Defining a TikZ style to scale the content of a node

Question

I am trying to define a scale content=<factor> style to be applied to node objects in TikZ pictures. It's purpose is to scale the content of the node without changing the node's outer dimensions. I got relatively far with the following definition:

% Key to scale the content of a node by a factor #1
\tikzset{scale content/.style={
  execute at begin node={
    \numdef{\scale@content@nest}{\scale@content@nest+1} % increase nesting counter
    \ifnumequal{\scale@content@nest}{1}{                % do not nest 
      \pgfmathsetmacro{\scale@content}{#1}
      \begin{lrbox}{\@tempboxa}
      \ifx\tikz@text@width\pgfutil@empty\else
        % if 'text width' has been specified it must been scaled reciprocally
        % we achieve this by typesetting in a accordingly dimensioned minipage
        \pgfmathsetlength{\@tempdima}{\tikz@text@width/#1}
        \begin{minipage}{\@tempdima}
      \fi
    }{}
  }, 
  execute at end node={
    \ifnumequal{\scale@content@nest}{1}{
      \ifx\tikz@text@width\pgfutil@empty\else
        \end{minipage}
      \fi
      \end{lrbox}
      \scalebox{\scale@content}{\usebox\@tempboxa}
    }{}
    \numdef{\scale@content@nest}{\scale@content@nest-1} % decrease nesting counter
  }
}}

This works well, unless I specify text width depending on \textwidth as shown in the third example below (complete MWE):

\documentclass{article}
\usepackage{tikz,etoolbox}
\makeatletter

% Key to scale the content of a node by a factor #1
\tikzset{scale content/.style={
  execute at begin node={
    \numdef{\scale@content@nest}{\scale@content@nest+1} % increase nesting counter
    \ifnumequal{\scale@content@nest}{1}{                % do not nest 
      \pgfmathsetmacro{\scale@content}{#1}
      \begin{lrbox}{\@tempboxa}
      \ifx\tikz@text@width\pgfutil@empty\else
        % if 'text width' has been specified, scale it accordingly by using a minipage
        \pgfmathsetlength{\@tempdima}{\tikz@text@width/#1}
        \begin{minipage}{\@tempdima}
      \fi
    }{}
  }, 
  execute at end node={
    \ifnumequal{\scale@content@nest}{1}{
      \ifx\tikz@text@width\pgfutil@empty\else
        \end{minipage}
      \fi
      \end{lrbox}
      \scalebox{\scale@content}{\usebox\@tempboxa}
    }{}
    \numdef{\scale@content@nest}{\scale@content@nest-1} % decrease nesting counter
  }
}}
\makeatother

\begin{document}
  \tikzset{every node/.style={draw, fill=yellow!40}}

  \tikz\node[text width=3cm]{Not scaled, text width is 3cm};
  \par
  \tikz\node[scale content=0.7, text width=3cm]{scaled by 0.7, text width is 3cm, everything is fine};
  \par
  \tikz\node[scale content=0.7, text width=0.5\textwidth, text=red]{scaled by 0.7, text width is 0.5\textbackslash textwidth, so why aren't we using it?};
  \par
  \pgfmathsetlengthmacro{\mywidth}{0.5\textwidth}
  \tikz\node[scale content=0.7, text width=\mywidth]{scaled by 0.7, text width is 0.5\textbackslash textwidth, manually expanded before, so this is how it should look like};
\end{document}

Apparently, TikZ does not expand text width directly, which would solve the problem:

% original definition from tikz.code.tex:779
\tikzoption{text width}{\def\tikz@text@width{#1}}

% Alternative definition that would work
\tikzoption{text width}{\pgfmathsetlengthmacro{\tikz@text@width}{#1}}

However, once I am inside execute at begin node it seems already to be to late, apparently \textwidth has already be transformed somehow.

So how can I get the expanded value of a node's text width?

Or should I take a completely different route?

Answer 1

The \textwidth macro is changed by a minipage to the given width. Now, TikZ already uses a minipage itself if text width is used. The solution would be to use the new \textwidth for scaling instead of \tikz@text@width. Otherwise a text width value relative to the outer \textwidth would be taken into account twice!

You can find the relevant code in tikz.code.tex starting at line 3116:

\ifx\tikz@text@width\pgfutil@empty%
\else%
  \begingroup%
    \pgfmathsetlength{\pgf@x}{\tikz@text@width}%
    \pgfutil@minipage[t]{\pgf@x}\leavevmode\hbox{}%
      \tikz@text@action%
\fi%
\tikz@atbegin@node%

Here \pgfutil@minipage is the normal \minipage macro (i.e. ~\begin{minipage}) and \tikz@atbegin@node is where the execute at begin node is inserted.

Applied to your MWE:

\documentclass{article}
\usepackage{tikz,etoolbox}
\makeatletter

% Key to scale the content of a node by a factor #1
\tikzset{scale content/.style={
  execute at begin node={
    \numdef{\scale@content@nest}{\scale@content@nest+1} % increase nesting counter
    \ifnumequal{\scale@content@nest}{1}{                % do not nest 
      \pgfmathsetmacro{\scale@content}{#1}
      \begin{lrbox}{\@tempboxa}
      \ifx\tikz@text@width\pgfutil@empty\else
        % if 'text width' has been specified, scale it accordingly by using a minipage
        % Here `\textwidth` now represents the original text width of the node.
        \pgfmathsetlength{\@tempdima}{\textwidth/\scale@content}
        \begin{minipage}{\@tempdima}
      \fi
    }{}
  }, 
  execute at end node={
    \ifnumequal{\scale@content@nest}{1}{
      \ifx\tikz@text@width\pgfutil@empty\else
        \end{minipage}
      \fi
      \end{lrbox}
      \scalebox{\scale@content}{\usebox\@tempboxa}
    }{}
    \numdef{\scale@content@nest}{\scale@content@nest-1} % decrease nesting counter
  }
}}
\makeatother

\begin{document}
  \tikzset{every node/.style={draw, fill=yellow!40}}

  \tikz\node[text width=3cm]{Not scaled, text width is 3cm};
  \par
  \tikz\node[scale content=0.7, text width=3cm]{scaled by 0.7, text width is 3cm, everything is fine};
  \par
  \tracingassigns=1
  \tikz\node[scale content=0.7, text width=0.5\textwidth, text=red]{scaled by 0.7, text width is 0.5\textbackslash textwidth, so why aren't we using it?};
  \par
  \pgfmathsetlengthmacro{\mywidth}{0.5\textwidth}
  \tikz\node[scale content=0.7, text width=\mywidth]{scaled by 0.7, text width is 0.5\textbackslash textwidth, manually expanded before, so this is how it should look like};
\end{document}