fitting side by side figures with floatrow

Question

I'm trying to fit two figures side by side (with the same size) in the width of the text, using the floatrow package. I use the showframe option and the \fbox in mwe for better visualization. how to fit perfectly on text width?

\documentclass[]{report}
\usepackage[a4paper,showframe]{geometry}
\usepackage{subcaption}
\usepackage{lipsum}
\usepackage{graphicx}
\usepackage{floatrow} 

\begin{document}


\lipsum[1]
\begin{figure}[htb!]
\ffigbox[\textwidth]
{\begin{subfloatrow}[2]\setlength\fboxsep{0pt}\setlength\fboxrule{0.75pt}\fbox{\ffigbox[0.49\textwidth]
{
    \caption{sidewalk perforation}
    \label{subfig:furadeira}
}
{   \setlength\fboxsep{0pt}
    \setlength\fboxrule{0.75pt}
    \fbox{\includegraphics[width=0.48\textwidth]{image1.jpg}}}
    \setlength\fboxsep{0pt}
    \setlength\fboxrule{0.75pt}}
    \fbox{\ffigbox[0.49\textwidth]
{
\caption{volatile organic compounds measurement}
\label{subfig:medicaomalha}
}
{
    \setlength\fboxsep{0pt}
    \setlength\fboxrule{0.75pt}
    \fbox{\includegraphics[width=0.48\textwidth]{image2.jpg}}
}}
\end{subfloatrow}
}
{
    \caption{Hot spot investigation}
    \label{fig:hotspot}
}
\end{figure}%
\lipsum[1]
\end{document}

Answer 1

You need to change the default floatseparator (\columnsep) used. None of the predefined separators will be of use here given the widths of your figures, but you can easily define one using \DeclareFloatSeparators:

\documentclass[]{report}
\usepackage[a4paper,showframe]{geometry}
\usepackage{subcaption}
\usepackage{lipsum}
\usepackage{graphicx}
\usepackage{floatrow} 

\DeclareFloatSeparators{myfill}{\hskip.013\textwidth plus1fill}

\begin{document}

\lipsum[1]
\floatsetup[subfloat]{floatrowsep=myfill}
\begin{figure}[htb!]
\setlength\fboxsep{0pt}\setlength\fboxrule{0.75pt}
\ffigbox[\textwidth]
{
\begin{subfloatrow}[2]
%\fbox{
\ffigbox[.49\textwidth]
  {
    \caption{sidewalk perforation}
    \label{subfig:furadeira}
  }
  {
    \includegraphics[width=\linewidth]{example-image-a}%
  }
%}
%\fbox{
\ffigbox[.49\textwidth]
  {
    \caption{volatile organic compounds measurement}
    \label{subfig:medicaomalha}
  }
  {
    \includegraphics[width=\linewidth]{example-image-b}%
  }
%}
\end{subfloatrow}%
}
{
    \caption{Hot spot investigation}
    \label{fig:hotspot}
}
\end{figure}%
\lipsum[1]

\end{document}

Notice that there's still some spurious blank space to the left of the first subfligure; I suspect this is a bug in floatrow.sty (most probably a missing % to kill a surious space).

To have the subfigures perfectly flushed to the margins, I'd suggest you a work-around using floatrow instead of subfloatrow and using \captionof{subfigure}{text...} to provide the captions:

\documentclass[]{report}
\usepackage[a4paper,showframe]{geometry}
\usepackage{subcaption}
\usepackage{lipsum}
\usepackage{graphicx}
\usepackage{floatrow} 

\captionsetup[subfigure]{labelformat=parens,labelsep=space}

\begin{document}

\lipsum[1]

\begin{figure}[htb!]
\ffigbox[\textwidth]
  {
    \begin{floatrow}
    \ffigbox[\linewidth]
      {\captionof{subfigure}{sidewalk perforation}
      \label{subfig:furadeira}}
      {\includegraphics[width=\linewidth]{example-image-a}}
    \ffigbox[\linewidth]
      {\captionof{subfigure}{volatile organic compounds measurement}
      \label{subfig:medicaomalha}}
      {\includegraphics[width=\linewidth]{example-image-b}}
    \end{floatrow}%
  }
  {\caption{Hot spot investigation}\label{fig:hotspot}}
\end{figure}
\lipsum[1]

\end{document}