make text invisible

Question

I have a manuscript I lecture from using class scrbook.

Now for distance teaching I want to redefine some environments (such as proof) so that I get gaps for the proofs on the presentation that I can fill handwritten.

One possibility probably would be to just choose a textcolor of white in these environments. However, then the text remains copy-pasteable on the page. And it might be irritating, if I handwrite over the text, and the invisible characters in the background interfere. Another drawback would be, that if my text contains a \textcolor{red}{redly emphasized} text, it would not vanish:

\documentclass{scrbook}
\usepackage{color}
\usepackage{xcolor}
\usepackage{amsthm,amsfonts,amssymb}
\begin{document}
\begin{proof}
  \textcolor{red}{This} is important
\end{proof}
\begin{proof}
  \color{white}
  \textcolor{red}{This} is important
\end{proof}
\end{document}

Numbering should not depend on whether the proofs and their content are hidden or not.

I was wondering, if there were a possibility, to make a font-manipulation, so that instead of the pdf-characters, only empty (and invisible) boxes with the char's size were put into the pdf. That way, numbering would stay intact, and the text were no longer copyable.

thanks, sven

Answer 1

You can transform the proofs into invisible text by using the tcolorbox package. In the example below, the background yellow color is stressing the place where the text becomes invisible. This is the original document (with the text visible).

In the code you find the definition of the box onoffbox that depends on an optional argument; if empty, the text in the box is visible. If non-empty, the text is invisible.

The environment onoffproof is defined using the usual proof and the onoffbox. It inherits the argument of the latter.

\documentclass[11pt, a4paper]{article}
\usepackage{amsmath, amsthm}
\usepackage{tcolorbox}
\tcbuselibrary{skins,breakable}
\usepackage{ifthen}
\usepackage{lipsum}
\newtheorem{pro}{Proposition}
\newenvironment{onoffbox}[1][]{%
  \ifthenelse{\equal{#1}{}}{\def\onoff{visible}}{\def\onoff{invisible}}
  \tcolorbox[%
  empty,
  \onoff,
  parbox=false,
  noparskip,
  enhanced,
  breakable,
  frame hidden, % default frame hidden
  boxrule=0pt, % default frame hidden
  colback=white, % yellow,
  left=-.5ex, right=-.5ex,
  before skip=0ex plus 2pt,
  after skip=1ex plus 2pt]
}{\endtcolorbox}
\newenvironment{onoffproof}[1][]{%
  \begin{onoffbox}[#1]\begin{proof}}{\end{proof}\end{onoffbox}}
\title{Using tcolorbox package for invisible text}
\begin{document}
\maketitle
\lipsum[1-2]
\begin{pro}
 $b^2+c^2=a^2$ 
\end{pro}
\begin{onoffproof}[off]
  It seems that {\color{red}this is an important result}.
  \lipsum[3-4]
  This is the end of our test.
\end{onoffproof}
\lipsum[5-7]
\end{document}