How to correctly decorate an ellipse with random steps

Question

To create something that looks like the shape of a lake, the following code transforms the border of a blue ellipse with smooth random steps. The idea is taken from another answer.

\documentclass[tikz]{standalone}
\usetikzlibrary{decorations.pathmorphing}
\pgfmathsetseed{42}
% The seed is only set explicitly to always generate the same picture.
% The problem occurs independent of seed.

\begin{document}
\begin{tikzpicture}
  \fill[blue, draw=brown,
    decoration={random steps,segment length=1cm,amplitude=.5cm},
    decorate,
    rounded corners=.3cm
  ] (0, 0) ellipse (3 and 2);
  \draw[ultra thick, red] (3.1, 0) circle (.3);
\end{tikzpicture}
\end{document}

As marked with a red circle, at the point where the ellipse begins (at 0 degrees) the random step is not correctly smoothed but adds an ungly corner. How can this be fixed?

Answer 1

You can use the options pre and post to control what's happening at the beginning and the end of the path you're decorating. More information in the Tikz/PGF manual, section 21.4.2, page 247.

I suggest this:

decoration={random steps,segment length=1cm,amplitude=.5cm,
            pre=lineto,pre length=.25cm,post=lineto,post length=.25cm}

A complete source file and the result follow.

\documentclass{article}
\usepackage[a4paper,margin=2cm]{geometry}
\usepackage{tikz}
\usetikzlibrary{decorations.pathmorphing}

\newcommand\rndellipse[2][]{%
  \pgfmathsetseed{#2}
  \begin{tikzpicture}
    \fill[blue, draw=brown,
      decoration={random steps,segment length=1cm,amplitude=.5cm,#1},
      decorate,
      rounded corners=.3cm
    ] (0, 0) ellipse (3 and 2);
  \end{tikzpicture}}
\newcommand\try[1]{%
  \noindent
  \textbf{Seed #1} \\
  \begin{tabular}{@{}cc@{}}
    \rndellipse{#1} &
    \rndellipse[pre=lineto,pre length=.25cm,post=lineto,post length=.25cm]{#1}
  \end{tabular}
  \par
}

\begin{document}
\try{42}
\try{54}
\try{666}
\try{4217}
\end{document}