Guiding Self-Organization in Systems of Cooperative Mobile Agents - Advancing Artificial Intelligence Through Biological Process Applications - page 283

as a non-linear, weighted combination of a number
of component forces, such as cohesion, align-
ment, various repulsive influences (for avoiding
obstacles), random forces for resolving situations
of persistent interference, attractive/repulsive
forces for guiding blocks up/down staircases,
and attractive forces for approaching local goal
locations.
These goal locations are determined via the use
of stigmergic rule s (Adam, 2005, Bonabeau, 1999,
Theraulaz, 1995) whose antecedents are matched
against local arrangements of blocks already part
of the emerging structure. While no block b has
an explicit representation of the target structure
or a preassigned unique location within it, this
structure is implicitly encoded by these rules. If
a part of the emerging structure falls within b 's
local neighborhood, b will match it against the

antecedents of stigmergic rules, to determine
whether there is some adjacent location where b
itself can belong. For example, consider the rule
illustrated on the left side of Figure 10, which is
used by medium blocks for the construction of
a temporary staircase (shown on the right side)
employed during the assembly of a building-like
structure. The two locations in the center of
the rule are unmarked, denoting that they must
be empty, in order to leave room for a deposit-
ing medium block (solid rectangle). Solid line
squares indicate that stationary blocks must exist
just “behind” this goal location, both at the same
level and above. Dashed lines denote “wildcard”
locations, where a block can be present or absent.
This rule creates a natural procedure (somewhat
similar to Hosokawa, 1998) for stair assembly,
where the steps are deposited as needed: since a

Figure 10. The use of stigmergy for staircase construction. On the left, a stigmergic rule is illustrated,
with the antecedent (to the left of the arrow) showing the arrangement around a potential goal loca-
tion before the matching block deposits itself, and the consequent (to the right of the arrow) showing
the arrangement immediately after. The rule is drawn in three horizontal cross sections above (top), at
the same level as (center), and below (bottom) the goal location. Further explanations are given in the
text. On the right, the block marked with an asterisk (*) is about to deposit itself into a goal location
determined by this rule, as a step of the currently assembling layer of the staircase.