\usepackage{overpic}

%********************
%* Graphic callouts *
%********************

% Where to put the <co> in the graphic
\newdimen\picfactorx
\newdimen\picfactory
\newdimen\picx
\newdimen\picy

% Evaluates the percent factors for x and y dimensions. The principle is that
% overpic gives a 100% for the biggest length (width or height).
% The coordonates given to \calspair being in percentage of the graphic dimension
% itself, the picfactors translate them to overpic percentages.
% 
% Variables set by overpic:
% \@tempcnta = width
% \@tempcntb = height

\def\picfactoreval{%
  \ifnum\@tempcnta>\@tempcntb%
    \picfactorx=1pt%
    \picfactory=\@tempcntb pt%
    \divide\picfactory by \@tempcnta%
  \else%
    \picfactory=1pt%
    \picfactorx=\@tempcnta pt%
    \divide\picfactorx by \@tempcntb%
  \fi%
}

\newcommand{\calspair}[3]{
  \sbox{\z@}{#3}%
  \picx=#1\picfactorx%
  \picy=#2\picfactory%
  \put(\strip@pt\picx,\strip@pt\picy){\makebox(0,0){\usebox{\z@}}}
}

%*********************
%* Verbatim callouts *
%*********************

% Counters for cross-referencing callouts
\newcounter{cocnt}
\newcounter{colref}

% How to represent the <co> markup.
% Big thanks to Jean-Côme Charpentier for his help.
%
\newlength{\co@width}
\newlength{\co@height}
\newlength{\balldiam}
\newlength{\ballcentre}

\def\conum#1{%
  \sbox{\z@}{\color{white}\sffamily\bfseries\tiny#1}% 
  % Box sizes for any number with two digits
  \sbox{\@ne}{\color{white}\sffamily\bfseries\tiny00}%
  \settowidth{\co@width}{\usebox{\@ne}}%
  \settoheight{\co@height}{\usebox{\@ne}}%
  % Find out the biggest length, to define the circle diameter
  \ifnum\co@width>\co@height
    \setlength{\balldiam}{\co@width}%
  \else
    \setlength{\balldiam}{\co@height}%
  \fi
  \balldiam=1.45\balldiam
  \ballcentre=0.5\balldiam
  \setlength{\unitlength}{1pt}% In the case it has been changed
  \begin{picture}(\strip@pt\balldiam,\strip@pt\balldiam) %
  \put(\strip@pt\ballcentre,\strip@pt\ballcentre){\circle*{\strip@pt\balldiam}}
  \put(\strip@pt\ballcentre,\strip@pt\ballcentre){\makebox(0,0){\usebox{\z@}}}
  \end{picture}%
}

% How to represent a <co> embedded in a listing
\def\co#1{\refstepcounter{cocnt}\conum{#1}}

% Make the <co> text and the label to link to
\def\coref#1#2{\co{#1}\label{#2}}

% Make only the <co> label to link to
\def\colabel#1{\refstepcounter{cocnt}\label{#1}}

% Make the <callout> label to link to
\def\collabel#1{\refstepcounter{colref}\label{#1}}
