memoir label and ref subfigures inside bigger image

Question

I have a big image containing several subfigures, each with its own single letter caption already overlayed.

I'd like to reference these subcaptions from the text. As far as I recall something like this was achieavable with subcaption and its phantom commands, but I'm not that sure they covered this particular case.

Is there a way to do it from memoir?

Here's a sample figure to better explain what I'd like to do

It's a single image that I'd prefer not to split. And I'd like to reference the subimages as they were separate figures.

EDIT Below what I'd do if I could use subcaption, unfortunately as soon as I load it in a document using memoir's own subfigures it messes up with all the caption handling and styling.

So the question remains, is there a way to do phantom subcaptions within memoir?

\documentclass{memoir}
\usepackage{subcaption}

\begin{document}
\begin{figure}[t]
  \centering
  \includegraphics{figure.png}
  \begingroup
  \phantomsubcaption
  \label{fig:somefiga}
  \phantomsubcaption
  \label{fig:somefigb}
  \phantomsubcaption
  \label{fig:somefigc}
  \phantomsubcaption
  \label{fig:somefigd}
  \endgroup
  \caption{some caption}
  \label{fig:somefig}
\end{figure}
\noindent Figure~\ref{fig:somefig} has four subfigures:
figure~\ref{fig:somefiga} is a nice fat rectangle, figure\ref{fig:somefigb}
looks good too. Figures~\ref{fig:somefigc}~and~\ref{fig:somefigd} could do
better.
\end{document}

Answer 1

AFAIU, this cannot be achieved with just native Memoir macros. My own quick-and-dirty (absolutely no warranty) solution was to introduce slightly modified versions of the original native macros (side-by-side diff):

\documentclass[a4paper,14pt]{memoir}
\usepackage{graphicx}
\usepackage{hyperref}
\hypersetup{
    colorlinks=true,
    linkcolor=blue,
    filecolor=magenta,      
    urlcolor=cyan,
}

\usepackage[all]{hypcap}

% =============== defining the \subcaptionphantom ===================================
\makeatletter
% \begin{macro}{\subcaptionphantom}
% \cs{subcaptionphantom}\oarg{list-entry}\marg{caption} is a hidden
% non-printed subcaption. Designed for the case if "(a)", "(b)" are
% already embedded in the figure itself. 
% Roughtly equivalent to \cs{phantomsubcaption} from the \Lpack{subcaption} package.
%
%    \begin{macrocode}
\newcommand{\subcaptionphantom}{%
  \bgroup    
    \let\label=\memsub@label
    \ifdonemaincaption\else
      \advance\csname c@\@captype\endcsname\@ne
    \fi
    \refstepcounter{sub\@captype}\@contkeep
    \@ifnextchar [%
      {\@memsubcapphantom{sub\@captype}}%
      {\@memsubcapphantom{sub\@captype}[\@empty]}}
%    \end{macrocode}
% \end{macro}

% \begin{macro}{\@memsubcapphantom}
% Quick-and-dirty analog of \Lpack{memoir} \cs{memsubcap}, adapted
% for use in \cs{subcaptionphantom}.
%    \begin{macrocode}
\long\def\@memsubcapphantom#1[#2]#3{%
  \@tempdima=\hsize
  \vskip\subfloatcapskip
  \ifx \@empty #2
    \@memsubcaptionphantom{#1}{#3}{#3}%
  \else
    \@memsubcaptionphantom{#1}{#2}{#3}%
  \fi
  \vskip\subfloatcapskip
  \egroup}
%    \end{macrocode}
% \end{macro}
%
%
% \begin{macro}{\@memsubcaptionphantom}
% Quick-and-dirty analog of \Lpack{memoir} \cs{memsubcaption}, adapted
% for use in \cs{memsubcapphantom}.
%    \begin{macrocode}
\newcommand{\@memsubcaptionphantom}[3]{%

  \ifx \relax#2\relax \else
    \bgroup
      \let\label\@gobble
      \let\protect\string
      \def\@memsubcaplabel{\@nameuse{@@the#1}}%
      \xdef\@memsubfigcaptionlist{%
        \@memsubfigcaptionlist,%
  {\protect\numberline{\@memsubcaplabel}\noexpand{\ignorespaces #2}}}%
    \egroup
  \fi
  \@makesubfloatcaptionphantom{\@nameuse{@the#1}}{#3}%
  }

%    \end{macrocode}
% \end{macro}

% \begin{macro}{\@makesubfloatcaptionphantom}
% Quick-and-dirty analog of \Lpack{memoir} \cs{makesubfloatcaption}, adapted
% for use in \cs{memsubcaptionphantom}.
%    \begin{macrocode}
\newcommand{\@makesubfloatcaptionphantom}[2]{%
  \setbox\@tempboxa\hbox{%
    \@subcapsize
    {\phantom{\@subcaplabelfont#1}}{\ignorespaces #2}\unskip}%
  \@tempdimb=-\subfloatcapmargin
  \multiply\@tempdimb\tw@
  \advance\@tempdimb\@tempdima
  \hb@xt@\@tempdima{%
    \hss
    \ifdim \wd\@tempboxa >\@tempdimb
      \phantom{\memsubfig@caption{#1}}{#2}%
    \else
      \if@shortsubcap
        \phantom{\memsubfig@caption{#1}}{#2}%
      \else
        \box\@tempboxa
      \fi
    \fi
    \hss}}
%    \end{macrocode}
% \end{macro}
\makeatother
% ===================================================================================

\newsubfloat{figure} % allows to use "subbottom"

\begin{document}


\begin{figure}
\centering

\hfill%
\begin{minipage}[b][][b]{0.6\linewidth}
\centering
    \subcaptionphantom{\label{sf:1}}
    \subcaptionphantom{\label{sf:2}}
    \framebox[\linewidth]{\textit{subfigure with embedded (a)}}
    \framebox[\linewidth]{\textit{subfigure with embedded (b)}}
\end{minipage}
\hfill%
\begin{minipage}{0.3\linewidth}
    \centering
    \subbottom[regular subfigure \label{sf:3}]%
    {\includegraphics[width=0.7\linewidth]{example-image}}
\end{minipage}


\caption{Figure with three subfigures\label{fig:1}}
\end{figure}        

Example refs:\newline
Full fig: \ref{fig:1}. \newline
Sub1: \subcaptionref{sf:1}, same as \ref{sf:1} (the hidden caption). \newline 
Sub2: \subcaptionref{sf:2}, same as \ref{sf:2} (the hidden caption).  \newline
Sub3: \subcaptionref{sf:3}, same as \ref{sf:3}. \newline



\clearpage
This text is here to add more space for scrolling.

\end{document}

Same in Overleaf.