How to reduce size of line in tikz L-system?

Question

I am trying to make a L-system in TikZ using the lindenmayersystems library:

I have this code:

\documentclass[border=1cm]{standalone}
\usepackage{tikz}
\usetikzlibrary{lindenmayersystems}
\pgfdeclarelindenmayersystem{A}{
\rule{A->F[+A][-A]}
}
\begin{document}
\begin{tikzpicture}[scale=10,rotate=90]
\draw
    [blue,opacity=0.5,line width=0.5cm,line cap=round]
    l-system [l-system={A,axiom=A
    ,order=4,angle=45,step=0.25cm}];
\end{tikzpicture}
\end{document}

This code is producing this output:

But as you can see, the top branches of the tree are touching each other. I do not want this to happen. I want that the size of the line should be reduced with every step, like in the following wikipedia image:

Is there any way to do this in TikZ?

Answer 1

One solution is to define the symbol F in a way which reduces the value of \pgflsystemstep, i.e:

\pgfdeclarelindenmayersystem{A}{
  \symbol{F}{\pgflsystemstep=0.6\pgflsystemstep\pgflsystemdrawforward}
  \rule{A->F[+A][-A]}
}

Which gives:

Complete code for the above figure:

\documentclass{article}
\usepackage{tikz}
\usepackage[active,tightpage]{preview}\PreviewEnvironment{tikzpicture}
\usetikzlibrary{lindenmayersystems}
\pgfdeclarelindenmayersystem{A}{
\symbol{F}{\pgflsystemstep=0.6\pgflsystemstep\pgflsystemdrawforward}
\rule{A->F[+A][-A]}
}
\begin{document}
\foreach \n in {1,...,8} {
\begin{tikzpicture}[scale=10,rotate=90]
\draw (-.1,-.2) rectangle (.4,0.2);
\draw
    [blue,opacity=0.5,line width=0.1cm,line cap=round]
    l-system [l-system={A,axiom=A
    ,order=\n,angle=45,step=0.25cm}];
\end{tikzpicture}
}

\end{document}

Update

A better solution: Define a new symbol (eg, S for "scale") which changes the scale, but draws nothing. The advantage is that it can be used in any part of the rule.

Using this approach, your example would be:

\pgfdeclarelindenmayersystem{A}{
  \symbol{S}{\pgflsystemstep=0.6\pgflsystemstep}
  \rule{A->SF[+A][-A]}
}

Another example, which shows how the "scaling" can be used in a part of a rule, could be:

\pgfdeclarelindenmayersystem{A}{
  \symbol{S}{\pgflsystemstep=0.5\pgflsystemstep}
  \rule{A->F[+A][-A]SA}
}

which, used this way:

\foreach \n in {1,...,6} {
\begin{tikzpicture}[scale=10,rotate=90]
\draw (-.03,-.17) rectangle (.35,0.17);
\draw
    [blue,opacity=0.5,thin,line cap=round]
    l-system [l-system={A,axiom=A
    ,order=\n,angle=15,step=0.05cm}];
\end{tikzpicture}
}

produces