CMUnited-98 agents are equipped with two different defending modes: opponent tracking and opponent marking. In both cases, a particular opponent player is selected as the target against which to defend. This opponent can either be selected individually or as a defensive unit via communication (the latter is the case in CMUnited-98).
In either case, the agent defends against this player by observing its position over time and position itself strategically so as to minimize its usefulness to the other team. When tracking, the agent stays between the opponent and the goal at a generous distance, thus blocking potential shots. When marking, the agent stays close to the opponent on the ball-opponent-goal angle bisector, making it difficult for the opponent to receive passes and shoot towards the goal. Defensive marking and tracking positions are illustrated in Figure 6.
Figure 6: Positioning for defensive tracking and marking.
When marking and tracking, it is important for the agent to have accurate knowledge about the positions of both the ball and the opponent (although the ball position isn't strictly relevant for tracking, it is used for the decision of whether or not to be tracking). Thus, when in the correct defensive position, the agent always turns to look at the object (opponent or ball) in which it is least confident of the correct position. The complete algorithm, which results in the behavior of doggedly following a particular opponent and glancing back and forth between the opponent and ball, is as follows:
This defensive behavior is locally optimal in that it defends according to the opponent's current position, following it around rather than predicting its future location. However, in both cases, the defensive positioning is chosen in anticipation of the opponent's future possible actions, i.e. receiving a pass or shooting.