blog/posts/lsystems.org

---
title: Generating and representing L-systems
author: Dimitri Lozeve
date: 2018-01-18
tags: visualization, haskell
toc: true
---

L-systems are a formal way to make interesting visualisations. You can
use them to model a wide variety of objects: space-filling curves,
fractals, biological systems, tilings, etc.

See the Github repo: [[https://github.com/dlozeve/lsystems]]

* What is an L-system?

** A few examples to get started

[[../images/lsystems/dragon.png]]

[[../images/lsystems/gosper.png]]

[[../images/lsystems/plant.png]]

[[../images/lsystems/penroseP3.png]]

** Definition

An [[https://en.wikipedia.org/wiki/L-system][L-system]] is a set of
rewriting rules generating sequences of symbols. Formally, an L-system
is a triplet of:
+ an /alphabet/ $V$ (an arbitrary set of symbols)
+ an /axiom/ $\omega$, which is a non-empty word of the alphabet
  ($\omega \in V^+$)
+ a set of /rewriting rules/ (or /productions/) $P$, each mapping a
  symbol to a word: $P \subset V \times V^*$. Symbols not present in
  $P$ are assumed to be mapped to themselves.

During an iteration, the algorithm takes each symbol in the current
word and replaces it by the value in its rewriting rule. Not that the
output of the rewriting rule can be absolutely /anything/ in $V^*$,
including the empty word! (So yes, you can generate symbols just to
delete them afterwards.)

At this point, an L-system is nothing more than a way to generate very
long strings of characters. In order to get something useful out of
this, we have to give them /meaning/.

** Drawing instructions and representation

Our objective is to draw the output of the L-system in order to
visually inspect the output. The most common way is to interpret the
output as a sequence of instruction for a LOGO-like drawing
turtle. For instance, a simple alphabet consisting only in the symbols
$F$, $+$, and $-$ could represent the instructions "move forward",
"turn right by 90°", and "turn left by 90°" respectively.

Thus, we add new components to our definition of L-systems:
+ a set of /instructions/, $I$. These are limited by the capabilities of
  our imagined turtle, so we can assume that they are the same for
  every L-system we will consider:
  + ~Forward~ makes the turtle draw a straight segment.
  + ~TurnLeft~ and ~TurnRight~ makes the turtle turn on itself by a
    given angle.
  + ~Push~ and ~Pop~ allow the turtle to store and retrieve its
    position on a stack. This will allow for branching in the turtle's
    path.
  + ~Stay~, which orders the turtle to do nothing.
+ a /distance/ $d \in \mathbb{R_+}$, i.e. how long should each forward segment should be.
+ an /angle/ $\theta$ used for rotation.
+ a set of /representation rules/ $R \subset V \times I$. As before,
  they will match a symbol to an instruction. Symbols not matched by
  any rule will be associated to ~Stay~.

Finally, our complete L-system, representable by a turtle with
capabilities $I$, can be defined as \[ L = (V, \omega, P, d, \theta,
R). \]

One could argue that the representation is not part of the L-system,
and that the same L-system could be represented differently by
changing the representation rules. However, in our setting, we won't
observe the L-system other than by displaying it, so we might as well
consider that two systems differing only by their representation rules
are different systems altogether.

* Implementation details

** The ~LSystem~ data type

The mathematical definition above translate almost immediately in a
Haskell data type:

#+BEGIN_SRC haskell
  -- | L-system data type
  data LSystem a = LSystem
    { name :: String
    , alphabet :: [a] -- ^ variables and constants used by the system
    , axiom :: [a] -- ^ initial state of the system
    , rules :: [(a, [a])] -- ^ production rules defining how each
			  -- variable can be replaced by a sequence of
			  -- variables and constants
    , angle :: Float -- ^ angle used for the representation
    , distance :: Float -- ^ distance of each segment in the representation
    , representation :: [(a, Instruction)] -- ^ representation rules
					   -- defining how each variable
					   -- and constant should be
					   -- represented
    } deriving (Eq, Show, Generic)
#+END_SRC

Here, ~a~ is the type of the literal in the alphabet. For all
practical purposes, it will almost always be ~Char~.

~Instruction~ is just a sum type over all possible instructions listed
above.

** Iterating and representing

From here, generating L-systems and iterating is straightforward. We
iterate recursively by looking up each symbol in ~rules~ and replacing
it by its expansion. We then transform the result to a list of ~Instruction~.

** Drawing

The only remaining thing is to implement the virtual turtle which will
actually execute the instructions. It goes through the list of
instructions, building a sequence of points and maintaining an
internal state (position, angle, stack). The stack is used when ~Push~
and ~Pop~ operations are met. In this case, the turtle builds a
separate line starting from its current position.

The final output is a set of lines, each being a simple sequence of
points. All relevant data types are provided by the
[[https://hackage.haskell.org/package/gloss][Gloss]] library, along
with the function that can display the resulting ~Picture~.

* Common file format for L-systems

In order to define new L-systems quickly and easily, it is necessary
to encode them in some form. We chose to represent them as JSON
values.

Here is an example for the [[https://en.wikipedia.org/wiki/Gosper_curve][Gosper curve]]:
#+BEGIN_SRC json
{
  "name": "gosper",
  "alphabet": "AB+-",
  "axiom": "A",
  "rules": [
    ["A", "A-B--B+A++AA+B-"],
    ["B", "+A-BB--B-A++A+B"]
  ],
  "angle": 60.0,
  "distance": 10.0,
  "representation": [
    ["A", "Forward"],
    ["B", "Forward"],
    ["+", "TurnRight"],
    ["-", "TurnLeft"]
  ]
}
#+END_SRC

Using this format, it is easy to define new L-systems (along with how
they should be represented). This is translated nearly automatically
to the ~LSystem~ data type using
[[https://hackage.haskell.org/package/aeson][Aeson]].

* Variations on L-systems

We can widen the possibilities of L-systems in various ways. L-systems
are in effect deterministic context-free grammars.

By allowing multiple rewriting rules for each symbol with
probabilities, we can extend the model to
[[https://en.wikipedia.org/wiki/Probabilistic_context-free_grammar][probabilistic
context-free grammars]].

We can also have replacement rules not for a single symbol, but for a
subsequence of them, thus effectively taking into account their
neighbours (context-sensitive grammars). This seems very close to 1D
cellular automata.

Finally, L-systems could also have a 3D representation (for instance
space-filling curves in 3 dimensions).

* Usage notes

1. Clone the repository: =git clone [[https://github.com/dlozeve/lsystems]]=
2. Build: =stack build=
3. Execute =stack exec lsystems-exe -- examples/penroseP3.json= to see the list of options
4. (Optional) Run tests and build documentation: =stack test --haddock=

Usage: =stack exec lsystems-exe -- --help=
#+BEGIN_SRC
lsystems -- Generate L-systems

Usage: lsystems-exe FILENAME [-n|--iterations N] [-c|--color R,G,B]
                    [-w|--white-background]
  Generate and draw an L-system

Available options:
  FILENAME                 JSON file specifying an L-system
  -n,--iterations N        Number of iterations (default: 5)
  -c,--color R,G,B         Foreground color RGBA
                           (0-255) (default: RGBA 1.0 1.0 1.0 1.0)
  -w,--white-background    Use a white background
  -h,--help                Show this help text
#+END_SRC

Apart from the selection of the input JSON file, you can adjust the
number of iterations and the colors.

=stack exec lsystems-exe -- examples/levyC.json -n 12 -c 0,255,255=

[[../images/lsystems/levyC.png]]

* References

1. Prusinkiewicz, Przemyslaw; Lindenmayer, Aristid (1990). /The Algorithmic Beauty of Plants./ Springer-Verlag. ISBN 978-0-387-97297-8. [[http://algorithmicbotany.org/papers/#abop]]
2. Weisstein, Eric W. "Lindenmayer System." From MathWorld--A Wolfram Web Resource. [[http://mathworld.wolfram.com/LindenmayerSystem.html]]
3. Corte, Leo. "L-systems and Penrose P3 in Inkscape." /The Brick in the Sky./ [[https://thebrickinthesky.wordpress.com/2013/03/17/l-systems-and-penrose-p3-in-inkscape/]]