be coordinated so that they occupy a set of good defensive positions that surround the player's current location, if that is possible. In the game an AI character will often make a desperate run past the player in order to take up a flanking position. Unless your enemy characters always rush the player, as in the original Doom, you will probably need to implement a pathfinding layer. The indoor levels of most shooters can be represented with