Strange interaction of caption and textwidth

Question

I am trying to create some nice-looking captions for some photos. I do not want to adjust the aspect ratio of these photos, so I am including the keepaspectratio option in \includegraphics.

Inspired by Martin Scharrer's answer to the question Automatic width of floats, I have used a savebox to capture the width of my photos, and then I use this width in the \captionsetup command. This approach works well, except that I am noticing a rather strange phenomenon:

• If the width option in \includegraphics is set to width=1\textwidth, then everything compiles as it should.
• If the width option in \includegraphics is set to width=a\textwidth, where a is less than 1, then the caption will be offset to the right by about 2pt, and will thus not be correctly centered under the image.
• If the width option in \includegraphics is set to width=b\textwidth, where b is greater than 1, then there will be a thin amount of white space between the caption and the image (although the caption will not be offset in this case).

Below is a MWE:

\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{graphicx}
\usepackage{xcolor}
\usepackage{tgbonum}
\usepackage{tikz}
\usepackage{caption}
\usepackage{float}
\usepackage{tcolorbox}
\usepackage{calc}
%include own graphics path \graphicspath{{/Users/...}}
%%captions formatting%%
\definecolor{captionColor}{RGB}{103,143,150}
\DeclareCaptionFormat{customCaption}{%
  \begin{tcolorbox}[
    nobeforeafter,
    colback=captionColor,
    arc=0pt,
    outer arc=0pt,
    boxrule=0pt,
    colupper=white,
    %fontupper=\normal,
    boxsep=0pt
  ]
  #1#2#3
  \end{tcolorbox}%
}
\begin{document}
The beginning of the document stretches the full text width.
\begin{minipage}[t]{0.60\textwidth}\vspace{0pt}
There will be some text on the left side of the page.  The start of this text needs to be vertically aligned with the top of the other minipage.
\end{minipage}
\quad \quad \quad
\begin{minipage}[t]{0.29\textwidth}\vspace{0pt}
%FIRST PHOTO
%use savebox to get the width of photo1
\newsavebox\placeHolderOne
\sbox\placeHolderOne{\includegraphics[width=.9\linewidth,height=.9\linewidth,keepaspectratio]{examplePhoto1}}
%set the width of the caption to the width of photo1
\captionsetup{type=figure,width=\wd\placeHolderOne,format=customCaption,labelformat=empty,justification=raggedright,position=bottom,skip=0pt,textfont={it}}
%include photo1, with caption
\includegraphics[width=.9\linewidth,height=.9\linewidth,keepaspectratio]{examplePhoto1}
{\caption[]{Caption is offset to the side when width < linewidth}}
\vspace{6mm}
%SECOND PHOTO
%use savebox to get the width of photo2
\newsavebox\placeHolderTwo
\sbox\placeHolderTwo{\includegraphics[width=1.2\linewidth,height=.9\linewidth,keepaspectratio]{examplePhoto2}}
%set the width of the caption to the width of photo2
\captionsetup{type=figure,width=\wd\placeHolderTwo,format=customCaption,labelformat=empty,justification=raggedright,position=bottom,skip=0pt,textfont={it}}
%include photo2, with caption
\includegraphics[width=1.2\linewidth,height=.9\linewidth,keepaspectratio]{examplePhoto2}
{\caption[]{A white space appears between image and caption when width>linewidth}}
\end{minipage}
\end{document}

Answer 1

This answer only solves the first problem,

Caption is offset to the side when width < linewidth

The caption option width=<length> not only sets the width of captions, but also centers the caption. What you see is the effect of centering.

To control both the caption width and the left margin, use margin={0pt, \dimexpr\linewidth-<desired width>}.

An example demonstrating the centering alignment of width=<length>:

\documentclass{article}
\usepackage{caption}
\usepackage[pass, showframe]{geometry}
\usepackage{graphicx}
\usepackage[T1]{fontenc}
\begin{document}
  \captionsetup{type=figure, width=3cm}
  \includegraphics[width=3cm]{example-image}
  \caption{|text\hfill text|}
\captionsetup{type=figure, width=6cm}
  \includegraphics[width=6cm]{example-image}
  \caption{|text\hfill text|}
\end{document}

Your example, using margin={0pt,\dimexpr\linewidth-\wd\placeHolder(One|Two)} instead of width=\wd\placeHolder(One|Two), document class article, and images example-image-[ab] instead of examplePhoto[12]:

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{graphicx}
\usepackage{xcolor}
\usepackage{tgbonum}
\usepackage{tikz}
\usepackage{caption}
\usepackage{float}
\usepackage{tcolorbox}
\usepackage{calc}
%include own graphics path \graphicspath{{/Users/...}}
%%captions formatting%%
\definecolor{captionColor}{RGB}{103,143,150}
\DeclareCaptionFormat{customCaption}{%
  \begin{tcolorbox}[
    nobeforeafter,
    colback=captionColor,
    arc=0pt,
    outer arc=0pt,
    boxrule=0pt,
    colupper=white,
    %fontupper=\normal,
    boxsep=0pt
  ]
  #1#2#3
  \end{tcolorbox}%
}
\begin{document}
The beginning of the document stretches the full text width.
\begin{minipage}[t]{0.60\textwidth}\vspace{0pt}
There will be some text on the left side of the page.  The start of this text needs to be vertically aligned with the top of the other minipage.
\end{minipage}
\quad \quad \quad
\begin{minipage}[t]{0.29\textwidth}\vspace{0pt}
%FIRST PHOTO
%use savebox to get the width of photo1
\newsavebox\placeHolderOne
\sbox\placeHolderOne{\includegraphics[width=.9\linewidth,height=.9\linewidth,keepaspectratio]{example-image-a}}
%set the width of the caption to the width of photo1
\captionsetup{type=figure,margin={0pt,\dimexpr\linewidth-\wd\placeHolderOne},format=customCaption,labelformat=empty,justification=raggedright,position=bottom,skip=0pt,textfont={it}}
%include photo1, with caption
\includegraphics[width=.9\linewidth,height=.9\linewidth,keepaspectratio]{example-image-a}
{\caption[]{Caption is offset to the side when width < linewidth}}
\vspace{6mm}
%SECOND PHOTO
%use savebox to get the width of photo2
\newsavebox\placeHolderTwo
\sbox\placeHolderTwo{\includegraphics[width=1.2\linewidth,height=.9\linewidth,keepaspectratio]{example-image-b}}
%set the width of the caption to the width of photo2
\captionsetup{type=figure,margin={0pt,\dimexpr\linewidth-\wd\placeHolderTwo},format=customCaption,labelformat=empty,justification=raggedright,position=bottom,skip=0pt,textfont={it}}
%include photo2, with caption
\includegraphics[width=1.2\linewidth,height=.9\linewidth,keepaspectratio]{example-image-b}
{\caption[]{A white space appears between image and caption when width>linewidth}}
\end{minipage}
\end{document}

Answer 2

Another approach in which both the figure and its caption are typeset by a customed \tcbox. Note tcolorbox won't use caption settings done by caption package, so you have to do it yourself.

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{tgbonum}
\usepackage{tcolorbox}
\tcbuselibrary{skins}
\definecolor{captionColor}{RGB}{103,143,150}
\newtcbox[use counter*=figure, list inside=lof]\tcbfigure[1]{
  enhanced,
  nobeforeafter,
  colback=captionColor,
  colbacktitle=captionColor,
  arc=0pt,
  outer arc=0pt,
  boxrule=0pt,
  boxsep=0pt,
  leftupper=0pt, rightupper=0pt,
  top=0pt, bottom=0pt,
  toptitle=3mm,
  bottomtitle=3mm,
  flip title,
  fonttitle=\itshape,
  % before title=\RaggedRight, % need ragged2e package
  #1
}
\begin{document}
The beginning of the document stretches the full text width.
\begin{minipage}[t]{0.60\textwidth}\vspace{0pt}
  There will be some text on the left side of the page.  The start of this text needs to be vertically aligned with the top of the other minipage.
\end{minipage}
%
\quad \quad \quad
%
\begin{minipage}[t]{0.29\textwidth}\vspace{0pt}
  \tcbfigure{title=Caption is offset to the side when width < linewidth}
    {\includegraphics[width=.9\linewidth]{example-image}}\par
  \vspace{6mm}
\tcbfigure{title=A white space appears between image and caption when width > linewidth}
    {\includegraphics[width=1.2\linewidth]{example-image}}
\end{minipage}
\end{document}