How to Contiguously Shade a Portion of a Paragraph with Color

Question

Consider the code:

\documentclass{book}
\usepackage{xcolor,soul}
\colorlet{soulred}{red!25}

\begin{document}
\thispagestyle{empty}
\LARGE

\begingroup
  \sethlcolor{soulred}%
  Here is some text. \hl{Here is some text. Here is some text. Here is some text. Here is some text. Here is some text. Here is some text. Here is some text. Here is some text. Here is some text. Here is some text. Here is some text. Here is some text.  Here is some text. Here is some text. Here is some text.}%
\endgroup
\end{document}

producing the output

QUESTION: I would like to have no white space showing between the red highlighting; but rather, have the displayed highlighted text appear as a "color-filled polygon." What might be the best way to accomplish this; and how? My MWE may not be the best way to start. Remark: I know that if I wanted the entire paragraph "blocked off", I could use the mdframed package, but I don't know how to extend it to a specific portion(s) of a given paragraph. I compiled the code with pdflatex. Thank you.

Answer 1

This uses \tikzmark to locate the start and end of the highlighted area. Each \tikzmark requires a unique name. These are stored on the aux file, so it takes two runs to locate them.

Note that the tikzpicture comes before the text.

\documentclass{book}
\usepackage{xcolor}
\colorlet{soulred}{red!25}
\usepackage{tikzpagenodes}
\usetikzlibrary{tikzmark,calc}

\newlength{\markheight}

\begin{document}
\thispagestyle{empty}
\LARGE

\begin{tikzpicture}[overlay, remember picture]
\coordinate (A) at (pic cs:start);
\coordinate (B) at (pic cs:end);
\pgfextracty{\markheight}{\pgfpointdiff{\pgfpointanchor{B}{center}}{\pgfpointanchor{A}{center}}}% difference in baselines
\ifdim\markheight<\baselineskip% same line
  \fill[color=soulred] ($(A)+(-2pt,\ht\strutbox)$) rectangle ($(B)+(2pt,-\dp\strutbox)$);
\else
  \ifdim\markheight<3\baselineskip
    \fill[color=soulred] ($(A)+(-2pt,\ht\strutbox)$) rectangle ($(A -| current page text area.east)+(2pt,-\dp\strutbox)$);
    \fill[color=soulred] ($(B -| current page text area.west)+(-2pt,\ht\strutbox)$) rectangle ($(B)+(2pt,-\dp\strutbox)$);
  \else
    \fill[color=soulred] ($(A)+(-2pt,\ht\strutbox)$) rectangle ($(A -| current page text area.east)+(2pt,-\dp\strutbox)$);
    \fill[color=soulred] ($(A -| current page text area.west)+(-2pt,-\dp\strutbox)$) rectangle ($(B -| current page text area.east)+(2pt,\ht\strutbox)$);
    \fill[color=soulred] ($(B -| current page text area.west)+(-2pt,\ht\strutbox)$) rectangle ($(B)+(2pt,-\dp\strutbox)$);
  \fi
\fi
\end{tikzpicture}

  Here is some text. \tikzmark{start}Here is some text. Here is some text. Here is some text. Here is some text. Here is some text. Here is some text. Here is some text. Here is some text. Here is some text. Here is some text. Here is some text. Here is some text.  Here is some text. Here is some text. Here is some text.\tikzmark{end}

\end{document}