10

I would like to use a mark connection node decoration on a line that also has an arrow tip. Unfortunately, using the decoration causes the arrow tip to point in the wrong direction (always straight up). For example:

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}

\begin{document}
\begin{tikzpicture}
  \node (A) at (0, 0) {A} ;
  \node (B) at (3, 0) {B} ;
  \draw [->, decorate, decoration={
    markings,
    mark connection node=label,
    mark=at position .5 with {
      \node (label) {label};
    },
  }]
  (A) -- (B)
  ;
\end{tikzpicture}
\end{document}

output showing incorrect arrow tip direction

Notice the arrow tip, just to the left of node B, pointing up instead of to the right.

The Tikz documentation mentions that “the decoration usually destroys the path. However, this is no longer the case when the [mark connection node] key is set.” If the path is not being destroyed, then why is the arrow tip oriented incorrectly? Is there any way to get arrow tips and mark connection node to coexist on the same path?

Ben Liblit
  • 1,433

4 Answers4

6

I don't really know what is happening here with the ->. But you can use mark=at position 1 with {\arrow{>}}; to get the desired result. Also, you can define a style label decorate so that you can use decorate option for other elements in your tikzpicture or document.

\documentclass[border=5,convert={density=150},resize=200]{standalone}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}
\tikzset{label decorate/.style={
    decorate,
    decoration={
    markings,
    mark connection node=label,
    mark=at position .5 with {
      \node (label) {label};
    },
    mark=at position 1 with {\arrow{>}};
    }}}

\begin{document}
\begin{tikzpicture}
  \node (A) at (0, 0) {A} ;
  \node (B) at (3, 0) {B} ;
  \draw [label decorate] (A) -- (B);
\end{tikzpicture}
\end{document}

enter image description here

hpesoj626
  • 17,282
  • This looks good when small. At higher magnifications, one can see that the right edge of the line extends slightly beyond the arrow tip, leading to something like --->- (though not as exaggerated). I suppose I can use shorten >=.5pt to trim off the excess. But I've never really liked using shorten >. It requires manually guessing the proper amount to shorten and needs to be tweaked whenever I change the arrow style, line thickness, etc. Not sure how to automate that better. – Ben Liblit Feb 26 '13 at 23:30
  • Yes, I see your point. Indeed, a shorten >-.5pt will do the trick. It is up to you now if you want to do this. It looks to me like @percusse's solution is neater. I don't know though if you still have some other uses for mark connection node. But, then again, choose what you think is the best solution. :) – hpesoj626 Feb 26 '13 at 23:47
  • I do like the way your approach neatly tucks everything away in a style. And I could get over my distaste for shorten >. But now I am seeing a different problem. If the line is supposed to curve, your solution instead draws two straight line segments. Try changing (A) -- (B) to (A) to [bend right] (B) and you should see what I mean. The label is correctly shifted downward, but the line segments from A to the label and from the label to B are both straight. Without label decorate, the line from A to B is curved as desired. – Ben Liblit Feb 27 '13 at 00:05
  • @BenLiblit Perhaps this necessitates a separate question. – hpesoj626 Feb 27 '13 at 00:24
  • bend right has the same problematic effect on edge. In both cases, mark connection node turns the bend into two straight segments. But I'm inclined to agree that this is a separate question. – Ben Liblit Feb 27 '13 at 00:27
  • @BenLiblit Okay. You can refer to this post for background. – hpesoj626 Feb 27 '13 at 00:36
  • let us continue this discussion in chat – hpesoj626 Feb 27 '13 at 00:42
5

Looking at the figure, my code would be very different. I don't see the need of decorations for that:

\documentclass{standalone}
\usepackage{tikz}
\tikzset{
  label/.style = {font=\footnotesize, midway, fill=white, anchor=center}
}

\begin{document}
\begin{tikzpicture}
  \node (A) at (0, 0) {A} ;
  \node (B) at (3, 0) {B} ;
  \draw [->] (A) -- (B) node[label] {label};
\end{tikzpicture}
\end{document}

Result

JLDiaz
  • 55,732
  • 2
    This works well for my minimal example, and is the simplest-looking solution yet offered. In a more complex diagram, though, the fill=white strategy will create problems if any other background content is present. An ideal solution would create a transparent gap in the line where it passes through the label. That's what I'd hoped mark connection node would do. – Ben Liblit Feb 26 '13 at 22:19
  • My background content concerns are really hypothetical, as I have no such background material in the diagram I actually want to make. I do have curves, though, and of the three solutions proposed so far, this is the only one that lets curves remain curved. I'm going to mark this as the accepted answer. I will probably post a new question with the challenge of interrupting a curved line with a label without obscuring background content. – Ben Liblit Feb 28 '13 at 15:56
4

The path is not destroyed but also not available after the decoration application. In other words, the decoration uses the path just to plug in the node but the actual arrow placement does not have access to it anymore. So after the decoration applied the arrow placement code sees a zero length path and the arrow is misplaced.

A similar effect can be obtained by 

\documentclass[tikz]{standalone}
\usetikzlibrary{calc}
\begin{document}
\begin{tikzpicture}
  \node (A) at (0, 0) {A} ;
  \node (B) at (3, 0) {B} ;
\draw[->] (A) node[append after command={--(label)--(B)}] (label) at ($(A)!0.5!(B)$) {label};
\end{tikzpicture}
\end{document}

But if the path is complicated curve etc. marking with an additional arrow head is better.

enter image description here

percusse
  • 157,807
  • Unfortunately I do indeed need a curved path. I omitted it from my initial example because I did not realize it would an extra difficulty, but I need the line from A to B to bend right. Do you know a way to do that? hpesoj626 posted a suggestion that marks with an additional arrow head, but that approach also breaks when using a bend right line from A to B. – Ben Liblit Feb 27 '13 at 00:10
0

For straight lines, use node on line which just needs you to do

\draw[->] (A) to [node on line] node {label} (B);

This will place the node first and then only draw the connections

  (A) -- (label) -- (B)

which automatically skips the line through label (that's the whole point of nodes).

For any other curved or straight lines that do have more than one component (i.e. |- and -|), this will not work anymore since the path from A to B will not be the same as the one from A to a node on that path and from that node to B.

However, we can use the same concept from node on line to solve this with the help of the spath3 library. For this, we do create the original curve (but don't draw it) and do place the nodes (if needed even drawn) and both paths gets saved so that spath3 can then be used to

  • find the intersections,
  • break the curved path up at these points (both these points are done by split at intersections with = {<curve path>}{<node path>}),
  • remove that component between these intersections, i.e. the 2nd, 4th, … one (unless you place a node so that it does not fully lie on the curve) and then
  • use it (or append it when we're not talking about connections between nodes).

Since that edge that is used for the original curve and node is its own group, it is unfortunately needed to globalize these paths. I couldn't find a way to set up a path where I can add the paths of multiple nodes on it.

There are some distinction to be made between

  • curves between coordinates (→ node on curve, always appends) and
  • curves between nodes (→ node on curve', always uses) and then
    • curves with to (needs to reinsert the target as the next start node)
    • curves with edge (doesn't need it but it wouldn't hurt much anyway, I guess).

This will work for mulitple nodes along the curve but it pollutes a few global macros for that.

Connections between both coordinates and nodes might get tricky with only node on curve and node on curve' but feel free to test it and talk to me about it.

I don't know what the speed impact is of all the needed work spath3 and intersection have to do but you can always externalize these diagrams.

With the update that allows multiple nodes on a path, the second picture actually takes a bit longer than the others, don't know what that's about.

As a last nodenote, the intersections library (which is used by spath3) finds the literal intersection between the curve and the node's path which means that the curve parts doesn't just touch the node's border but protrudes into it (this also means that the outer sep values have no impact on the drawn curve) but luckily, the node (on the edge) is placed on top of the parent path so that small snippet of the curve is just covered and it looks almost better
… unless you use opacity (but neither to arrow tips look good then):

enter image description here

I could also see some messing around with an inverse and remembered clip after another answer of mine but a bit more housekeeping around it is needed.

Code

\documentclass[tikz, border=5pt]{standalone}
\usetikzlibrary{spath3, intersections, quotes}
\makeatletter
\newcounter{@nodeoncurve@}
\tikzset{
  spath/split multiple at intersections/.style n args={3}{
    /utils/temp/.style={
      /tikz/spath/split at intersections with={#1}{#2##1}},
    /utils/temp/.list={#3}},
  node on curve/.default=line to,
  node on curve'/.default=line to,
  node on curve/.style={@node on curve={#1}{append}{}},% normal path
  node on curve'/.style={@node on curve={#1}{use}{% for edges and tos
    \ifx\tikz@to@or@edge@function\tikz@do@to(\tikztotarget)\fi}},
  @node on curve/.style n args={3}{
    to path={
      \pgfextra{%
        \setcounter{@nodeoncurve@}{0}%
        \edef\tikz@temp{% rescuing nodes and target for edge
          edge[%
            #1, path only,% path only = no draw, no fill, …
            every edge quotes/.append style={auto=false},% node *on* the line
            nodes={
              /utils/exec=\noexpand\stepcounter{@nodeoncurve@},
              spath/save global=@node@nodeoncurve@\noexpand\the\c@@nodeoncurve@,
            },
            spath/save global=@curve@nodeoncurve@
          ]
          \unexpanded\expandafter{\tikz@tonodes}(\tikztotarget)
        }\expandafter}\tikz@temp
        [
          spath/.cd,
            split multiple at intersections/.expanded={@curve@nodeoncurve@}
                                                      {@node@nodeoncurve@}
                                               {1,...,\the\c@@nodeoncurve@},
            remove components/.expanded={@curve@nodeoncurve@}{2\ifnum\c@@nodeoncurve@
              >1 ,4,...,\pgfinteval{2*\the\c@@nodeoncurve@}\fi},
            #2=@curve@nodeoncurve@
        ]
        #3
     }
  }
}
\makeatother
\usetikzlibrary{backgrounds}
\tikzset{every picture/.append style={show background rectangle},
background rectangle/.append style={draw=none, left color=green, right color=violet}}
\begin{document}
\begin{tikzpicture}[ultra thick]
  \node (A) at (0, 0) {A} ;
  \node (B) at (3, 0) {B} ;
  \path [red, ->, node on curve'=bend left]
    (A) edge node[blue,draw]{label} (B)
    (B) edge ["label"] (A);
\end{tikzpicture}
\begin{tikzpicture}[ultra thick]
  \node (A) at (0, 0) {A} ;
  \node (B) at (3, 0) {B} ;
  \draw [red, ->, node on curve'=bend left]
    (A) to node[blue,draw]{label} (B)
        to ["X" {sloped, very near start},
            "Y" {sloped, very near end, inner sep=.1em},
            "label"] (A);
\end{tikzpicture}
\tikz[inner sep=.15em, circle, nodes={draw, green}, sloped, ultra thick]
  \draw[->, node on curve=bend left] (0,0) to["0"]               (1,1)
                                           to["1"]               (2,0)
                           to["2" near start, "3", "4" near end] (4,1)
                                           -- ++(down:1);
\end{document}

Output

enter image description here enter image description here enter image description here

Qrrbrbirlbel
  • 119,821