How to create code in the preamble to give the same result as a prescribed boxed figure with nested caption and numbering

Question

Can a routine be created - similar to those for \chapter, \section, \subsection i.e., containing a number of #2, #1 parameters - to control the boxes used to highlight a series of figures? In my MWE below, I include an example of how I would like the end result to look. If possible, could the lengths (widths) of the images, and numbered captions be automatically detected by the routine?

\documentclass[english,11pt]{book}
%
\usepackage[fleqn]{amsmath} %
\usepackage{amsthm,amssymb} %
\usepackage[explicit]{titlesec}
\usepackage{titletoc} %
\usepackage[english]{babel} %
\usepackage{xcolor} %
\usepackage{colortbl} %
\usepackage{graphicx}
\usepackage{tikz}
\usepackage[skins,theorems,most]{tcolorbox} %
\usepackage{caption} %
\usepackage[margin=6pt,font+=smaller,labelformat=parens,labelsep=space,skip=2.4pt,list=false,hypcap=false]{subcaption}
\usepackage[paperwidth=170mm,paperheight=240mm,bindingoffset=12.5mm,left=12.5mm,right=12.5mm,top=8mm,bottom=8mm,showframe,showcrop]{geometry}
\usepackage[a4,frame,cam,center]{crop} %
\usepackage{newtxtext} %
\usepackage{cellspace}
\usepackage{mwe} %
\usepackage{refcount} %
\usepackage{hyperref}

\frontmatter

\addto\captionsenglish{
\renewcommand{\figurename}{Fig\hspace{0.8pt}.}
\renewcommand{\thefigure}{\thechapter\hspace{1.0pt}.\hspace{0.6pt}\arabic{figure}} }

\newtcolorbox{sectionbox}[2][]{nobeforeafter, boxsep=4.4pt,boxrule=1.6pt,arc=3pt,outer arc=3.2pt,
width=\linewidth, coltitle=white,colbacktitle=red!60!black,after=\hfill,colframe=black,colback=white,
title={#2},fonttitle=\bfseries,#1} %

\titleformat{\section}{}{}{0pt} %
{\begin{sectionbox}
{\fontsize{16}{18}\selectfont{Section\hspace{5.6pt}\thetitle}}
{\normalfont\fontsize{12}{12.8}\selectfont{#1}\vspace{-2.4pt}} %
\end{sectionbox}}

\renewcommand{\thesection}{\arabic{section}} %
\titlespacing{\section}{0pt}{15.35pt}{4pt}[0pt] % from left, before, after, [0pt]


\newtcolorbox[]{outerbox}[2][]
{boxrule=1.6pt,arc=1.2pt,coltitle=red!50!black,colbacktitle=pink,colback=gray,colframe=red,
every float=\centering,title={#2},#1}

\newtcolorbox[]{innerbox}[2][]
{boxsep=2pt,boxrule=0.7pt,arc=2pt,coltitle=red!50!black,colbacktitle=gray,
colback=white,colframe=black,fonttitle=\bfseries, title={#2},#1}


\begin{document}

\setcounter{chapter}{4}
\chapter{Chapter $\thechapter$}
\section{Using my graphic - please edit example-image-a} % my image=127.5mm wide

\begin{figure}[htp]
\begin{outerbox}[width=\textwidth-2mm]
\noindent\includegraphics[scale=1.00]{../math_text/chap_01_intro_to_number_figs/fig_40_step_forward}
%\noindent\includegraphics[width=45mm]{example-image-a}
\begin{innerbox}[width=80mm]
\caption{ - \hspace{0.6pt}using the preamble - gives errors}
\end{innerbox}
\end{outerbox}
\label{fig:a_figure}
\end{figure}

\begin{figure}[htp]
\centering
     \begin{tcolorbox}
[width=131mm,colframe=red!50!black,colback=gray,top=0pt,bottom=0pt,left=1.0pt,right=1.0pt,boxrule=1.6pt]
\centering
\noindent\includegraphics[scale=1.00]{../math_text/chap_01_intro_to_number_figs/fig_40_step_forward}
%\noindent\includegraphics[width=45mm]{example-image-a}
        \begin{tcolorbox}[center,width=87mm,height=7.2mm,top=1pt,left=1pt,right=1pt,colframe=black,colback=white,boxrule=0.7pt]
                    \caption{- \hspace{0.6pt}using direct coding after begin document}
        \end{tcolorbox}
    \end{tcolorbox}
    \label{fig:another_figure}
\end{figure}

\end{document}

Answer 1

Your MWE gives

Please explain what is your problem! I will delete this answer after your response.

Answer 2

I'm not sure exactly what you want, because you've changed the colors you use in your code a couple times, and the options for the innerbox and outerbox in your attempts are not the same, but I think this will get you close, and give you a place to start.

Your code does not work well in part because you're using the title= option but don't seem to want to use titles.

But I suggest going an even more compact route.

Defining custom commands that have, inside of them, environments created with \newtcolorbox does not work particularly well. I suggest just using \NewDocumentCommand with a regular tcolorbox environment inside; since it's inside a defined command, you don't need to add the options separately each time; the same ones will be used each time you call the command.

The following code defines a command \myimagebox which takes three arguments: the name of an image, the caption text, and a label to use for crossreferencing.

It determines the widths of the image and the caption and creates boxes of the appropriate sizes.

You can, but do not have to, pass it an optional fourth argument inside [ and ] as well if you want to pass additional arguments to the \includegraphics command inside. Using this you can manually tweak the size and other properties of the image if need be.

(With this setup, images larger than 120mm or so do not look good since there isn't enough room for the boxes to have similar padding to other boxes, so sometimes you may want to use this tweak the image size. In other cases, it will use the image's natural size and calculate the box from it.)

\documentclass[english,11pt]{book}
%
\usepackage[fleqn]{amsmath} %
\usepackage{amsthm,amssymb} %
\usepackage[explicit]{titlesec}
\usepackage{titletoc} %
\usepackage[english]{babel} %
\usepackage{xcolor} %
\usepackage{colortbl} %
\usepackage{graphicx}
\usepackage{tikz}
\usepackage[skins,theorems,most]{tcolorbox} %
\usepackage{caption} %
\usepackage[margin=6pt,font+=smaller,labelformat=parens,labelsep=space,skip=2.4pt,list=false,hypcap=false]{subcaption}
\usepackage[paperwidth=170mm,paperheight=240mm,bindingoffset=12.5mm,left=12.5mm,right=12.5mm,top=8mm,bottom=8mm,showframe,showcrop]{geometry}
\usepackage[a4,frame,cam,center]{crop} %
\usepackage{newtxtext} %
\usepackage{cellspace}
\usepackage{mwe} %
\usepackage{refcount} %
\usepackage{hyperref}


\addto\captionsenglish{
\renewcommand{\figurename}{Fig\hspace{0.8pt}.}
\renewcommand{\thefigure}{\thechapter\hspace{1.0pt}.\hspace{0.6pt}\arabic{figure}} }

\newtcolorbox{sectionbox}[2][]{nobeforeafter, boxsep=4.4pt,boxrule=1.6pt,arc=3pt,outer arc=3.2pt,
width=\linewidth, coltitle=white,colbacktitle=red!60!black,after=\hfill,colframe=black,colback=white,
title={#2},fonttitle=\bfseries,#1} %

\titleformat{\section}{}{}{0pt} %
{\begin{sectionbox}
{\fontsize{16}{18}\selectfont{Section\hspace{5.6pt}\thetitle}}
{\normalfont\fontsize{12}{12.8}\selectfont{#1}\vspace{-2.4pt}} %
\end{sectionbox}}

\renewcommand{\thesection}{\arabic{section}} %
\titlespacing{\section}{0pt}{15.35pt}{4pt}[0pt] % from left, before, after, [0pt]

% some lengths and a counter we'll need later
\newlength{\captionwidth}
\newlength{\innerboxwidth}
\newlength{\imagewidth}
\newlength{\outerboxwidth}
\newcounter{fakefigure}

\NewDocumentCommand{\myimagebox}{mmmo}{
    % a fake counter to use to calculate width of caption
    % which will be one more than current
    \setcounter{fakefigure}{\value{figure}}
    \stepcounter{fakefigure}
    % determine width of caption
    \settowidth{\captionwidth}{Fig.~\thefakefigure: #2}
    % determine innerbox size by adding to captionwidth
    \setlength{\innerboxwidth}{\captionwidth}
    \addtolength{\innerboxwidth}{10mm}
    % determine width of image
    \IfValueTF{#4}{%
        \settowidth{\imagewidth}{\includegraphics[#4]{#1}}%
    }{%
        \settowidth{\imagewidth}{\includegraphics{#1}}%
    }%
    % outerbox determined by max of those two widths
    \setlength{\outerboxwidth}{\maxof{\innerboxwidth}{\imagewidth}}
    \addtolength{\outerboxwidth}{20mm}
    % but not above \textwidth
    \setlength{\outerboxwidth}{\minof{\outerboxwidth}{\textwidth}}
    \begin{figure}[htp]%
        \centering%
        % outerbox
        \begin{tcolorbox}[
            center,
            width=\outerboxwidth,
            coltitle=red!50!black,
            colframe=red!50!black,
            colbacktitle=pink,
            colback=gray,
            top=0pt,
            bottom=0pt,
            every float=\centering
        ]%
            \centering%
            \IfValueTF{#4}{%
                \includegraphics[#4]{#1}%
            }{%
                \includegraphics{#1}%
            }%
            % innerbox
            \begin{tcolorbox}[center,top=1pt,bottom=-10pt,left=1pt,right=1pt,%
                width=\innerboxwidth,
                colframe=black,colback=white,boxrule=0.7pt]%
            \caption{#2}\label{#3}
            \end{tcolorbox}
        \end{tcolorbox}
    \end{figure}
}

\usepackage{lipsum}% for dummy text

\title{Document Title}
\author{Someone P. Someone}
\date{\today}

\begin{document}
\frontmatter
\maketitle

\mainmatter

\setcounter{chapter}{4}
\chapter{Chapter \thechapter}
\section{Using my graphic - please edit example-image-a} % my image=127.5mm wide

\myimagebox{sample100mm.png}{An 100mm example}{fig:samp1}

\lipsum[1]% dummy text

\myimagebox{sample50mm.png}{A 50mm example}{fig:samp2}

\lipsum[9]

\myimagebox{example-image-a}{This is my example with a longer caption}{fig:ex1}[width=50mm]

\lipsum[2]% dummy text

As seen in figures \ref{fig:samp1} and \ref{fig:ex1} ...

\end{document}

The stuff I added starts around the comment % some lengths and a counter we'll need later.

I moved \frontmatter because it doesn't make any sense in the preamble.