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# 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.

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/ ...

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