Draw Tikz highlighting on the background of text

Question

I was recently referred to Cool Text Highlighting in LaTeX for Tikz highlighting-underlining.

I've managed to modify Antal answer to my needs but I can't seem to achieve the following: I'd like the highlight to be drawn on the background of text and not over it, as it is now.

I've prepared a MWE for better explaining what I mean:

\documentclass{minimal}
\usepackage{soul}
\usepackage{tikz}
\usetikzlibrary{calc}
\usetikzlibrary{decorations.pathmorphing}

\makeatletter

\newcommand{\defhighlighter}[3][]{%
  \tikzset{every highlighter/.style={color=#2, fill opacity=#3, #1}}%
}

\defhighlighter{yellow}{.5}

\newcommand{\highlight@DoHighlight}{
  \fill [ decoration = {random steps, amplitude=1pt, segment length=15pt}
        , outer sep = -15pt, inner sep = 0pt, decorate
        , every highlighter, this highlighter ]
        ($(begin highlight)+(0,8pt)$) rectangle ($(end highlight)+(0,-3pt)$) ;
}

\newcommand{\highlight@BeginHighlight}{
  \coordinate (begin highlight) at (0,0) ;
}

\newcommand{\highlight@EndHighlight}{
  \coordinate (end highlight) at (0,0) ;
}

\newdimen\highlight@previous
\newdimen\highlight@current

\DeclareRobustCommand*\highlight[1][]{%
  \tikzset{this highlighter/.style={#1}}%
  \SOUL@setup
  %
  \def\SOUL@preamble{%
    \begin{tikzpicture}[overlay, remember picture]
      \highlight@BeginHighlight
      \highlight@EndHighlight
    \end{tikzpicture}%
  }%
  %
  \def\SOUL@postamble{%
    \begin{tikzpicture}[overlay, remember picture]
      \highlight@EndHighlight
      \highlight@DoHighlight
    \end{tikzpicture}%
  }%
  %
  \def\SOUL@everyhyphen{%
    \discretionary{%
      \SOUL@setkern\SOUL@hyphkern
      \SOUL@sethyphenchar
      \tikz[overlay, remember picture] \highlight@EndHighlight ;%
    }{%
    }{%
      \SOUL@setkern\SOUL@charkern
    }%
  }%
  %
  \def\SOUL@everyexhyphen##1{%
    \SOUL@setkern\SOUL@hyphkern
    \hbox{##1}%
    \discretionary{%
      \tikz[overlay, remember picture] \highlight@EndHighlight ;%
    }{%
    }{%
      \SOUL@setkern\SOUL@charkern
    }%
  }%
  %
  \def\SOUL@everysyllable{%
    \begin{tikzpicture}[overlay, remember picture]
      \path let \p0 = (begin highlight), \p1 = (0,0) in \pgfextra
        \global\highlight@previous=\y0
        \global\highlight@current =\y1
      \endpgfextra (0,0) ;
      \ifdim\highlight@current < \highlight@previous
        \highlight@DoHighlight
        \highlight@BeginHighlight
      \fi
    \end{tikzpicture}%
    \the\SOUL@syllable
    \tikz[overlay, remember picture] \highlight@EndHighlight ;%
  }%
  \SOUL@
}
\makeatother

\begin{document}
\highlight[yellow]{The highlight is drawn over the text}

\bigskip
\begin{tikzpicture}
\node[fill=yellow!50]{the highlight is drawn below text};
\end{tikzpicture}
\end{document}

Is it possible to move the highliting below the text as in standard filling for Tikz nodes?

Answer 1

With this solution, I leave the OP's solution in place and add another layer in the form of \Ohighlight, in the form of

\newsavebox\tmpbox
\usepackage{tabto}
\newcommand\Ohighlight[2][]{%
  \tabto*{0in}\edef\tmpx{\TabPrevPos}\tabto*{\tmpx}%    SAVES THE CURRENT x POSITION
  \savebox\tmpbox{%                            SETS THE TEXT, STARTING AT x, IN A SAVED MINIPAGE
  \begin{minipage}[b]{\textwidth}
    \tabto*{\tmpx}#2\strut%
  \end{minipage}}%
  {\color{\highlightcolor!0}\highlight[#1]{#2}}%         STEP #1, BELOW
  \vspace*{\dimexpr-\dp\tmpbox-\ht\tmpbox+\dp\strutbox}% STEP #2, BELOW
  \tabto*{\tmpx}#2\unskip\ %                             STEP #3, BELOW
}

which tries to accomplish the following:

1) do the original \highlighting, while making the text the same color as the highlight (aka invisible);

2) back up to the original location (horizontally by way of tabto package) and vertically by way of \vspace on a measured minipage;

3) retype the text, in the clear, atop the prior highlighting, which leaves the page position where it ought to be.

This solution was EDITED when some kerning differences were noted between the output of \highlight and text in the clear. The pre-\highlight is now done in a blank color, to avoid the issue of double-striked text.

This approach will not work across page boundaries. In the MWE, it works across paragraph boundaries, but I imagine that inserted glue on a page could mess that up, unless \raggedbottom were used.

The fact that some kerning issues were noted that prompted an edit to my code leaves open the possibility that for some cases, the \highlight linebreaking would differ from text in the clear, which would obviously represent a failure of the method. Since the issue seemed related to trailing blank spaces, adding an \unskip\ to the final text in the clear (step 3.) seemed to fix the kerning issue, but I won't say that it can't pop up under another guise.

If one wants to check the kerning issue, then edit this line {\color{\highlightcolor!0}\highlight[#1]{#2}}% to {\color{red}\highlight[#1]{#2}}%, which will produce a double-strike of the text. If they don't match up, you will see it there.

\documentclass{minimal}
\usepackage{soul}
\usepackage{tikz}
\usetikzlibrary{calc}
\usetikzlibrary{decorations.pathmorphing}

\makeatletter

\newcommand{\defhighlighter}[3][]{%
  \tikzset{every highlighter/.style={color=#2, fill opacity=#3, #1}}%
}

\def\highlightcolor{yellow}
\defhighlighter{\highlightcolor}{.5}

\newcommand{\highlight@DoHighlight}{
  \fill [ decoration = {random steps, amplitude=1pt, segment length=15pt}
        , outer sep = -15pt, inner sep = 0pt, decorate
        , every highlighter, this highlighter ]
        ($(begin highlight)+(0,8pt)$) rectangle ($(end highlight)+(0,-3pt)$) ;
}

\newcommand{\highlight@BeginHighlight}{
  \coordinate (begin highlight) at (0,0) ;
}

\newcommand{\highlight@EndHighlight}{
  \coordinate (end highlight) at (0,0) ;
}

\newdimen\highlight@previous
\newdimen\highlight@current

\DeclareRobustCommand*\highlight[1][]{%
  \tikzset{this highlighter/.style={#1}}%
  \SOUL@setup
  %
  \def\SOUL@preamble{%
    \begin{tikzpicture}[overlay, remember picture]
      \highlight@BeginHighlight
      \highlight@EndHighlight
    \end{tikzpicture}%
  }%
  %
  \def\SOUL@postamble{%
    \begin{tikzpicture}[overlay, remember picture]
      \highlight@EndHighlight
      \highlight@DoHighlight
    \end{tikzpicture}%
  }%
  %
  \def\SOUL@everyhyphen{%
    \discretionary{%
      \SOUL@setkern\SOUL@hyphkern
      \SOUL@sethyphenchar
      \tikz[overlay, remember picture] \highlight@EndHighlight ;%
    }{%
    }{%
      \SOUL@setkern\SOUL@charkern
    }%
  }%
  %
  \def\SOUL@everyexhyphen##1{%
    \SOUL@setkern\SOUL@hyphkern
    \hbox{##1}%
    \discretionary{%
      \tikz[overlay, remember picture] \highlight@EndHighlight ;%
    }{%
    }{%
      \SOUL@setkern\SOUL@charkern
    }%
  }%
  %
  \def\SOUL@everysyllable{%
    \begin{tikzpicture}[overlay, remember picture]
      \path let \p0 = (begin highlight), \p1 = (0,0) in \pgfextra
        \global\highlight@previous=\y0
        \global\highlight@current =\y1
      \endpgfextra (0,0) ;
      \ifdim\highlight@current < \highlight@previous
        \highlight@DoHighlight
        \highlight@BeginHighlight
      \fi
    \end{tikzpicture}%
    \the\SOUL@syllable
    \tikz[overlay, remember picture] \highlight@EndHighlight ;%
  }%
  \SOUL@
}
\makeatother

\newsavebox\tmpbox
\usepackage{tabto}
\newcommand\Ohighlight[2][]{%
  \tabto*{0in}\edef\tmpx{\TabPrevPos}\tabto*{\tmpx}%
  \savebox\tmpbox{%
  \begin{minipage}[b]{\textwidth}
    \tabto*{\tmpx}#2\strut%
  \end{minipage}}%
  {\color{\highlightcolor!0}\highlight[#1]{#2}}%
  \vspace*{\dimexpr-\dp\tmpbox-\ht\tmpbox+\dp\strutbox}%
  \tabto*{\tmpx}#2\unskip\ %
}
\textwidth=5in

\begin{document}
\highlight{The original highlight is drawn over the text.
The original definition works over... 

Multi paragraphs, so my fix should try to as well.}

\bigskip
\begin{tikzpicture}
\node[fill=yellow!50]{with tikz, the highlight is drawn below text but not fancy};
\end{tikzpicture}

Now here is my fix. \Ohighlight{The highlight is drawn under the text}
Does it work multiline?
\Ohighlight[yellow]{The highlight is drawn under the text The highlight is drawn under the text The highlight is drawn under the text The highlight is drawn under the text

A new paragraph}
And more...
\Ohighlight{short}

\end{document}

The image below shows a zoom.