Subfigures in RevTex

I’m in the process of writing a paper (yay!). Merrily I add a formula here a plot there; but my supervisor zeroes in immediately on a two-panel figure with sub figures called “a” and “b” saying:

The sub figure labels go in the top-left corner. You’ve read papers before, haven’t you?

Having indeed read papers before, I must have neglected to pay proper attention to sub-figure labels – an inexcusable blunder – so this was news to me. Unwilling to fix the issues by monkeying around in inkscape, I turned to stack overflow. We have the pleasure to be using the RevTeX document class and annoyingly this class happens to be incompatible with both the subcaption package and the part of subfloat that controls where the captions go. The aforementioned packages are “the way” to do sub figures and most other solutions I found involved monkeying around with minipages.

So that’s what I did; but I wanted to do properly so I’d be able to give the sub figures referenceable labels. I came up with a surprisingly short piece of hacked-together LaTeX package which we will now walk through. If you’re impatient, you can just grab the sty file and jump ahead to the usage example.

We begin by declaring the package.

\NeedsTeXFormat{LaTeX2e}
\ProvidesPackage{poormanssubfig}[2025/02/14 A subfigure environment
  with working labels in the top-left corner that is compatible with revtex.]

Then, we create a new counter for the subfigures to be able to generate use labels to reference them.

\newcounter{subfigure}

The multifig environment resets the subfigure counter¹, and redefines the \thesubfigure macro to display references to the subfigure as [Figure Number] ([figure sublabel]). The environment temporarily increases the Figure counter so that \thefigure prints the right number. The three commands at the top of the below snipped allow the user to overwrite the way the label is printed and formatted.

\newcommand{\subfiglabelformat}[1]{\raggedright{#1}}
\newcommand{\subfiglabelstyle}[1]{\alph{#1}}
\newcommand{\subfiglabelsize}{\small}
\newenvironment{multifig}%
{\addtocounter{figure}{1}%
  \setcounter{subfigure}{0}%
  \renewcommand\thesubfigure{\thefigure~(\subfiglabelstyle{subfigure})}%
}
{\addtocounter{figure}{-1}}

This is the point where the real hackery begins. We define an internal variables \if@insubfig whose use will become clear later. The subfigure environment takes two arguments, the first one being the width of the subfigure and the second being an optional label string (it’s also where you put \label{[label]}). The environment then creates a minipage with the appropriate width and prints the label of the figure in the top-left corner.

\makeatletter
\ExplSyntaxOn
\newif\if@insubfig
\NewDocumentEnvironment{subfigure}{mO{}}{%
  \@insubfigtrue%
  \refstepcounter{subfigure}%
  \begin{minipage}{#1}%
    \subfiglabelformat{{\subfiglabelsize (\subfiglabelstyle{subfigure})~#2}}%
    }
    {\end{minipage}%
  \@insubfigfalse%
}
\ExplSyntaxOff

This final bit is truly horrendous. I wanted to be able to refer to the subfigures in the figure caption using their labels, but a standard \ref{[label]} would print the figure number, too. So I stole some code from the cleveref package to redefine the \label macro to write a new label format (with the label name [youlabel]@multifig) to the auxiliary (.aux) file upon compilation. LaTeX reads the auxiliary file on the second pass and evaluates the \newlabel{[label]} macros therein. These tell latex which text to print when we use the macro\ref{[label]}. In our case this is just the sufigure counter value, without the figure number. I define a new reference command \subref to expand \ref but append @multifig to its argument, so that the text we stored using\newlabel is used. These extra labels will only be generated for subfigures thanks to \if@insubfig.

\let\multifig@old@label\label%
\def\label#1{%
  \multifig@old@label{#1}%
  \if@insubfig%
    \@bsphack%
    \protected@write\@auxout{}%
    {\string\newlabel{#1@multifig}{{(\subfiglabelstyle{subfigure})}%
        {\thepage}%
        {\@currentlabelname}%
        {\@currentHref}{}%
      }}%
    \@esphack%
  \fi
}%
\def\subref#1{\ref{#1@multifig}}
\makeatother

With these details out of the way, let’s get to the part that you actually wanted to read about²: The package is used in a similar way to – who’d have thought it – the subfigure package, but puts your label it the top left corner – where it ought to be! Just put the .sty file somewhere TeXwill find it. For global installation, you can put it in your$TEXMFHOME/tex/latex folder. For usage with a specific project, you can dump it next to your .tex file. And now, behold the below usage example.

\begin{figure*}
  \begin{multifig}
    \begin{subfigure}{\columnwidth}{\label{subfig:descriptive-label}Some
        text label.}
      \begin{center}
        \includegraphics[width=.8\columnwidth]{example-image-a}
      \end{center}
    \end{subfigure}
    \begin{subfigure}{\columnwidth}{\label{subfig:another-descriptive-label}Another
        text label.}
      \begin{center}
        \includegraphics[width=.8\columnwidth]{example-image-b}
      \end{center}
    \end{subfigure}
  \end{multifig}
  \caption{\label{fig:beautiful-figures}As the discerning eye may have
    spied, the figures \subref{subfig:descriptive-label} and
    \subref{subfig:another-descriptive-label} have the potential to
    rock the foundations of society.}
\end{figure*}
Total nonsense, but I couldn't think of anything better to demonstrate
Figure \ref{subfig:descriptive-label} and \ref{subfig:another-descriptive-label}.

This produces the below result...