How to overlap subfigure with caption

Question

Consider the following

\documentclass{article}
\usepackage{subcaption}
\usepackage{graphicx}
\begin{document}
\begin{figure}[htpb]
\centering
\begin{minipage}{0.3\linewidth}
    \begin{subfigure}[t]{\textwidth}
        \includegraphics[width=\linewidth]{example-image-a}
            \caption{}
            \label{figa}
    \end{subfigure}
\end{minipage}
\begin{minipage}{0.3\linewidth}
    \begin{subfigure}[t]{\textwidth}
        \includegraphics[width=\linewidth]{example-image-b}
            \caption{}
            \label{figb}
    \end{subfigure}
\end{minipage}
\par\medskip
\caption{Example caption.}
\label{fig}
\end{figure}
\end{document}

which gives

How can I position the captions of the subfigures, (a) and (b), in any position overlapping the image? For example, how can I get

Edit: For example, using something like

\captionsetup[subfigure]{font={bf,small}, skip=-100pt, margin=0.1cm, singlelinecheck=false}

almost does the trick

but not quite. Putting \caption before \includegraphics also does not seem to help, as the caption moves to the back layer. Any ideas?

Answer 1

In those kind of scenarios, I think tikz provides good solutions, at least to start with. The only thing is in this case you have to split actions of a regular \caption because the latter cannot be used as a argument of \node (see section 7.3 of documentation).

Below is one example. Referencing to each subfigure should work, as usually. Since you keep them on the same page, I just stuck \captionlistentrys to generate anchors, so that you can reference each subfigure. Then, \captiontext* is just to produce empty labels. This way there shouldn't be a problem to append more images.

\documentclass{article}
\usepackage{graphicx}
\usepackage{subcaption}
\usepackage{tikz}
\usepackage{kantlipsum}
\usetikzlibrary{positioning}
\newcounter{subfig}
\begin{document}
\kant[1]
\begin{figure}[tbh]
  \setcounter{subfig}{0}
  \newcommand\typecap{\stepcounter{subfig}\captiontext*[\value{subfig}]{}}
  \captionsetup{position=bottom, skip=3pt}
  \centering
  \begin{subcaptiongroup}
    \captionlistentry{}\label{figa}   % Required to reference subfigures
    \captionlistentry{}\label{figb}
    \tikz[
      node distance=3pt,
      caption/.style={
        anchor=north west,
        font=\bfseries,
        outer sep=6pt,
      },
      relpos/.style={
        right=of #1.south east,
        anchor=south west,
      },
    ] {
      \node (A) {\includegraphics[width=0.3\textwidth]{example-image-a}};
      \node (B) [relpos=A] {\includegraphics[width=0.2\textwidth]{example-image-b}};
      \node (C) [relpos=B] {\includegraphics[width=0.35\textwidth]{example-image-b}};
      \node at (A.north west) [caption] {\typecap};
      \node at (B.north west) [caption] {\typecap};
      \node at (C.north west) [caption] {\typecap};
    }
  \end{subcaptiongroup}
  \caption{Example caption.}\label{fig}
\end{figure}
\kant[2]
\end{document}