TikZ get current unit length of x-coordinate

Question

I am searching for a macro which prints the current x-unit-length. For instance the following document:

\documentclass[border=2pt]{standalone}

\usepackage{tikz}
\usetikzlibrary{calc}

\begin{document}

  \begin{tikzpicture}[x=2cm,y=3cm]
    \node at (0,0) {};
  \end{tikzpicture}
\end{document}

Now I want a macro which can return the '2cm' of 'x' or '3cm' of y.

Paul Gaborit · Accepted Answer · 2018-04-04T14:45:40.107

8

Here is a solution using the calc library:

\documentclass[border=2pt]{standalone}

\usepackage{tikz}
\usetikzlibrary{calc}

\def\getxlengthincm#1{
  \path let \p{x}=(1,0), \n{xlencm}={scalar(veclen(\x{x},\y{x})/1cm)}
  in \pgfextra{\xdef#1{\n{xlencm}}};
}

\begin{document}
\begin{tikzpicture}[x=2cm,y=3cm]
  \getxlengthincm{\myxlength}
  \node[align=center] at (0,0) {x length:\myxlength cm};
\end{tikzpicture}
\end{document}

Here is a variant with explicit pt unit:

\def\getxlength#1{
  \path let \p{x}=(1,0), \n{xlen}={veclen(\x{x},\y{x})}
  in \pgfextra{\xdef#1{\n{xlen}}};
}

edited Apr 04 '18 at 14:45

answered Apr 04 '18 at 12:22

Paul Gaborit

70,770
10
176
283

Why not simply use \path let \p{x}=(1,0), \n{xlencm}={scalar(\x{x}/1cm)} without veclen ? – Kpym Apr 04 '18 at 13:51
@Kpym the value of x (and y) can be a length or a coordinate. – Paul Gaborit Apr 04 '18 at 13:54
I also want the unit, so, cm to be retrieved. Here you added it as text. – Arne Apr 04 '18 at 14:15
@PaulGaborit you use it here with \p{x}=(1,0) so veclen(\x{x},\y{x}) and \x{x} are the same, no ? – Kpym Apr 04 '18 at 14:30
@Kpym Try with x={(1cm,1cm)} for example... – Paul Gaborit Apr 04 '18 at 14:41
@Arne The natural unit is pt. I provided a variant with an explicit pt. – Paul Gaborit Apr 04 '18 at 14:46
@PaulGaborit ok, thanks for the explanation. – Kpym Apr 04 '18 at 15:34

score 5 · Answer 2 · answered Apr 04 '18 at 11:57

5

Using the printlen package and https://tex.stackexchange.com/a/15996/36296

\documentclass[border=2pt]{standalone}

\usepackage{tikz}
\usetikzlibrary{calc}
\usepackage{printlen}

\newlength{\pgfx}
\newlength{\pgfy}

\uselengthunit{cm}
\begin{document}

  \begin{tikzpicture}[x=2cm,y=3cm]
    \node at (0,0) {};
    \pgfpointxy{1}{1};
    \makeatletter
    \setlength{\pgfx}{\pgf@x}
    \setlength{\pgfy}{\pgf@y}
    \makeatother
    \node {\printlength{\pgfx}~\printlength{\pgfy}}; 
  \end{tikzpicture}
\end{document}

answered Apr 04 '18 at 11:57

samcarter_is_at_topanswers.xyz

158,329

2

So there is no simple, native macro which can do this. Feels a bit weird to me ;) – Arne Apr 04 '18 at 12:00
@Arne I did not say that, just that you could do it like this – samcarter_is_at_topanswers.xyz Apr 04 '18 at 12:01
If I replace y=3cm by y={(-2cm,1cm)}, I got a wrong result! – Paul Gaborit Apr 04 '18 at 13:53
What does y={(-2cm,1cm)} mean? I thought y is a coordinate... – Arne Apr 04 '18 at 14:17
@Arne As far as I understand this would result in a tilted coordinate system? – samcarter_is_at_topanswers.xyz Apr 04 '18 at 14:18
2

@Arne x defines the x-vector. You can use x=1cm (equivalent to x={(1cm,0)}) or a true vector as x={(1cm,5mm)} (see pp.357-358, pgfmanual, v3.0.1a). – Paul Gaborit Apr 04 '18 at 14:51

Tarass · Answer 3 · 2018-04-04T14:52:43.900

\documentclass[border=2pt]{standalone}

\usepackage{tikz,luatex85,siunitx}

\makeatletter
\newcommand{\NodeDist}[3][\MyDist]{%
    \pgfpointdiff{\pgfpointanchor{#2}{center}}
                 {\pgfpointanchor{#3}{center}}
    % no need to use a new dimen
    \pgf@xa=\pgf@x
    \pgf@ya=\pgf@y
    % to convert from pt to cm   
    \pgfmathparse{veclen(\pgf@xa,\pgf@ya)/28.45274}
    \global\let#1\pgfmathresult % we need a global macro    
}

\newcommand{\TZUnits}{
    \coordinate (@0) at (0,0) ;
    \coordinate (@X) at (1,0) ;
    \coordinate (@Y) at (0,1) ;
    \NodeDist[\Xunit]{@0}{@X}\edef\Xunit{\SI{\Xunit}{\cm}}
    \NodeDist[\Yunit]{@0}{@Y}\edef\Yunit{\SI{\Yunit}{\cm}}
}

\makeatother

\begin{document}

  \begin{tikzpicture}[x=2cm,y=3cm]
    \TZUnits
    \node {\Xunit~\Yunit}; 
  \end{tikzpicture}

\end{document}

Here, also the unit (cm) is missing. – Arne Apr 04 '18 at 14:16 — Arne, Apr 04 '18 at 14:16
Vos désirs font désordre... – Tarass Apr 04 '18 at 14:53 — Tarass, Apr 04 '18 at 14:53

TikZ get current unit length of x-coordinate

3 Answers3

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