How to put a background letter in a table?

Question

I have a table with some numbers in it, such as

\documentclass{article}

\begin{document}

\begin{tabular}{llll}
1 & 2 & 3 & 4 \\
1 & 2 & 3 & 4 \\
1 & 2 & 3 & 4 \\
1 & 2 & 3 & 4 \\
1 & 2 & 3 & 4
\end{tabular}

\end{document}

I would like to have a "background" for the table (I actually want to be able to specify it for specific blocks in the table) -- the background should be an alphabet letter (such as A) in light shaded gray that doesn't hide the numbers in black.

How would it be possible to do that?

Answer 1

Without absolute positioning using tikzmark library for the tikz package.

\documentclass{article}
\usepackage{tikz, lmodern}
\usetikzlibrary{calc,tikzmark}
\begin{document}

\begin{tikzpicture}[overlay,remember picture]
  \node [yshift=.6ex] at ( $(pic cs:A) !.5! (pic cs:B)$ ){ \fontsize{60}{60}\selectfont\textbf{\color{gray!20}A} };
\end{tikzpicture}

\begin{tabular}{llll}
\tikzmark{A}%
1 & 2 & 3 & 4 \\
1 & 2 & 3 & 4 \\
1 & 2 & 3 & 4 \\
1 & 2 & 3 & 4 \\
1 & 2 & 3 & 4\tikzmark{B}%
\end{tabular}

\end{document}

Observe that the tikzpicture is drawn here before the table text is typeset and before its coordinates are defined. The purpose here is to let the text go on top of the picture. This will work in most cases, because the coordinates are calculated at the first run and the picture is positioned correctly at subsequent runs.

If, for some reason, this doesn't work, then you can add \usepackage{everypage} in the preamble and put the tikzpicture below the table using \AddThispageHook{...} like this:

\AddThispageHook{%
\begin{tikzpicture}[overlay,remember picture]
    \node [yshift=.6ex] at ( $(pic cs:A) !.5! (pic cs:B)$ ){ \fontsize{60}{60}\selectfont\textbf{\color{gray!20}A} };
\end{tikzpicture}
}

Answer 2

\documentclass{article}
\usepackage{color}
\usepackage{lmodern}
\begin{document}


\begin{picture}(0,0)
\put(7,-20){\fontsize{70pt}{70pt}\bfseries\color[gray]{.9}{A}}
\end{picture}%
\begin{tabular}{llll}
1 & 2 & 3 & 4 \\
1 & 2 & 3 & 4 \\
1 & 2 & 3 & 4 \\
1 & 2 & 3 & 4 \\
1 & 2 & 3 & 4
\end{tabular}

\end{document}

By reducing the font size and altering the coordinates you can of course just place the letter (or letters) behind any sub-matrix block.

Answer 3

Here's a TikZ solution which uses a matrix of nodes for the table, rather than a tabular, and hence gets the positions for the letters automatically from the TikZ code. It uses the matrix library for that and the backgrounds library to put the letters on a layer drawn in the background i.e. behind the numbers in the matrix.

\documentclass[border=5pt, multi, tikz]{standalone}
\usepackage[rm={lining},sf={lining},tt={lining,tabular,monowidth}]{cfr-lm}
\usetikzlibrary{backgrounds,matrix}
\begin{document}
\begin{tikzpicture}
  [
    font=\tstyle,
    big alph/.style={text opacity=.2, text=gray, font=\Huge}
  ]
  \matrix (m) [matrix of nodes]
  {
    1 & 2 & 3 & 4 \\
    1 & 2 & 3 & 4 \\
    1 & 2 & 3 & 4 \\
    1 & 2 & 3 & 4 \\
  };
  \begin{scope}[on background layer, big alph]
    \node at (m-1-1.south east) {A};
    \node at (m-1-3.south east) {B};
    \node at (m-3-1.south east) {C};
    \node at (m-3-3.south east) {D};
  \end{scope}
\end{tikzpicture}
\end{document}

If your table includes more complex mathematics, use matrix of math nodes to have the nodes automatically switch to maths mode.

The (m) gives the matrix its name which is then used to position the letters by referencing the matrix's name, the row and the column (e.g. m-1-1 etc.). The anchor of the node south east is the relevant position on the border of the individual node within the matrix i.e. bottom left for south east. So (m-3-3.north west) would refer to the top right corner of the node in the third column and third row of the matrix named m.

Answer 4

I introduce \grayblock{tabular code}{x-fractional offset}{y-fractional offset}{scale of inset}{inset}. The macro can be nested (as it is in the MWE below) to place multiple insets, where a \grayblock can be placed where tabular code would go.

\documentclass{article}
\usepackage{stackengine,xcolor,graphicx}
\newsavebox{\tmpbox}
\newcommand\grayblock[5]{%
\savebox{\tmpbox}{#1}%
\stackinset{c}{}{c}{}{\usebox{\tmpbox}}%
  {\stackinset{l}{#2\wd\tmpbox}{b}{#3\dimexpr\ht\tmpbox+\dp\tmpbox}{%
    \color{black!20}\scalebox{#4}{#5}}{%
    \rule{\wd\tmpbox}{0pt}\rule[-\dp\tmpbox]{0pt}{\dimexpr\ht\tmpbox+\dp\tmpbox}}}%
}
\begin{document}
\fboxsep=0pt
x
\grayblock{%
\grayblock{\begin{tabular}{|cccc|}
\hline
a & b & c & d\\
d & e & f & g\\
g & h & i & j\\
j & k & l & m\\\hline
\end{tabular}}{.1}{.53}{3}{B}%
}{.57}{.08}{3}{C}
x
\end{document}

Answer 5

Lazy way: tcolorbox

\documentclass{article}
\usepackage{tcolorbox}
\tcbuselibrary{skins}
\newtcbox{\mytable}[1][X]{enhanced, blanker, on line, watermark text=#1}
\begin{document}
My tables 
\mytable[A]{
\begin{tabular}{llll}
1 & 2 & 3 & 4 \\
1 & 2 & 3 & 4 \\
1 & 2 & 3 & 4 \\
1 & 2 & 3 & 4 \\
1 & 2 & 3 & 4
\end{tabular}} 
with watermarks
\mytable{\begin{tabular}{lll}  1 & 2 \\3 & 4 \end{tabular}}
\end{document}