[Tex/LaTex] TikZ finishes a morphed path with a straight segment

decorationspathstikz-pgf

When, in TikZ, I apply a path morphing decoration (say, "snake") and specify the segment length option, I often finish up with a straight segment at the end of the path, such as

enter image description here

which often is not the desired behavior. It happens in case there is no room for the whole decoration segment at the end of path. If I do not specify the segment length, it's all right, but I want to have some control over the decoration parameters.

Do you know if there is a way to set the segment length softly in such a way that it is modified a bit automatically in order to get rid of these straight segments?

The picture was produced with the following code:

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{decorations.pathmorphing}

\begin{document}
\begin{tikzpicture}
    \draw[decorate, decoration = {snake, segment length = .4cm}] (0,0) -- (2,0);
\end{tikzpicture}
\end{document}

Best Answer

This is a slightly adapted version of my answer to Nicer wavy line with TikZ. This version doesn't insist on drawing a full sine wave, but will be happy with half periods as well (i.e. it can start and end with an upward arch, while the previous version would always start with an upward and end with a downward arch):

\documentclass[a4paper,12pt]{article}
\usepackage{tikz}
\usetikzlibrary{decorations}
\begin{document}

\pgfdeclaredecoration{complete sines}{initial}
{
    \state{initial}[
        width=+0pt,
        next state=upsine,
        persistent precomputation={\pgfmathsetmacro\matchinglength{
            \pgfdecoratedinputsegmentlength / int(\pgfdecoratedinputsegmentlength/\pgfdecorationsegmentlength)}
            \setlength{\pgfdecorationsegmentlength}{\matchinglength pt}
        }] {}
    \state{upsine}[width=\pgfdecorationsegmentlength,next state=downsine]{
        \pgfpathsine{\pgfpoint{0.25\pgfdecorationsegmentlength}{0.5\pgfdecorationsegmentamplitude}}
        \pgfpathcosine{\pgfpoint{0.25\pgfdecorationsegmentlength}{-0.5\pgfdecorationsegmentamplitude}}
    }
    \state{downsine}[width=\pgfdecorationsegmentlength,next state=upsine]{
        \pgfpathsine{\pgfpoint{0.25\pgfdecorationsegmentlength}{-0.5\pgfdecorationsegmentamplitude}}
        \pgfpathcosine{\pgfpoint{0.25\pgfdecorationsegmentlength}{0.5\pgfdecorationsegmentamplitude}}
}
    \state{final}{}
}

\begin{tikzpicture}[
    every path/.style={
        decoration={
            complete sines,
            segment length=1cm,
            amplitude=1cm
        },
        decorate,
        thick
    }]
\draw (0,0) -- (2,0);
\draw [yshift=-1.2cm] (0,0) -- (2.5,0);
\draw [yshift=-2.4cm] (0,0) -- (3,0);
\draw [yshift=-3.6cm] (0,0) -- (3.5,0);
\end{tikzpicture}
\end{document}

Related Solutions

[Tex/LaTex] How to animate a path or how to draw starting fraction of a complex but linear path

Here's the decoration Caramdir came up with, adapted to draw only the first fraction of an arbitrary path:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations}

% A simple empty decoration, that is used to ignore the last bit of the path
\pgfdeclaredecoration{ignore}{final}
{
\state{final}{}
}

% Declare the actual decoration.
\pgfdeclaremetadecoration{middle}{initial}{
    \state{initial}[
        width={0pt},
        next state=middle
    ]
    {\decoration{moveto}}

    \state{middle}[
        width={\pgfdecorationsegmentlength*\pgfmetadecoratedpathlength},
        next state=final
    ]
    {\decoration{curveto}}

    \state{final}
    {\decoration{ignore}}
}

% Create a key for easy access to the decoration
\tikzset{middle segment/.style={decoration={middle},decorate, segment length=#1}}

\begin{document}\noindent
\foreach \step in {0.2,0.4,...,1} {%
\begin{tikzpicture}
    \draw[middle segment=\step,ultra thick,red,->] (0,0) to[out=30,in=150] (3,0) -- (7,1) -- (8,0);
\end{tikzpicture}\\
}
\end{document}

[Tex/LaTex] Rotate a node but not its content: the case of the ellipse decoration

The source of the difficulty is that ellipses are constructed in a particular way in TikZ. They are paths that start from the x-axis and proceed counter-clockwise around their centre. The vast majority of the time, the exact parametrisation doesn't matter. You appear to have found the one situation where it does!

In the actual question, you only want to be able to mirror the ellipse, and so draw it starting from the negative x-axis (the title of the question suggests a more flexible approach). That's actually not too hard since we can exploit the symmetry of the ellipse. The key is to provide it with a negative x-radius, since then it will start from the negative x-axis (and proceed clockwise, but we could correct for that by negating the y-radius as well). To do this, we interrupt the call from the node shape to the drawing command and flip the sign of the x-radius. The simplest way to do this is to redefine the \pgfpathellipse macro to do the negation and then call the original macro. The following code does this.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations,shapes,decorations.markings}
\makeatletter
\let\origpgfpathellipse=\pgfpathellipse
\def\revpgfpathellipse#1#2#3{%
  #2%
  \pgf@xa=-\pgf@x
  \origpgfpathellipse{#1}{\pgfqpoint{\pgf@xa}{0pt}}{#3}}
\makeatother

\tikzset{
  reversed ellipse/.style={
    ellipse,
    reverse the ellipse%
  },
  reverse the ellipse/.code={
    \let\pgfpathellipse=\revpgfpathellipse
  }
}
\begin{document}
\begin{tikzpicture}
\node[ellipse,
  draw,
  postaction={
    decorate,
    decoration={
      markings,
      mark=at position 1 with {
        \arrow[line width=5pt,blue]{>}
      }
    }
  }
] at (0,0) {hello world};

\node[reversed ellipse,
  draw,
  postaction={
    decorate,
    decoration={
      markings,
      mark=at position 1 with {
        \arrow[line width=5pt,blue]{>}
      }
    }
  }
] at (0,-2) {hello world};
\end{tikzpicture}

\end{document}

Here's the result:

rotated ellipse

(the arrow got clipped, but you can see where it lies)

Best Answer

Related Solutions

[Tex/LaTex] How to animate a path or how to draw starting fraction of a complex but linear path

[Tex/LaTex] Rotate a node but not its content: the case of the ellipse decoration

Related Question