[Tex/LaTex] Two parallel lines between nodes of arbitrary size and position

tikz-pgf

I would like to draw two parallel arrows between two nodes in TikZ.
The difficulty is that the nodes can be of any size and can be arranged in any order.

Related:

The question Parallel arrows between nodes of varying size deals with a similar problem, but the positions of the nodes are not arbitrary.

I would also like to be able to customize the arrows individually (e.g., different colors or show them one after the other in the presentation).
Therefore, the answers to the question Connect Tikz nodes via two parallel arrows are not applicable.

First Approaches and description of MWE:

Here is a MWE with first approaches.

Using the angle results in non-parallel lines for different sizes (if the angles are simply set to the same sizes).
The bend left option also uses angles, but calculates them automatically.
The approach with a fixed shift up and down with [yshift=...] requires manual effort and does not draw the arrows up to the nodes. In addition, there would be problems with this approach if the nodes were at an arbitrary angle to each other, as in 4.
This is the desired result. However, the code was adjusted manually.

MWE:

\documentclass[border=3mm]{standalone}

\usepackage{tikz}
\usetikzlibrary{positioning}
\usetikzlibrary{arrows}

\begin{document}

\begin{tikzpicture}[
    every node/.style={draw, circle},
    >=stealth',
]

    \node (a) {1};
    \node[right=of a] (b) {bbb};

    \draw[red] (a) -- (b);
    \draw[->] (a.20) -- (b.160);
    \draw[<-] (a.-20) -- (b.-160);

    \begin{scope}[yshift=-18mm]
        \node (a) {2};
        \node[right=of a, yshift=5mm] (b) {bbb};

        \draw[red] (a) -- (b);
        \draw[->] (a) to[bend left] (b);
        \draw[->] (b) to[bend left] (a);
    \end{scope}

    \begin{scope}[xshift=30mm]
        \node (a) {3};
        \node[right=of a] (b) {bbb};

        \draw[red] (a) -- (b);
        \draw[->] ([yshift=1mm]a.east) -- ([yshift=1mm]b.west);
        \draw[<-] ([yshift=-1mm]a.east) -- ([yshift=-1mm]b.west);
    \end{scope}

    \begin{scope}[xshift=30mm, yshift=-18mm]
        \node (a) {4};
        \node[right=of a, yshift=5mm] (b) {bbb};

        \draw[red] (a) -- (b);
        \draw[->] (a.30) -- (b.185);
        \draw[<-] (a.00) -- (b.205);
    \end{scope}
\end{tikzpicture}

\end{document}

Goal:

I would like to have an option move left=#1 (like bend left=#1) which gives a result as in part 4 of the above figure.

Below is a test example with differently arranged nodes of different sizes.
The two lines which should draw the arrows are commented out.

Minimal test example:

\documentclass[border=3mm]{standalone}

\usepackage{tikz}
\usetikzlibrary{positioning}
\usetikzlibrary{arrows}
\usetikzlibrary{calc}

\begin{document}

\begin{tikzpicture}[
    every node/.style={draw, circle},
    >=stealth',
]

    \foreach \d in {1,...,16} {
        \node (a\d) at ($(0,0)+(90-\d*22.5:25mm)$) {\footnotesize $a_{\d}$};
        \node[minimum size=10+\d mm] (b\d) at ($(0,0)+(90-\d*22.5:50mm)$) {\footnotesize $b_{\d}$};
        \draw[red] (a\d) to (b\d);

        % TODO:
        %\draw[blue] (a\d) to[move left=30] node{x} (b\d);
        %\draw[yellow] (b\d) to[move right=30] node{y} (a\d);
    }
\end{tikzpicture}

\end{document}

Best Answer

For circle nodes it is rather easy.

\documentclass[tikz,border=3.14mm]{standalone}
\usetikzlibrary{positioning}
\usetikzlibrary{arrows}
\usetikzlibrary{calc}
\tikzset{shifted path/.style args={from #1 to #2 by #3}{insert path={
let \p1=($(#1.east)-(#1.center)$),
\p2=($(#2.east)-(#2.center)$),\p3=($(#1.center)-(#2.center)$),
\n1={veclen(\x1,\y1)},\n2={veclen(\x2,\y2)},\n3={atan2(\y3,\x3)} in
(#1.{\n3+180+asin(#3/\n1)}) to (#2.{\n3-asin(#3/\n2)})
}}}
\begin{document}

\begin{tikzpicture}[
    every node/.style={draw, circle},
    >=stealth',
]

    \node (a) {1};
    \node[right=of a] (b) {bbb};

    \draw[red] (a) -- (b);
    \draw[blue,->,shifted path=from a to b by 3pt];
    \draw[blue,->,shifted path=from b to a by 3pt];

    \begin{scope}[yshift=-18mm]
        \node (a) {2};
        \node[right=of a, yshift=5mm] (b) {bbb};

        \draw[red] (a) -- (b);
        \draw[blue,bend left,->,shifted path=from a to b by 3pt];
        \draw[blue,bend left,->,shifted path=from b to a by 3pt];
    \end{scope}

    \begin{scope}[xshift=30mm]
        \node (a) {3};
        \node[right=of a] (b) {bbb};

        \draw[red] (a) -- (b);
        \draw[blue,->,shifted path=from a to b by 3pt];
        \draw[blue,->,shifted path=from b to a by 3pt];
    \end{scope}

    \begin{scope}[xshift=30mm, yshift=-18mm]
        \node (a) {4};
        \node[right=of a, yshift=5mm] (b) {bbb};

        \draw[red] (a) -- (b);
        \draw[blue,bend left,->,shifted path=from a to b by 3pt];
        \draw[blue,bend left,->,shifted path=from b to a by 3pt];
    \end{scope}
\end{tikzpicture}
\end{document}

Or a version that supports edge labels. (I'm not a big fan of quotes.)

\documentclass[tikz,border=3.14mm]{standalone}
\usetikzlibrary{positioning}
\usetikzlibrary{arrows}
\usetikzlibrary{calc}
\tikzset{dexteritas/.cd,
shifted path label/.style={pos=0.5,draw=none,rectangle,auto,sloped}}
\tikzset{shifted path/.style args={from #1 to #2 by #3}{insert path={
let \p1=($(#1.east)-(#1.center)$),
\p2=($(#2.east)-(#2.center)$),\p3=($(#1.center)-(#2.center)$),
\n1={veclen(\x1,\y1)},\n2={veclen(\x2,\y2)},\n3={atan2(\y3,\x3)} in
(#1.{\n3+180+asin(#3/\n1)}) to 
(#2.{\n3-asin(#3/\n2)})
}}}
\tikzset{labeled shifted path/.style args={from #1 to #2 by #3 label #4}{insert path={
let \p1=($(#1.east)-(#1.center)$),
\p2=($(#2.east)-(#2.center)$),\p3=($(#1.center)-(#2.center)$),
\n1={veclen(\x1,\y1)},\n2={veclen(\x2,\y2)},\n3={atan2(\y3,\x3)} in
(#1.{\n3+180+asin(#3/\n1)}) to node[dexteritas/shifted path label]{#4}
(#2.{\n3-asin(#3/\n2)})
}}}

\makeatother
\begin{document}

\begin{tikzpicture}[
    every node/.style={draw, circle},
    >=stealth',
]

    \node (a) {1};
    \node[right=of a] (b) {bbb};

    \draw[red] (a) -- (b);
    \draw[blue,->,shifted path=from a to b by 3pt];
    \draw[blue,->,shifted path=from b to a by 3pt];

    \begin{scope}[yshift=-18mm]
        \node (a) {2};
        \node[right=of a, yshift=5mm] (b) {bbb};

        \draw[red] (a) -- (b);
        \draw[blue,bend left,->,shifted path=from a to b by 3pt];
        \draw[blue,bend left,->,shifted path=from b to a by 3pt];
    \end{scope}

    \begin{scope}[xshift=30mm]
        \node (a) {3};
        \node[right=of a] (b) {bbb};

        \draw[red] (a) -- (b);
        \draw[blue,->,shifted path=from a to b by 3pt];
        \draw[blue,->,shifted path=from b to a by 3pt];
    \end{scope}

    \begin{scope}[xshift=30mm, yshift=-18mm]
        \node (a) {4};
        \node[right=of a, yshift=5mm] (b) {bbb};

        \draw[red] (a) -- (b);
        \draw[blue,bend left,->,labeled shifted path=from a to b by 3pt label hi];
        \draw[blue,bend left,->,shifted path=from b to a by 3pt];
    \end{scope}
\end{tikzpicture}
\end{document}

Related Solutions

[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)

[Tex/LaTex] How to define the default vertical distance between nodes

I don't really get the question so I hope this is what you wanted. If you include a full document (such that we copy paste and see the problem on our systems) things are much more easier.

Here, you can change the default setting within a scope but your block style had a node distance which was resetting every time it is issued. I've made it 2mm such that we can see the difference easier.

\documentclass[tikz]{standalone}
\usetikzlibrary{arrows,shapes.geometric,positioning}

\begin{document}
\begin{tikzpicture}[decision/.style={diamond, draw, text width=4.5em, text badly centered, node distance=3.5cm, inner sep=0pt},
                    block/.style   ={rectangle, draw, text width=6em, text centered, rounded corners, minimum height=4em, minimum height=2em},
                    cloud/.style   ={draw, ellipse, minimum height=2em},
                    line/.style    ={draw,-latex'},
                    node distance = 1cm, 
                    auto]
    \node [block] (1st) {1st};
    \node [block, right= of 1st]  (2nd1) {2nd1};
\begin{scope}[node distance=2mm and 10mm]%Here we change it for everything inside this scope
    \node [block, above= of 2nd1] (2nd2) {2nd2};
    \node [block, below= of 2nd1] (2nd3) {2nd3};
    \node [block, right= of 2nd1] (3rd1) {3rd1};
    \node [block, above= of 3rd1] (3rd2) {3rd2};
    \node [block, above= of 3rd2] (3rd3) {3rd3};
\end{scope}
    \node [block, below= of 3rd1] (3rd4) {3rd4};
    \node [block, below= of 3rd4] (3rd5) {3rd5};

    \path [line] (1st) -- (2nd1);
    \path [line] (2nd1) -- (2nd2);
    \path [line] (2nd1) -- (2nd3);
    \path [line] (2nd2) -- (3rd3);
    \path [line] (2nd1) -- (3rd1);
    \path [line] (1st) -- (2nd1);
\end{tikzpicture}
\end{document}

enter image description here

Best Answer

Related Solutions

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

[Tex/LaTex] How to define the default vertical distance between nodes

Related Question