[Tex/LaTex] coordinate along decorated path

pgf-decorationstikz-pgf

I'm using the brace decoration from the TikZ decorations.pathreplacing library to draw a brace around some nodes.
Now I would like to connect the tip of the brace to another path (use case: labelling terms in an equation).
First, I used the LaTeX command \overbrace, but realised it would be difficult to connect a tikz-path to the tip.
It should be easier using the TikZ decoration brace.
However, if I place node[midway] along the path, the node is placed midway along the unaltered path, not midway along the decorated path.
Below is code along with the undesired output.

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations.pathreplacing}
\begin{document}
\begin{tikzpicture}
\node (spam) at (1, 1) {Spam};
\node (eggs) at (4, 1) {Eggs};
\draw[decoration={brace,amplitude=5mm}, decorate] (spam.north) -- (eggs.north) coordinate[midway] (mid);
\draw (mid) -- ++(0,1);
\end{tikzpicture}
\end{document}

Second path not connected properly

Of course, in the present example, I could simply replace the second \draw-command by \draw (mid) ++(0,5mm) -- ++(0,1);.
But I would like something more stable that I don't need to tweak by hand, and maybe even work for arbitrary decorated paths.

So, my question is:

How do I place a coordinate at some point along a decorated path?

UPDATE 2012-12-17: To show the use case, I add a picture of the solution here.

enter image description here

Best Answer

This is a funny one! :)

You can accomplish this by realizing that the decoration is a path in itself.

What you just then need to do is access that path!

You can do this by applying a postaction on the path and decorate it again!

But as the decoration is in a separate key directory you need to explicitly tell to use /tikz/postaction. You can then access any coordinate however you wish, i choose to use the decoration library markings.

So here it is:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations.pathreplacing}
\usepgflibrary{decorations.markings}
\begin{document}
\begin{tikzpicture}
  \node (spam) at (1, 1) {Spam};
  \node (eggs) at (4, 1) {Eggs};
  \node (bacon) at (4, 3) {Bacon};
  \draw[decoration={brace,amplitude=5mm}, decorate] (spam.north) -- (eggs.north) coordinate[midway] (mid);
  \draw (mid) -- ++(0,1);
  \draw[decoration={brace,amplitude=5mm,
      /tikz/postaction={
          decoration={
              markings,
              mark=at position 0.45 with \coordinate (Mb);,
              mark=at position 0.5 with \coordinate (M);,
              mark=at position 0.55 with \coordinate (Ma);
          },decorate
      }
  },decorate] (spam.north) -- (bacon.north);
  \draw[red] (Mb) circle (2pt);
  \draw[blue] (M) circle (2pt);
  \draw[green] (Ma) circle (2pt);
\end{tikzpicture}
\end{document}

An the output becomes: output of tex

Related Solutions

[Tex/LaTex] Drawing an arrow tip to every node of a path

Your first command only creates one path and TikZ will apply the arrowheads to the end of that path. So you need to create separate paths for the individual bits. One relatively quick way to insert subpaths into a statement is the edge command:

\documentclass{article}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}
    \path[|<->|]
             (0.4,2.4) edge node[right] {$L$} +(0,-1.5)
            ++(0,-1.5) edge node[right] {$d$} +(0,-1)
            ++(0,-1)   edge node[right] {$h$} +(0,-1);
    \end{tikzpicture}
\end{document}

The only drawback is that edge doesn't advance the current position, so that you have to repeat the coordinates. Also you need to use \path instead of \draw to avoid the drawing of an empty path with arrowheads.

[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] Drawing an arrow tip to every node of a path

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

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