position subcaption within `subfigure` at the top (inset)

Question

I've asked a similar question but I did not realize that the question and solution relied on the caption being positioned at the bottom.

Consider the following example:

\documentclass{article}
\usepackage{graphicx}
\usepackage{caption,subcaption}
\begin{document}
\begin{figure}
    \centering
    % from caption documentation:
    % Please note that position=top does NOT mean that the caption is actually placed at the top of the figure or table
    % Instead the caption is usually placed where you place the \caption command.
    \captionsetup[subfigure]{singlelinecheck=false,skip=-6pt,position=top}
    \begin{subfigure}{0.4\textwidth}
        \caption{}
        \includegraphics[width=1.0\textwidth]{example-image-a}
        \label{fig:a}
    \end{subfigure}
    \begin{subfigure}{0.4\textwidth}
        \caption{}
        \includegraphics[width=1.0\textwidth]{example-image-b}
        \label{fig:b}
    \end{subfigure}
\end{figure}
\end{document}

The subcaption package mentions options that are described in further detail in the caption documentation. In the source, \caption{} should appear before \includegraphics{... to affect the ordering. Unfortunately, this has an unintended influence that the label is rendered below the figure, which means that the figure occludes it. In the example, I include a skip that partially occludes the labels:

How can I get captions positioned on the top, but within the figure? I am beginning to understand why these answers are so much more involved for captions placed within the top and not the bottom of the figure.

Answer 1

The foreground must always be drawn after the background, which means raising the caption into position. (If you were wondering, TikZ does layers in order automatically.)

\caption must go inside a minipage, \parbox or \vbox, and \setbox0=\vbox{} is the simplest way to put one inside a \raisebox. The \vbox gets its width from the current environment.

You can tweak the \raisebox to change the location.

\documentclass{article}
\usepackage{graphicx}
\usepackage{caption,subcaption}
\begin{document}
\begin{figure}
    \centering
    % from caption documentation:
    % Please note that position=top does NOT mean that the caption is actually placed at the top of the figure or table
    % Instead the caption is usually placed where you place the \caption command.
    \captionsetup[subfigure]{singlelinecheck=false,skip=0pt}%
    \begin{subfigure}{0.4\textwidth}
      \setbox0=\vbox{\caption{}\label{fig:a}}% label uses local values
      \sbox1{\includegraphics[width=1.0\textwidth]{example-image-a}}% measure height
        \leavevmode\rlap{\usebox1}% use horizontal overlap
        \raisebox{\dimexpr \ht1-\ht0}{\usebox0}
    \end{subfigure}\hfil
    \begin{subfigure}{0.4\textwidth}
      \setbox0=\vbox{\caption{}\label{fig:b}}% label uses local values
      \sbox1{\includegraphics[width=1.0\textwidth]{example-image-b}}% measure height
        \leavevmode\rlap{\usebox1}% use horizontal overlap
        \raisebox{\dimexpr \ht1-\ht0}{\usebox0}
    \end{subfigure}
\end{figure}
\end{document}