"Drill holes" using glyphs or TikZ's even odd rule for text

Question

I know how to use TikZ even and odd rule, but what I need is to apply it on a node text. Compare

I need the text to be punched out such that when I use the resulting pdf on top of a background, the background will be visible where the glyphs are located.

\documentclass{standalone}
\usepackage{palatino}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
  \path[use as bounding box] (-.1,-.1) rectangle (2.4,1.1);
  \filldraw[fill=blue,even odd rule]
  (0,0) rectangle (1,1) (0.5,0.5) circle (0.4cm);
  \begin{scope}[xshift=1.3cm]
    \filldraw[fill=blue,even odd rule]
    % the node text is black (I guess foreground colour)
    (0,0) rectangle (1,1) (0.5,0.5) node  {as};
  \end{scope}
\end{tikzpicture}
\end{document}

I also tried node[text opacity=0], but this makes the glyphs vanish.

I guess I can export the text to svg, and the use tikz2svg to have it available as path, but this is quite inconvenient.

Answer 1

I guess this is "proof-of-concept" rather than any kind of practical solution (it certainly isn't practical). It does some rather naughty stuff with low level PDF literals:

\documentclass[border=5]{standalone}
\usepackage{tikz}
\usetikzlibrary{fadings}

\tikzset{
  outline text/.style={
    execute at begin node={%
      \pgfsetfillopacity{0}%
      \pgfsetlinewidth{\pgflinewidth}%
      \pgfsetstrokecolor{#1}%
      \special{pdf:literal 1 Tr }%    
    }
  }
}

\begin{tikzfadingfrompicture}[name=Aa] 
\node [text=transparent!100, font=\Large\bfseries, 
  minimum size=1cm, fill=transparent!0] {Aa};
\end{tikzfadingfrompicture}

\begin{document}
\begin{tikzpicture}
\path [left color=yellow, right color=red, middle color=purple, 
  shading angle=45] circle [radius=2cm/3];

\node [fill=blue!20,fit fading=false, draw, minimum size=1cm,
  path fading=Aa, font=\Large\bfseries, outline text=black] {Aa};

\end{tikzpicture}    
\end{document}

And here's an sort of automatic version, which (perhaps unsurprisingly) requires some low level hacking.

\documentclass[tikz,border=5]{standalone}
\usetikzlibrary{fadings}
\makeatletter

\tikzset{
  outline text/.style={
    execute at begin node={%
      \pgfsetfillopacity{0}%
      \pgfsetlinewidth{\pgflinewidth}%
      \pgfsetstrokecolor{#1}%
      \special{pdf:literal 1 Tr }%      
    },
  },
  knockout text fading/.code={%
    \tikz@addmode{%
      % Interrupt the picture to create a fading.
      \pgfinterruptpicture%
        \let\tikz@atbegin@node=\relax%
        \begin{tikzfadingfrompicture}[name=.]
          \node [node contents=, #1,text=transparent!100, fill=transparent!0];%
          \xdef\fadingboundingbox{{\noexpand\pgfpoint{\the\pgf@picminx}{\the\pgf@picminy}}%
            {\noexpand\pgfpoint{\the\pgf@picmaxx}{\the\pgf@picmaxy}}}%
          \expandafter\pgfpathrectanglecorners\fadingboundingbox%
          \pgfusepath{discard}%
         \end{tikzfadingfrompicture}%
       \endpgfinterruptpicture%
       % Make the fading happen.
       \def\tikz@path@fading{.}%
       \tikz@mode@fade@pathtrue%
       \tikz@fade@adjustfalse%
       \pgfpointscale{0.5}{\expandafter\pgfpointadd\fadingboundingbox}%
       \def\tikz@fade@transform{shift={(\the\pgf@x,\the\pgf@y)}}%
    }%
    \tikzset{#1}% 
  }
} 
\makeatother

\begin{document}
\begin{tikzpicture}[line join=round] 
\path [left color=yellow, right color=red, middle color=purple, 
  shading angle=45] circle [radius=2cm];

\node [outline text=black,
   knockout text fading={
     rotate=-45,
     fill=blue!20, draw, text width=4.5cm, align=center,
     font=\sffamily\Large\bfseries, 
     node contents={The quick brown fox jumps over the lazy dog}
   }];

\end{tikzpicture}    
\end{document}

Answer 2

I have found examples in the TikZ & PGF manual that may help you. I have edited the two examples I found. The first picture is based on an example in section 23.4.1 Creating Fadings, the other two is based on an example in section 23.5 Transparency Groups in the TikZ & PGF manual.

        

Unfortunately, the text in the second and third example only was displayed in Adobe Reader (it didn't work with TeXworks and Evince).

You can download the background image I used in the third example here.

\documentclass[10pt]{article}

\usepackage{tikz}

\usetikzlibrary{fadings, patterns}

\newcommand{\normalfontsize}{\fontsize{10pt}{12pt}\selectfont}
\newcommand{\largefontsize}{\fontsize{1.5cm}{1.5cm}\selectfont}

\begin{document}

\normalfontsize

Example \#1

\largefontsize

\begin{tikzfadingfrompicture}[name=mypath]
\node [text=transparent!20, font=\bfseries] {Aa};
\end{tikzfadingfrompicture}

\begin{tikzpicture}
    \fill [black!20] (-1, -1) rectangle (1, 1);
    \pattern [pattern=checkerboard,%
        pattern color=black!30] (-1, -1) rectangle (1, 1);
    \shade[path fading=mypath,%
        fit fading=false,%
        left color=lime,%
        right color=orange] (-1, -1) rectangle (1, 1);
\end{tikzpicture}

\normalfontsize

Example \#2

\largefontsize

\begin{tikzpicture}
    \shade [left color=lime, right color=orange] (-1, -1) rectangle (1, 1);

    \begin{scope}[transparency group=knockout]
        \filldraw[lightgray] (-1, -1) rectangle (1, 1);
        \node[opacity=0, font=\bfseries] {Aa};
    \end{scope}
\end{tikzpicture}

\normalfontsize

Example \#3

\largefontsize

\begin{tikzpicture}
    \node[inner sep=0pt, outer sep=0pt] (background) at (0, 0)%
    {\includegraphics[width=2cm, height=2cm]{bg.png}};

    \begin{scope}[transparency group=knockout]
        \filldraw[lightgray] (-1, -1) rectangle (1, 1);
        \node[opacity=0, font=\bfseries] {Aa};
    \end{scope}
\end{tikzpicture}

\end{document}