[Tex/LaTex] Tikz arrow starting exactly in the center

arrowstikz-pgf

I have the following problem. I am using the tikz package to draw figures. I am drawing some sort of a data structure. This includes having nodes and lines which connect nodes. Sometimes, a line has to start with a dot and end with an arrow. I have the following code (using arrows and patterns tikz libraries):

\documentclass{article}

\usepackage{verbatim}
\usepackage{tikz}
\usepackage{pgfplots}
\usetikzlibrary{patterns}
\usetikzlibrary{arrows}
\begin{document}
\begin{figure}[h]

    \begin{tikzpicture}
      [internal/.style={rectangle,draw},
       bmp/.style={rectangle,draw,pattern=north east lines},
       ]
      \node(root)     at ( 0.00, 0.00)                           {\verb=root=};
      \node(in1)      at ( 0.00,-0.60)          [internal,minimum width=3mm,label=left:I1]        {};
      \draw[-stealth](root)--(in1);

      \node(cn1)      at ( 0.00,-1.2)           [internal,minimum width=6mm,label=left:C1]        {};
      \node(cn1bmp)   at (-0.15,-1.2)           [bmp,minimum width=3mm]             {};
      \node(cn1arr1)  at ( 0.15,-1.2)           [internal,minimum width=3mm]        {};
      \draw[*-stealth](in1.mid)--(cn1);

      \node(k1)       at ( 0.50,-1.8)                                               {$k_1$};
      \draw[*-stealth](cn1arr1.mid)--(k1);

      \node(cas)      at ( 0.8,-0.20)                              {\verb=CAS=};
      \draw[->, shorten >=2pt](cas)--(in1);

      \node(lab)      at (-0.70,-1.80) [internal]     {C};
    \end{tikzpicture}

\caption{Trie examples}
\label{f-tries}
\end{figure}
\end{document}

This yields the following picture:

enter image description here

Now, as you can see, the arrow pointing to k1 (corresponding to \draw[*-stealth](cn1arr1.mid)--(k1);) starts a little bit to the right. When I have more arrows, this looks nasty – they all seem to have offsets in various directions.

Is there any way to force the arrow to start in exactly the center (cn1arr1.center doesn't work – it puts it too low)?

Thanks!

Best Answer

Simple solution: Use the shorten < = <length> syntax, which will move the arrow tip (the circle in your case) a specified length along the path. Using

\draw[*-stealth,shorten <=-2.5pt](cn1arr1.center)--(k1);

works quite well in this case.

A more correct way would be to define a new arrow tip using \pgfdeclarearrow{start name}{end name}{extend code}{arrow tip code}, where the extend code (which moves the arrow tip along the line) is left empty. Here's an implementation that is based on the original * arrow tip code, so the circle diameter will be identical to the original one, but it will be centered on the start of the line:

\makeatletter
\pgfarrowsdeclare{new*}{new*}{}
{
  \pgfutil@tempdima=0.4pt%
  \advance\pgfutil@tempdima by.2\pgflinewidth%
  \pgfsetdash{}{+0pt}
  \pgfpathcircle{\pgfqpoint{0pt}{0pt}}{+4.5\pgfutil@tempdima}
  \pgfusepathqfillstroke
}
\makeatother

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