highlighter.sty

% This file from: https://tex.stackexchange.com/a/19716

\RequirePackage{atbegshi}
\RequirePackage{forloop}
\RequirePackage{ifthen}
\RequirePackage{tikz}
\RequirePackage{zref-abspage}
\RequirePackage{zref-abspos}

% customizable by package user
\tikzset{highlighter/.style = {yellow, line width = \baselineskip}}

% anchor placement, with @highlight counting upward to generate unique names
\newcounter{@highlight}
\newcommand{\@HighlightAnchor}[1]{\ensuremath{\vcenter{\zsavepos{highlight-#1}}}}
\newcommand{\HighlightFrom}[0]{\stepcounter{@highlight}\@HighlightAnchor{begin-\the@highlight}}
\newcommand{\HighlightTo}[0]{\@HighlightAnchor{end-\the@highlight}}

% highlight painting, with @@highlight counting upward to consider all defined highlights
\newcounter{@@highlight}
\newcommand{\@HighlightPage}[1]{\zref@extract{highlight-#1-\the@@highlight}{abspage}}
\newcommand{\@HighlightCoords}[2]{(#1\zposx{highlight-#2-\the@@highlight}sp, #1\zposy{highlight-#2-\the@@highlight}sp)}

\AtBeginShipout{
  \AtBeginShipoutUpperLeft{
    % consider every highlight until reaching one that is undefined
    \forloop{@@highlight}{1}{\@HighlightPage{begin} > 0}{
      % page highlight if it begins and ends on the current page
      \ifthenelse{\@HighlightPage{begin} = \value{abspage}}{
        \ifthenelse{\@HighlightPage{end} = \value{abspage}}{
          % drop an anchor here so we compute the proper (x, y) offsets
          \zsavepos{highlight-draw-\the@@highlight}%
          \tikz[overlay, shift={\@HighlightCoords{-}{draw}}]{
            \draw [highlighter] \@HighlightCoords{}{begin} -- \@HighlightCoords{}{end};
          }}
        {\PackageWarning{highlighter}{highlight \protect#\the@@highlight\space crosses from page \@HighlightPage{begin} to page \@HighlightPage{end}}}}
      {}}}}