[Tex/LaTex] Define a math symbol relative to font size

fontsizemath-modesymbolstikz-pgf

Idea

All characters, including the new defined symbol (here an arrow), should scale related to the current font size, e.g. here 12pt.

Implementation

I would like to define an arrow similar to here. The arrow should scale with the font size. The current MWE of my definition is:

\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{tikz}
\usetikzlibrary{calc,decorations.pathmorphing,shapes}

\newcounter{sarrow}
\newcommand\xrsquigarrow[1]{%
\mathrel{\begin{tikzpicture}[baseline={($(current bounding box.south)+(0,-0.5ex)$)}]
\node[inner sep=.5ex] (\thesarrow) {$\scriptstyle #1$};
\draw[<-,decorate,decoration={snake,amplitude=0.7pt,segment length=1.2mm,pre=lineto,pre length=4pt}] (\thesarrow.south east) -- (\thesarrow.south west);
\end{tikzpicture}}%
}

\begin{document}

$A\xrsquigarrow{f}B$

\end{document}

How does the definition have to look like in order to get dependent on the font size (from the document class)?

Best Answer

Define, as already suggested, the macro with "relative units":

\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{tikz}
\usetikzlibrary{calc,decorations.pathmorphing,shapes}

\newcounter{sarrow}
\newcommand\xrsquigarrow[1]{\mathrel{%
  \begin{tikzpicture}[baseline={($(current bounding box.south)+(0,-0.5ex)$)}]
    \node[inner sep=.5ex] (\thesarrow) {$\scriptstyle #1$};
    \draw[<-,decorate,
          decoration={snake,amplitude=0.135ex,segment length=0.34em,pre=lineto,pre length=0.4em}] 
         (\thesarrow.south east) -- (\thesarrow.south west);
  \end{tikzpicture}
}}

\begin{document}

$A\xrsquigarrow{f}B$

{\large$A\xrsquigarrow{f}B$\par}

{\Large$A\xrsquigarrow{f}B$\par}

{\LARGE$A\xrsquigarrow{f}B$\par}

{\small$A\xrsquigarrow{f}B$\par}

{\footnotesize$A\xrsquigarrow{f}B$\par}

\end{document}

enter image description here

In order to get automatic scaling for subscripts and superscripts (or \scriptstyle declarations), one has to work harder:

\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{tikz}
\usetikzlibrary{calc,decorations.pathmorphing,shapes}

\makeatletter
\newcommand\xrsquigarrow[1]{\mathrel{\mathchoice
  {\hbox{\fontsize{\tf@size}{\tf@size}\selectfont\@xrsquigarrow\scriptstyle{#1}}}
  {\hbox{\fontsize{\tf@size}{\tf@size}\selectfont\@xrsquigarrow\scriptstyle{#1}}}
  {\hbox{\fontsize{\sf@size}{\sf@size}\selectfont\@xrsquigarrow\scriptscriptstyle{#1}}}
  {\hbox{\fontsize{\ssf@size}{\ssf@size}\selectfont\@xrsquigarrow\scriptscriptstyle{#1}}}
}}
\newcommand\@xrsquigarrow[2]{%
  \begin{tikzpicture}[baseline={($(current bounding box.south)+(0,-0.5ex)$)}]
    \node[inner sep=.5ex] (A) {$#1#2$};
    \draw[<-,decorate,
          decoration={snake,amplitude=0.135ex,segment length=0.34em,pre=lineto,pre length=0.4em}] 
         (A.south east) -- (A.south west);
  \end{tikzpicture}%
}

\begin{document}

$A\xrsquigarrow{f}B_{A\xrsquigarrow{f}B}$

{\large$A\xrsquigarrow{f}B$\par}

{\Large$A\xrsquigarrow{f}B$\par}

{\LARGE$A\xrsquigarrow{f}B$\par}

{\small$A\xrsquigarrow{f}B$\par}

{\footnotesize$A\xrsquigarrow{f}B$\par}

\end{document}

enter image description here

Beware that this is slow, because for each instance of \xrsquigarrow all the four variants need to be typeset.

Note. I've removed \thesarrow that does nothing more than provide a node name.

\mathchoice requires four arguments: what's to be typeset in display, text, first level sub/superscript and second level sub/superscript styles respectively. All four text will be typeset and then TeX will decide which one to use. So we set four boxes in which the font is chosen to be of size \tf@size for display and text styles, \sf@size and \ssf@size for the other two styles (these sizes are automatically computed by LaTeX when a formula is being typeset).

The box then contains \@xrsquigarrow which has two arguments: the style to be applied for the label and the label itself. This style should be \scriptstyle when the arrow is in display or text styles, \scriptscriptstyle in the other cases.

The definition of \@xrsquigarrow, apart from the additional parameter, is just the same as the one proposed before.

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] Change font size relative to current font size

Have a look at the relsize package. Package description:

The basic command of the package is \relsize, whose argument is a number of \magsteps to change size; from this are defined commands \larger, \smaller, \textlarger, etc.