Set `\fboxsep` only for `\fbox` but not `\boxed`

Question

I’m using both \fbox and \boxed but for different uses. When I use \fbox, I need it to have \fboxsep set to -0.5pt, but when I used \boxed I want a \fboxsep of 2pt.

Is there a way to do that?

EDIT: As I was asked for my usage, I defined this in my preamble

\setlength\fboxrule{0.5pt}
\setlength\fboxsep{-0.5pt}
\newcommand{\schema}[1]{\fbox{\includegraphics[width=\linewidth]{#1}}}

This is in order to include schemes drawn on a white background in a colored background area (typically one from tcolorbox) and have them neatly separated by a thin black margin.

OTOH, I used \boxed around all types of mathematical formulas.

Answer 1

You should do it the other way around:

\newcommand{\schema}[1]{%
  \begingroup % localize the changes in the parameters
  \setlength{\fboxsep}{-0.5pt}%
  \setlength{\fboxrule}{0.5pt}%
  \fbox{\includegraphics[width=\linewidth]{#1}}%
  \endgroup
}

This way, any other \fbox (and also \boxed) will use the default values (or the ones you fix for them).

Answer 2

With xpatch:

\documentclass{article} %
\usepackage{amsmath}
\setlength{\fboxsep}{-0.5pt}
\usepackage{xpatch} 
\pretocmd{\boxed}{\setlength{\fboxsep}{2pt}}{}{}

\begin{document}

    \[ \boxed{a = \frac{b + c}{2}} \]%

    \centering\fbox{$a = \dfrac{b + c}{2}$}

\end{document} 

Answer 3

\fbox simply does not work with negative \fboxsep To see what happens see

\documentclass{article}

\usepackage{color}

\begin{document}

\fbox{\textcolor{red}{\rule{1cm}{1cm}}}

\bigskip

\setlength\fboxsep{5pt}

\fbox{\textcolor{red}{\rule{1cm}{1cm}}}

\bigskip

\setlength\fboxsep{-5pt}

\fbox{\textcolor{red}{\rule{1cm}{1cm}}}



\end{document}

The content over-prints the top and left rule and under-prints the right and bottom.

For the actual use case of boxing an image within \textwidth use

\noindent\fbox{%
  \includegraphics[width=\dimexpr\linewidth-2\fboxsep-2\fboxrule]{example-image}}

You could use the above with \fboxsep set to 0pt if you want to space.

To get a local setting with boxed, simplest is to define a custom command that locally sets \fboxsep

\newcommand\myboxed[1]{{{%
    \setlength\fboxsep{2pt}%
     \boxed{#1}}}