[Tex/LaTex] Problem with subcaption spacing

captionspositioningspacingsubcaptionsubfloats

When using the subcaption package to place tables1 side-by-side, the spacing between a table and its subtable caption is too small if the table caption is placed above the tables and the subtable captions are placed below the tables, as shown in the following example:

\documentclass{article}
\usepackage{booktabs,caption,subcaption}

\newcommand\exampletable{
  \begin{tabular}{ll}
    \toprule
    A & B\\
    \bottomrule
  \end{tabular}
}

\begin{document}

1) This looks OK:

\begin{table}[!h]
  \centering
  \begin{subtable}[t]{10em}
    \centering
    \exampletable
    \caption{Subcaption 1}
  \end{subtable}
  \begin{subtable}[t]{10em}
    \centering
    \exampletable
    \caption{Subcaption 2}
  \end{subtable}
  \caption{Main caption}
\end{table}

2) And this looks OK:

\begin{table}[!h]
  \caption{Main caption}
  \centering
  \begin{subtable}[t]{10em}
    \caption{Subcaption 1}
    \centering
    \exampletable
  \end{subtable}
  \begin{subtable}[t]{10em}
    \caption{Subcaption 2}
    \centering
    \exampletable
  \end{subtable}
\end{table}

3) Even this looks OK:

\begin{table}[!h]
  \centering
  \begin{subtable}[t]{10em}
    \caption{Subcaption 1}
    \centering
    \exampletable
  \end{subtable}
  \begin{subtable}[t]{10em}
    \caption{Subcaption 2}
    \centering
    \exampletable
  \end{subtable}
  \caption{Main caption}
\end{table}

4) But in this case, the spacing isn't nice:

\begin{table}[!h]
  \caption{Main caption}
  \centering
  \begin{subtable}[t]{10em}
    \centering
    \exampletable
    \caption{Subcaption 1}
  \end{subtable}
  \begin{subtable}[t]{10em}
    \centering
    \exampletable
    \caption{Subcaption 2}
  \end{subtable}
\end{table}

\end{document}

This produces something that looks like this:

Table 4 has too little spacing between tables and subtable captions

Table 4 is the problematic case. I would prefer it to look more like this instead:

This is what table 4 should look like

Any ideas on how to fix this? Ideally, I would want to have the right spacing "automagically" without having to insert explicit vspaces or anything similar inside the subtable environments.

1: the same would apply to figures, or other kinds of subfloats.

Best Answer

After Johannes_b pointed me in the right direction, it turned out that using \captionsetup[subtable]{position=auto} fixes the spacing in all four cases.

Long answer

After some experimentation, it seemed to me that the \caption of the outer table environment somehow affected the \caption used in the inner subtable environment. This led med to attempt wrapping the table \caption in a \begingroup-\endgroup pair to isolate any side-effects. In the following MWE, this was done with the problematic case 4:

\documentclass{article}
\usepackage{booktabs,caption,subcaption,blindtext}
\captionsetup[subtable]{skip=20pt}

\newcommand\exampletable{
  \begin{tabular}{ll}
    \toprule
    A & B\\
    \bottomrule
  \end{tabular}
}

\begin{document}

A single line of text that shows the spacing between the text and the float.

\begin{table}[!h]
  \caption{Main caption}
  \centering
  \begin{subtable}[t]{10em}
    \centering
    \exampletable
    \caption{Subcaption 1}
  \end{subtable}
\end{table}

A single line of text that shows the spacing between the text and the float.

\begin{table}[!h]
  \begingroup
  \caption{Main caption}
  \endgroup
  \centering
  \begin{subtable}[t]{10em}
    \centering
    \exampletable
    \caption{Subcaption 1}
  \end{subtable}
\end{table}

A single line of text that shows the spacing between the text and the float.

\end{document}

And the result confirms the suspicion:

Isolating caption in group

Table 2 in the image above corresponds to the case where \caption was used within a group.

After some digging through the caption source code, it turns out that the \caption@position macro is set as a result of using \caption in the table environment (it is undefined initially, unless overridden globally), and it is tested for with the \caption@iftop macro when \caption is used again in the subtable environment. In other words, the position attribute of the caption in the outer environment will override the caption position in nested environments, if their \caption commands succeed the \caption command of the outer environment, and the caption positions of the nested environments haven't been set explicitly.

To confirm this, I tried to reset the value of \caption@position after the table caption:

\begin{table}[!h]
  \caption{Main caption}
  \makeatletter
  \let\caption@position\@undefined
  \makeatother
  \centering
  \begin{subtable}[t]{10em}
    \centering
    \exampletable
    \caption{Subcaption 1}
  \end{subtable}
\end{table}

and it turned out that this had the same effect as wrapping the \caption in a group.

Note that the code above should not be used as a solution to this problem! Even the caption package source code states that the \caption@position macro should not be depended on.

So why does position=auto work?

It turns out that using \captionsetup[subtable]{position=auto} will result in a call to \caption@setposition{auto} before the subtable caption is output, and the auto argument will simply do \let\caption@position\@undefined, which is exactly what we wanted.

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