Killer Game Programming in Java

Comparison with L-Systems

L-systems consist of rewrite rules and have been widely used for plant modeling and simulation. Perhaps surprisingly, there is a direct mapping between the string expansions of a rule system and the visual representation of a plant. An example, using a bracketed L-system, will give an idea of how this works.

The L-system contains one start string F and rewrite rule:

    F --> F [-F] F [+F] F

The start symbol represents the initial plant form; in this case, F is a single plant limb. The rewrite rule specifies how the symbol on the left of the —> should be replaced or expanded to create the longer sequence of symbols on the right of the —>. The rewrite rule replaces F with the sequence F[-F]F[+F].

The visual characterization is obtained by thinking of each F in the sequence as a limb of the plant. The bracketed notation is viewed as a branch-creation operator; the - is a rotation to the right for the branch, and the + is a left rotation. Consequently, the rewrite of F to F[-F]F[+F] can be seen as the plant expansion in Figure 28-6.

Figure 28-6. First rewrite of F

The rewrite rule can be applied again to each F symbol in the F[-F]F[+F]sequence, producing a longer sequence. Repeated application of the rewrite rule creates ever larger sequences with more complex plant-like shapes as in Figure 28-7.

Perhaps the richest L-system language is cpfg, available from http://www.cpsc.ucalgary.ca/Research/bmv/ ...