Subfigure to subfigure caption

Question

I am new to LaTeX. I am not sure how to frame my question. But I will frame it so that it is understandable.

I would like to make a subfigure, but the caption must be

caption of subfigure a-1 caption of subfigure a-2

caption of subfigure b-1 caption of subfigure b-2

                 `caption  of entire figure`

I tried using subfigure, but I am getting continuously a,b,c,d, but i need to name them as as mentioned above (a-1, a-2, b-1, b-2). In my situation, there are chances that the sub figures might be in different pages.

Answer 1

This uses the same idea as subequations of amsmath.

\documentclass{article}

\usepackage{graphicx}
\usepackage{subcaption}

\makeatletter
\newcounter{parentsubcaption}
\newenvironment{subsubcaption}
 {\refstepcounter{sub\@captype}%
  \protected@edef\theparentsubcaption{\@nameuse{thesub\@captype}}%
  \setcounter{parentsubcaption}{\value{sub\@captype}}%
  \setcounter{sub\@captype}{0}%
  \@namedef{thesub\@captype}{\theparentsubcaption--\arabic{sub\@captype}}%
  \ignorespaces
}{%
  \setcounter{sub\@captype}{\value{parentsubcaption}}%
  \ignorespacesafterend
}
\makeatother

\begin{document}

\begin{figure}
\centering
\begin{subsubcaption}
\begin{subfigure}{5cm}
\includegraphics[width=\linewidth]{example-image}
\caption{caption of subfigure a--1}\label{a--1}
\end{subfigure}\qquad
\begin{subfigure}{5cm}
\includegraphics[width=\linewidth]{example-image}
\caption{caption of subfigure a--2}\label{a--2}
\end{subfigure}
\end{subsubcaption}

\begin{subsubcaption}
\begin{subfigure}{5cm}
\includegraphics[width=\linewidth]{example-image}
\caption{caption of subfigure b--1}\label{b--1}
\end{subfigure}\qquad
\begin{subfigure}{5cm}
\includegraphics[width=\linewidth]{example-image}
\caption{caption of subfigure b--2}\label{b--2}
\end{subfigure}
\end{subsubcaption}

\caption{caption  of entire figure}
\end{figure}

References:
\ref{a--1} and \subref{a--1};
\ref{b--1} and \subref{b--1}.

\end{document}

EDIT

The above code doesn't really work if more figure environments use subsubcaption. Here's a workaround that tries to work well also with \ContinuedFloat.

\documentclass{article}

\usepackage{graphicx}
\usepackage{subcaption}
\usepackage{etoolbox}
\makeatletter
\newif\ifsubsubcaption@ContinuedFloat
\newif\ifsubsubcaption@nonfirst
\patchcmd{\ContinuedFloat}
  {\caption@ContinuedFloat}
  {\subsubcaption@ContinuedFloattrue\caption@ContinuedFloat}
  {}{}

\newcounter{parentsubcaption}
\newenvironment{subsubcaption}
 {\refstepcounter{sub\@captype}%
  \protected@edef\theparentsubcaption{\@nameuse{thesub\@captype}}%
  \setcounter{parentsubcaption}{\value{sub\@captype}}%
  \setcounter{sub\@captype}{0}%
  \@namedef{thesub\@captype}{\theparentsubcaption--\arabic{sub\@captype}}%
  \ignorespaces
}{%
  \setcounter{sub\@captype}{\value{parentsubcaption}}%
  \ignorespacesafterend
}
\BeforeBeginEnvironment{subsubcaption}{%
  \ifsubsubcaption@ContinuedFloat
  \else
    \ifsubsubcaption@nonfirst
    \else
      \subsubcaption@nonfirsttrue
      \setcounter{sub\@captype}{0}%
    \fi
  \fi
}
\makeatother

\begin{document}

\begin{figure}
\centering
\begin{subsubcaption}
\begin{subfigure}{5cm}
\includegraphics[width=\linewidth]{example-image}
\caption{caption of subfigure a--1}\label{a--1}
\end{subfigure}\qquad
\begin{subfigure}{5cm}
\includegraphics[width=\linewidth]{example-image}
\caption{caption of subfigure a--2}\label{a--2}
\end{subfigure}
\end{subsubcaption}

\begin{subsubcaption}
\begin{subfigure}{5cm}
\includegraphics[width=\linewidth]{example-image}
\caption{caption of subfigure b--1}\label{b--1}
\end{subfigure}\qquad
\begin{subfigure}{5cm}
\includegraphics[width=\linewidth]{example-image}
\caption{caption of subfigure b--2}\label{b--2}
\end{subfigure}
\end{subsubcaption}

\caption{caption  of entire figure}
\end{figure}

References:
\ref{a--1} and \subref{a--1};
\ref{b--1} and \subref{b--1}.

\begin{figure}
\ContinuedFloat
\centering
\begin{subsubcaption}
\begin{subfigure}{5cm}
\includegraphics[width=\linewidth]{example-image}
\caption{caption of subfigure c--1}\label{c--1}
\end{subfigure}\qquad
\begin{subfigure}{5cm}
\includegraphics[width=\linewidth]{example-image}
\caption{caption of subfigure c--2}\label{c--2}
\end{subfigure}
\end{subsubcaption}

\begin{subsubcaption}
\begin{subfigure}{5cm}
\includegraphics[width=\linewidth]{example-image}
\caption{caption of subfigure d--1}\label{d--1}
\end{subfigure}\qquad
\begin{subfigure}{5cm}
\includegraphics[width=\linewidth]{example-image}
\caption{caption of subfigure d--2}\label{d--2}
\end{subfigure}
\end{subsubcaption}

\caption{caption  of entire figure (continued)}
\end{figure}

\begin{figure}
\centering
\begin{subsubcaption}
\begin{subfigure}{5cm}
\includegraphics[width=\linewidth]{example-image}
\caption{caption of subfigure a--1}\label{a--1-x}
\end{subfigure}\qquad
\begin{subfigure}{5cm}
\includegraphics[width=\linewidth]{example-image}
\caption{caption of subfigure a--2}\label{a--2-x}
\end{subfigure}
\end{subsubcaption}

\begin{subsubcaption}
\begin{subfigure}{5cm}
\includegraphics[width=\linewidth]{example-image}
\caption{caption of subfigure b--1}\label{b--1-x}
\end{subfigure}\qquad
\begin{subfigure}{5cm}
\includegraphics[width=\linewidth]{example-image}
\caption{caption of subfigure b--2}\label{b--2-x}
\end{subfigure}
\end{subsubcaption}

\caption{caption  of entire figure}
\end{figure}

\end{document}

The “continuation” figure has labels c and d, while the following “new” one has a and b again.