[Tex/LaTex] Multiple equally spaced arrows from one rectangle node to another rectangle node

tikz-pgf

I want to connect multiple arrows from one rectangle node to the other. I tried by having a few coordinate nodes declared in parallel and connecting these coordinate nodes together to form parallel arrows and then tried to place the rectangle node exactly on top of the coordinate list but I never succeded. My solution is as shown in the first picture.

I am trying to do this
enter image description here

Thanks

Best Answer

To access selected positions of the node you can use the name of the node together with an angle separated by a dot, as in the following example:

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

\begin{document}

\begin{tikzpicture}
  \node[draw, text badly centered, text width=2cm,minimum size=1cm] (a) {Frequency synthesizer};
  \node[draw, text badly centered, text width=2cm,minimum size=1cm,below=of a] (b) {Code generator};
  \draw[->] (b.140) -- (a.220);
  \draw[->] (b.40) -- (a.320);
\end{tikzpicture}

\end{document}

enter image description here

A Minimal Code

\documentclass[tikz]{standalone}
\usetikzlibrary{arrows.meta}

\begin{document}

\pgfkeys{
  /pgf/arrow keys/.cd,
  shear/.store in=\pgfarrowshear,
  US/.style={
    length = +1.05pt 1.925 1,
    shear = 1.7pt,
  },
  UK/.style={
    length = +1.05pt 1.925 1,
    shear = -1.7pt,
  },
}
\makeatletter
\newdimen\pgfarrowshear
\let\oldmacro\pgf@arrow@drawer@shift
\def\pgf@arrow@drawer@shift{\pgftransformyshift\pgfarrowshear\oldmacro}

\begin{tikzpicture}
    \path(1,3)node(A){A}(3,1)node(B){B};;
    \draw[double,double distance=3pt,{<[US]}-{>[US]},bend left](A)to(B){};
    \draw[double,double distance=3pt,{<[UK]}-{>[UK]},bend left](B)to(A){};
\end{tikzpicture}

\end{document}

Step by Step

Without lost of generality, assume arrows are pointing from left to right.

In pgfcorearrows.code.tex, Tikz shifts arrow tips using \pgf@arrow@drawer@shift, defined as

\def\pgf@arrow@drawer@shift#1#2#3{% tip end, back end, line end, sep
  \pgf@xb#2\pgftransformxshift{-\pgf@xb}%
  \pgf@xc#1%
  \advance\pgf@xc by\pgfarrowsep%
  \advance\pgf@xc by-\pgf@xb%
}

This step is necessary because sometimes we have shorten >= or sep= (in case there are more tips). But no y shift is applied because logically arrow tips are aligned to the main path.

So we have a dirty step which rewrites this macro as follows

\def\pgf@arrow@drawer@shift#1#2#3{
  \pgftransformyshift\pgfarrowshear% add this line
  \pgf@xb#2\pgftransformxshift{-\pgf@xb}%
  \pgf@xc#1%
  \advance\pgf@xc by\pgfarrowsep%
  \advance\pgf@xc by-\pgf@xb%
}

Next we want to make shear= acts like sep=. So I modified the code of sep/.code

\pgfkeys{
  /pgf/arrow keys/.cd,
  shear/.code={
    \pgfarrowsthreeparameters{#1}%
    \expandafter\pgfarrowsaddtooptions\expandafter{\expandafter\pgfarrowslinewidthdependent\pgfarrowstheparameters\pgfarrowshear\pgf@x}%
  },
  shear/.default = +0pt -.5 -.5
}

So now we can achieve the following by something like [-{>[shear=0pt .5 .5]>[shear]>[shear=0pt .5 .5]>[shear]}]

Line Width Dependent

However, \pgfarrowslinewidthdependent will never produce the right amount of y shift. If you look at the manual carefully, it calculates

wi = inner line width
wo = outer line width
wt = total line width = inner + 2*outer
w  = #3*wo + (1-#3)*wt = weighted line width
return #1 + #2*w

Which is, equivalently

#1 + #2 * [ (1-#3)*wi + (2-#3)*wo ]

While what we want is

0 + .5*wi + .5*wo

It leads to

#1 = 0
#2 = 0
#3 = ∞

I do not know why TikZ do so. Anyway, we can do it ourself.

\def\pgfarrowslinewidthdependentnew#1#2#3{%
  \pgf@x#1%
  \ifdim\pgfinnerlinewidth>0pt%
    \pgf@arrows@inner@line@width@depnew{#2}{#3}%
  \else%  
    \advance\pgf@x by#2\pgflinewidth%
  \fi%
}
\def\pgf@arrows@inner@line@width@depnew#1#2{%
  % #1 * outer line width + #2 * inner line width = our new one = the following
  % (#1/2) * full line width + (#2-#1/2) * inner line width)
  % Compute "real" line width
  \pgf@xa.5\pgflinewidth%
  \pgf@xa#1\pgf@xa%
  \advance\pgf@x by\pgf@xa%
  \pgf@xa\pgfinnerlinewidth%
  \pgf@xb.5\pgf@xa%
  \advance\pgf@x by#2\pgf@xa%
  \advance\pgf@x by-#1\pgf@xb%
}

For Convenience

We define

\pgfkeys{
  /pgf/arrow keys/.cd,
  Bidirectional/.style={
    length = +1.05pt 1.925 1,
    shear
  }
}

So we can have

\begin{tikzpicture}
    \path(0,4)(4,0);
    \node(A)at(1,3){A};
    \node(B)at(3,1){B};
    \draw[double,double distance=3pt,{<[Bidirectional]}-{>[Bidirectional]},bend left]
        (A)to(B){};
\end{tikzpicture}

Code

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{arrows.meta}

\begin{document}

\makeatletter

\pgfkeys{
  /pgf/arrow keys/.cd,
  Bidirectional/.style={
    length = +1.05pt 1.925 1,
    shear
  },
  shear/.code={
    \pgfarrowsthreeparameters{#1}%
    \expandafter\pgfarrowsaddtooptions\expandafter{\expandafter\pgfarrowslinewidthdependentnew\pgfarrowstheparameters\pgfarrowshear\pgf@x}%
  },
  shear/.default = +0pt -.5 -.5
}
\newdimen\pgfarrowshear
\pgfarrowshear0pt
\def\pgfarrowslinewidthdependentnew#1#2#3{%
  \pgf@x#1%
  \ifdim\pgfinnerlinewidth>0pt%
    \pgf@arrows@inner@line@width@depnew{#2}{#3}%
  \else%  
    \advance\pgf@x by#2\pgflinewidth%
  \fi%
}
\def\pgf@arrows@inner@line@width@depnew#1#2{%
  % #1 * outer line width + #2 * inner line width = our new one = the following
  % (#1/2) * full line width + (#2-#1/2) * inner line width)
  % Compute "real" line width
  \pgf@xa.5\pgflinewidth%
  \pgf@xa#1\pgf@xa%
  \advance\pgf@x by\pgf@xa%
  \pgf@xa\pgfinnerlinewidth%
  \pgf@xb.5\pgf@xa%
  \advance\pgf@x by#2\pgf@xa%
  \advance\pgf@x by-#1\pgf@xb%
}
\def\pgf@arrow@drawer@shift#1#2#3{
  \pgftransformyshift\pgfarrowshear%
  \pgf@xb#2\pgftransformxshift{-\pgf@xb}%
  \pgf@xc#1%
  \advance\pgf@xc by\pgfarrowsep%
  \advance\pgf@xc by-\pgf@xb%
}

\begin{tikzpicture}
    \draw[-{>[sep=1pt]>}](1,1)->(3,1);
\end{tikzpicture}

\begin{tikzpicture}[>={Classical TikZ Rightarrow[length = +1.05pt 1.925 1]}]
    \draw[double distance=3pt,-{>[shear=0pt .5 .5]>[shear]>[shear=0pt .5 .5]>[shear]}]
        (1,1)->(3,1);
\end{tikzpicture}

\begin{tikzpicture}
    \path(0,4)(4,0);
    \node(A)at(1,3){A};
    \node(B)at(3,1){B};
    \draw[double,double distance=3pt,{<[Bidirectional]}-{>[Bidirectional]},bend left]
        (A)to(B){};
\end{tikzpicture}

\end{document}

[Tex/LaTex] Drawing a block diagram with TiKz

Is this what you were looking for?

Fixes:

Added a node Measurement positioning it halfway between the nodes System and Model using this syntax: \node ... at ($(system)!.5!(model)$) {};. This requires calc to be added to the Tikz libraries.
Changed your diagonal path to \draw [->] (outfork) -| (sum.north) node [very near end] {$t$}; so that the node stops exactly at the north point of sum.
The [very near end] above ensures that the node appears very close to the arrow tip.
Removed minimal size for your nodes that makes them look square (it's a bit ugly), and replaced it with inner sep which adds space inside the node consistently so that the rectangle borders are equally far from the node text.
For the node u (the path on the left), I added the key [anchor=south west] so that it moves it right and up a bit and appears next to the path.
Used labels for the - and + symbols. Originally they were nodes but it looks better like this and the code is cleaner and shorter.

\documentclass{standalone}

\usepackage{tikz}       
\usetikzlibrary{arrows,positioning,patterns,decorations.pathmorphing,calc}

\begin{document}

\tikzstyle{block} = [draw, rectangle, inner sep=6pt]
\tikzstyle{joint} = [draw, circle,minimum size=1em]

\begin{tikzpicture}[>=stealth, auto, node distance=2cm]
    % Place nodes
    \node [block] (system) {System};
    \node [coordinate, left=of system] (infork) {};
    \node [coordinate, left=of infork] (input) {};
    \node [coordinate, right=of system] (outfork) {};
    \node [coordinate, right=of outfork] (output) {};
    \node [coordinate, above=of system] (disturbances) {};
    \node [block, below=of system] (model) {Model};
    \node [joint, right=of model, anchor=center,label={[shift={(2mm,-1mm)}]-},label={[shift={(-3mm,-5.5mm)}]\tiny +}] (sum) {};

    \node [coordinate, right=of sum] (error) {};
    \node [block, dashed, fill=gray, anchor=center, text width=7cm, align=center] at ($(system)!.5!(model)$) {\textsc{Measurement}};

    % Connect nodes
    \draw [->, decorate, decoration={snake, post length=1mm}] (disturbances) -- node {\(d'\)} (system);
    \draw [->] (input) -- node {\(u'\)} (system);
    \draw [->] (system) -- node {\(t'\)} (output);
    \draw [->] (model) -- node {\(y\)} (sum);
    \draw [->] (sum) -- node {\(\epsilon\)} (error);
    \draw [->] (infork) |- node [anchor=south west] {\(u\)} (model);
    \draw [->] (outfork) -| (sum.north) node [very near end] {\(t\)};
\end{tikzpicture}

\end{document}