# Lindenmayer Systems

## What is a Lindenmayer system?

• Designed to explore organic growth
• Creates complex shapes from simple rules
• For example, with the rules: $\Rule{0em}{3ex}{1.3ex}\color{blue}{0\rightarrow 1},\qquad\color{red}{1\rightarrow 01}$ we have this string transformation: $\Rule{0em}{2.7ex}{1ex}01101=\color{blue}{0}\;\color{red}{1\;1}\;\color{blue}{0}\;\color{red}{1} \longrightarrow\color{blue}{1}\;\color{red}{01\;01}\;\color{blue}{1}\;\color{red}{01}=10101101$
• All operations take place simultaneously
• L-systems are term-rewriting systems

## Fractals: real…

All these lovely fractal trees in the city of Melbourne, Australia, in the winter.

## Fractals are everywhere

Once you start looking, you can't stop seeing them

## Fractals: manufactured…

These are all examples from The Algorithmic Beauty of Plants by Aristid Lindenmayer and Przemysław Prusinkiewicz, available at http://algorithmicbotany.org/papers/abop/abop.pdf

## Turning strings of symbols into pictures

• Sets of rules describe how one string of symbols will be expanded to a new string
• Each symbol corresponds to a turtle graphics instruction:
• F: Move forward
• -: Turn left
• +: Turn right
• [: Memorize current position and heading
• ]: Move to most recently memorized position and heading

## An example

For example, this sequence of symbols:

F[+F]F[-F]F

has this output:

We can clearly alter the output by changing the angle of the turns, and the length of the move forward.

In this example, the angle is 26°

## How turtle graphics works

This shows how the turtle draws a path with branches:

## More on turtle graphics

It's all done from the point of view of the turtle. A side of Koch's snowflake can be computed by the rules:

• Start: F
• Modify: FF+F--F+F (with turns of 60°)
• At every further step, each F is replaced by the string F+F--F+F
• The second iteration produces

F+F--F+F+F+F--F+F -- F+F--F+F+F+F--F+F

• The third iteration produces:

F+F--F+F + F+F--F+F -- F+F--F+F + F+F--F+F+ F+F--F+F + F+F--F+F -- F+F--F+F + F+F--F+F -- F+F--F+F + F+F--F+F -- F+F--F+F + F+F--F+F+ F+F--F+F + F+F--F+F -- F+F--F+F + F+F--F+F

• and so on…

## Turtle graphics with pictures!

First iteration:

Second iteration:

Third iteration:

Fourth iteration:

## Some mathematics

Remember the F $$\rightarrow$$ F+F--F+F iteration? How many symbols are in the $$n$$th string?

Let $$f_n$$ be the number of F's, and $$k_n$$ be the number of other symbols in the $$n$$th string. We have:

It follows immediately that $f_n=4^n\mbox{ and }k_n=4+4^2+4^3+\cdots+4^n=\frac{4}{3}(4^n-1).$ The total length is thus $f_n+k_n=4^n+\frac{4}{3}(4^n-1)=\frac{1}{3}(7(4^n)-4).$

## The fractal plant in modern languages: Racket

Racket is a modern lisp; descended from Scheme.

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

(require furtle) ;; furtle is a simple but fast turtle graphics library
(: ltree_b (-> Real Real Real TurtleF))  ;; typed Racket so must declare types
(define (ltree level size angle)
(if (= level 0)
(turtles (forward size))
(turtles (ltree (- level 1) (/ size 3) angle)               ; F
(save)                                             ; [
(left angle) (ltree (- level 1) (/ size 3) angle)  ; +F
(restore)                                          ; ]
(ltree (- level 1) (/ size 3) angle)               ; F
(save)                                             ; [
(right angle) (ltree (- level 1) (/ size 3) angle) ; -F
(restore)                                          ; ]
(ltree (- level 1) (/ size 3) angle))))            ; F


## The fractal plant in modern languages: Python

import turtle as t  # "turtle" is a turtle graphics module

# Lindenmayer system (a) from ABOP figure 1.24(a), p 25
def edgetree(level, size, angle):
if (level==0):
t.fd(size)
else:
edgetree(level-1, size/3, angle)
t.lt(angle)
edgetree(level-1, size/3, angle)
t.bk(size/3)
t.rt(angle)
edgetree(level-1, size/3, angle)
t.rt(angle)
edgetree(level-1, size/3, angle)
t.bk(size/3)
t.lt(angle)
edgetree(level-1, size/3, angle)


## A couple of pictures:

F $$\rightarrow$$ FF-[-F+F+F]+[+F-F-F]

F $$\rightarrow$$ FF[+F][--FF][-F+F]

## Some more mathematics

Fractal dimension can be defined by the "box-counting measure":

Suppose our picture is subdivided into boxes of size $$b$$, and $$N(b)$$ boxes are needed to cover the shape. Its dimension can be defined as $\lim_{b\to 0}\frac{\log(N(b))}{\log(1/b)}.$ For example, take a curve of length $$k$$. As $$b\to 0$$, we would find that $N(b)\to \frac{k}{b}.$ Thus $\lim_{b\to 0}\frac{\log(N(b))}{\log(1/b)}=\lim_{b\to 0}\frac{\log(k/b)}{\log(1/b)} =\lim_{b\to 0}1-\frac{\log(k)}{\log(b)}=1.$ In general a fractal will have a non-integer dimension between 1 and 2.

## Some concluding remarks

• Lindenmayer systems provide a neat, elegant and simple way to explore fractal geometry…
• … and to explore natural shapes such as plants
• Fractals are everywhere in the world
• Their mathematics is subtle, interesting, and beautiful
• A wonderful mixture of mathematics, computing, algebra, and graphics
• Thank you all!

Created by Alasdair