The intention generator of a prey fish is a specialization of the generic intention generator of Fig. as shown in Fig. . The two characteristic behaviors of prey fish are schooling and evading predators. We briefly present the implementation of the two behaviors.
Figure: Portion of intention generator of prey.
Schooling is a complex behavior where all the fishes swim in generally the same direction (see Fig. ). Each fish constantly adjusts its speed and direction to match those of other members of the school. They maintain a certain distance from one another, roughly one body length from neighbors on average [Wilson and Wilson1985]. Each member of a school of artificial fish acts autonomously, and the schooling behavior is achieved through sensory perception and locomotion [Reynolds1987]. An inceptive school is formed when a few fish swim towards a lead fish (see Fig. ). Once a fish is in some proximity to some other schooling fish, the `schooling' behavior routine outlined in Fig. is invoked.
Figure: Schooling behavior routine.
The intention generator ensures that the fish do not get too close together, because the avoid collision intention has highest precedence. To create more compact schools, the collision sensitivity region of a schooling fish is decreased once the fish gets into formation. When a large school encounters an obstacle, the autonomous obstacle avoidance behavior of individual fishes may cause the school to split into two groups (obstacle avoidance behavior has higher priority than the schooling behavior). Once the obstacle is cleared, the behavior memory of individual fishes ensures that the schooling behavior routine regains control and hence the school rejoins (Fig. (a)-(d)).
Similar to the flocking of boids [Reynolds1987], the schooling behavior of artificial fish is achieved through the interaction of two behaviors--to stay in the neighborhood of other schooling fish (by using chasing-target) and to avoid being too close to any of them (by invoking avoiding-fish). Each fish acts independently based on its perception of its neighborhood. One can adjust the density of a school by adjusting the collision sensitivity regions of the schooling fish (Fig. (b)). For instance, increasing the sensitivity region can create more loosely coupled schools. In our implementation, there is a designated `leader fish' that swims in front of all other fish in the school. The general path that the school takes is determined by the leader fish. Of course, the leader fish could also be decided dynamically at run time, i.e. whichever fish is frontmost will become the leader.
Figure: A small school of angelfish.
Figure: An inceptive school.
When a predator gets too close to any fish the escaping behavior is triggered. This examines the relative position between the fish and the predator against a set of rules. Each rule suggests a proper action in a certain situation. For example, If predator is closely behind and swims in roughly the same direction and is to the right of fish, then turn to the left. One may consider all these rules as the specifications of one general rule, that is, to swim away from the predator as fast as possible. Rules are interpreted as detailed geometrical/mathematical conditions, and their quantities determine the values of motor control parameters such as the angle of a turn. Currently there are eleven rules and the resulting behavior looks natural. Fig. and show two instances of a small school of prey scattering in terror when a predator comes close. Note how each of the prey fish tries independently to flee from the pursuing predator.
Figure: School scatters in terror.
Figure: Fleeing from predator.
The eleven rules based on which a prey fish flees from a predator are listed below (note ``me'' or ``mine'' indicate the fleeing prey fish): if predator is far:
if predator is close:
|Xiaoyuan Tu||January 1996|