Extended figure caption and accompanying “List of”

Question

I want to write an extended figure caption section, and wondered if I could get LaTeX to do most of the hard work for me.

Ideally, it would take the form of a function, let say \extcaption, that we could use within a float, like

\begin{figure}
\includegraphics{example-image-a.pdf}
\caption{Important things.}
\extcaption{Less important things.}
\label{fig:bigA}
\end{figure}

and that we could print with

\printextcaptions

and that would return

Figure 1.1, page 2: Less important things.

Anyway to do it? Here is a MWE:

\documentclass[a4paper,10pt]{report}
\usepackage{fontspec}
\usepackage{graphicx}
\usepackage[titles]{tocloft}

\begin{document}
\chapter{A chapter}

\section{A section}

\begin{figure}
\includegraphics{example-image-a.pdf}
\caption{Picture of a big A}
%\extcaption{This picture is magnificent}
\label{fig:bigA}
\end{figure}

\begin{figure}
\includegraphics{example-image-b.pdf}
\caption[This picture is horrible]{Picture of a big B}
%\extcaption{This picture is horrible.}
\label{fig:bigB}
\end{figure}

%\printextcaptions

\end{document}

The best result I got was by setting a new list with the tocloft package

\newlistof[section]{extfig}{efc}{Extended figure captions}
\newcommand{\extcaption}[1]{%
\refstepcounter{extfig}
\par\noindent\textbf{\theextfig #1}
\addcontentsline{efc}{extfig}{\protect\numberline{\theextfig}#1}\par}
\renewcommand{\theextfig}{}

But this prints a new chapter while I want nothing, it does not print the extended caption in the format the way I want, and does not read the label of the figure (or table) float, so I would have to set everything manually.

Answer 1

The following writes out ToC-like entries to a new .efc file and reads it in using \printextcaptions:

\documentclass{report}
\usepackage{graphicx,lipsum}

\newcommand{\extcaption}[1]{% Write to .efc file
  \addcontentsline{efc}{extfig}{\thefigure, page~\thepage: #1}}
\makeatletter
\newcommand{\printextcaptions}{\@starttoc{efc}}% Read .efc file
\newcommand{\l@extfig}[2]{\noindent #1\par}% How each .efc entry is handled
\makeatother

\begin{document}

\sloppy% Just for this example

\chapter{A chapter}

\section{A section}

\printextcaptions

\lipsum[1-50]

\begin{figure}[ht]
  \includegraphics{example-image-a}
  \caption{Picture of a big A}
  \extcaption{This picture is magnificent}
\end{figure}

\lipsum[1-50]\lipsum[1-50]

\begin{figure}[ht]
  \includegraphics{example-image-b}
  \caption[This picture is horrible]{Picture of a big B}
  \extcaption{This picture is horrible}
\end{figure}

\end{document}