Math spacing in captions

Question

It seems that math in captions gets stretched out as illustrated in Figures 2, 3 and 4:

The problem shows up with pdflatex on the second run.

The solution given in Lock spacing in math mode of adding an additional curly brace ${y = mx + b}$ works but wondering if there is a better solution for captions.

A bit of stretch in order to justify text would be ok, but in this case that does not seem to be the issue and the stretching seems quite large.

References:

• Lock spacing in math mode
• Removing spaces between “words” in math mode

Code:

\documentclass{article}
\usepackage{showframe}
\usepackage{graphicx}
\usepackage{caption}
\DeclareCaptionStyle{mystyle}
  {format=plain,%
    font=footnotesize,
    textformat=period,
    justification=RaggedRight,
    singlelinecheck=true,
  }% all captions are left aligned
\DeclareCaptionStyle{singlelinecentered}
  [justification=Centering]% centered if single line and no singlelinecheck=false
  {style=mystyle}% other captions are left aligned
\DeclareCaptionStyle{singlelineraggedleft}
  [justification=RaggedLeft]% right aligned if single line and no singlelinecheck=false
  {style=mystyle}% other captions are left aligned
\captionsetup{style=singlelineraggedleft}
\begin{document}
\noindent
\begin{minipage}{0.5\linewidth}
    \centering
    \includegraphics[width=0.95\linewidth, height=2cm]{example-image}%
    \captionsetup{width=0.95\linewidth}%
    \captionsetup{singlelinecheck=false}%
    \captionof{figure}{$y=mx+b$ on Left}%
\end{minipage}%
\begin{minipage}{0.5\linewidth}
    \centering
    \includegraphics[width=0.95\linewidth, height=2cm]{example-image}%
    \captionsetup{width=0.95\linewidth}%
    \captionof{figure}{$y=mx+b$ on Right}%
\end{minipage}%
\medskip\noindent
\begin{minipage}[t]{0.5\linewidth}
    \centering
    \includegraphics[width=0.95\linewidth, height=2cm]{example-image}%
    \captionsetup{width=0.95\linewidth,style=singlelinecentered}%
    \captionof{figure}{$y=mx+b$}%
\end{minipage}%
\begin{minipage}[t]{0.5\linewidth}
    \centering
    \includegraphics[width=0.95\linewidth, height=2cm]{example-image}%
    \captionsetup{width=0.95\linewidth}%
    \captionof{figure}{$y=mx+b$}%
\end{minipage}%
\end{document}

Answer 1

I see nothing strange and it hasn't to do with math, but rather with how ragged2e does its work.

1. In a \RaggedRight, \RaggedLeft or \Centering context, the \spaceskip parameter is set to \spaceskip\fontdimen2\font, that is, the normal interword space for the current font, but with no stretch and shrink.

2. In a \RaggedRight context, the values of \rightskip is set to \RaggedRightRightskip (default 0pt plus 2em); similar settings are done for \RaggedLeft and \Centering.

3. A caption shorter than \linewidth-2em (for \RaggedRight) will need to be stretched.

4. The only stretchability is present in the spaces around binary operation and relation symbols.

5. Hey! Just math is stretched.

Solution: don't use ragged2e for this.

Alternative solution: remove the stretchability and shrinkability of \medmuskip and \thickmuskip:

\documentclass{article}
\usepackage{showframe}
\usepackage{graphicx}

\usepackage{ragged2e}
\usepackage{caption}
\usepackage{etoolbox}

\csappto{@raggedtwoe@everyselectfont}{%
  \medmuskip=1\medmuskip
  \thickmuskip=1\thickmuskip
}

\DeclareCaptionStyle{mystyle}{
  format=plain,
  font=footnotesize,
  textformat=period,
  justification=RaggedRight,
  singlelinecheck=true,
  }% all captions are left aligned

\DeclareCaptionStyle{singlelinecentered}
  [justification=Centering]% centered if single line and no `singlelinecheck=false`
  {style=mystyle}% other captions are left aligned

\DeclareCaptionStyle{singlelineraggedleft}
  [justification=RaggedLeft]% right aligned if single line and no `singlelinecheck=false`
  {style=mystyle}% other captions are left aligned

\captionsetup{style=singlelineraggedleft}

\begin{document}
\noindent
\begin{minipage}{0.5\linewidth}
    \captionsetup{width=0.95\linewidth}%
    \captionsetup{singlelinecheck=false}%
    \centering
    \includegraphics[width=0.95\linewidth, height=2cm]{example-image}
    \captionof{figure}{$y=mx+b$ on Left}
\end{minipage}%
\begin{minipage}{0.5\linewidth}
    \captionsetup{width=0.95\linewidth}%
    \centering
    \includegraphics[width=0.95\linewidth, height=2cm]{example-image}
    \captionof{figure}{$y=mx+b$ on Right}
\end{minipage}

\medskip\noindent
\begin{minipage}[t]{0.5\linewidth}
    \captionsetup{width=0.95\linewidth,style=singlelinecentered}%
    \centering
    \includegraphics[width=0.95\linewidth, height=2cm]{example-image}%
    \captionof{figure}{$y=mx+b$}%
\end{minipage}%
\begin{minipage}[t]{0.5\linewidth}
    \captionsetup{width=0.95\linewidth}
    \centering
    \includegraphics[width=0.95\linewidth, height=2cm]{example-image}
    \captionof{figure}{$y=mx+b$}
\end{minipage}

\end{document}

But you'll get “Underfull \hbox” messages for every short caption.