path with both mark connection node and arrow tip

Question

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}

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?

Answer 1

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}

Answer 2

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}

Answer 3

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.

Answer 4

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):

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