To determine if there is a threat of collision with a visible fish we
introduce a collision sensitivity region. It works in a similar
manner as the antenna ray 
used above. A collision
sensitivity region is a region of 
wherein any object
intersecting with it is considered to present a collision threat. The
different sizes of the sensitivity region associated with different
fishes can result in different `disposition': a large
sensitivity region results in a `timid' fish that takes evasive action
to avoid a potential collision well in advance, while a tight
sensitivity region yields a `courageous' fish that takes evasive
action at the last second.
Let the center line of a fish be the line through the mean
positions of the following sets of nodes: 
(see Fig. 
; larger fishes
need a finer sampling and this is done by interpolating the center
line). For efficiency we choose to represent the collision
sensitivity region as a rectangular box, see
Fig. . Fish i presents a threat of collision
to fish k if any node on i's center line falls within k's
collision sensitivity region. The size of a fish's collision
sensitivity region may be varied according to behavioral needs, as
will be seen in the next chapter. If more than one other fish present
a collision threat, the one with the largest portion of its center
line in fish i's sensitivity region is regarded as being the most
dangerous.
Figure: The fish's collision sensitivity region.
| Xiaoyuan Tu | January 1996 |