Using \phantom in subscript with the bayesnet TikZ library

Question

I'm using the bayesnet TikZ library to draw a graphical model. Node variables might have different size, say $v_0$ and $v_{N-1}$ and the result is that some bubbles are bigger than others. I am trying to use \phantom to create bubbles of the same size like this:

\node[obs] (v0) {$v_{0\phantom{N-}}$};

The problem with this solution is that the label is pushed all the way to the left inside the bubble. Referring to the above fragment, is there a way to compute the space taken by {N-} (note that it's a subscript) and distribute it evenly to the left and right of $v_0$?

Answer 1

Yes, you can measure the width. With

\sbox0{$\scriptstyle N-$}

and one

\hspace*{.5\wd0}

before and after the actual content would solve this.

But it is easier to use the text width key (or minimum width subtracting the /pgf/inner xsep values). You can use either \widthof (more robust) or PGFmath’s width function.

Furthermore you can also simply specify any arbitrary value for minimum width or text width.

And if you do not want to give the content before hand you can use the .aux file as in Dependent node size in TikZ.

Code

\documentclass[tikz,convert=false]{standalone}
\begin{document}
\begin{tikzpicture}[nodes=draw]
\node           {$v_{N-1}$};
\node at (0,-.5) {$v_0$};
\end{tikzpicture}
\begin{tikzpicture}[nodes={draw,align=center,text width=\widthof{$v_{N-1}$}}]
\node           {$v_{N-1}$};
\node at (0,-.5) {$v_0$};
\end{tikzpicture}
\begin{tikzpicture}[nodes={draw,align=center,text width=width("$v_{N-1}$")}]
\node           {$v_{N-1}$};
\node at (0,-.5) {$v_0$};
\end{tikzpicture}
\begin{tikzpicture}[nodes={draw,minimum width=+3.5em}]
\node           {$v_{N-1}$};
\node at (0,-.5) {$v_0$};
\end{tikzpicture}
\begin{tikzpicture}[nodes=draw]
\node           {$v_{N-1}$};
\node at (0,-.5) {\sbox0{$\scriptstyle N-$}\hspace*{.5\wd0}$v_0$\hspace*{.5\wd0}};
\end{tikzpicture}
\end{document}

Output

Original

\widthof{…}

width("…")

Arbitrary value

Measuring