Creating a table with a vertically separated header row and \hhline produces horizontal line distortion

Question

I want to create a table which marks some cells either with $\times$ or they simply stay empty. I have two header rows of which I would like to separate the first header row visually. I use \hhline{~======} to get this visual separation. I resorted to using this additinoal package because I found, that a double \hline does not seem to work correctly with \multicolumn{1}{c|}{}.

After finally having found a solution, I now get the problem, that the horizontal lines produced by \hhline are weirdly distorted:

Here's my minimal working example:

\documentclass{article}
\usepackage[utf8]{inputenc}

\usepackage{hhline}

\begin{document}

\begin{table}[h]
  \centering
  \begin{tabular}{|l|c|c|c|c|c|c|c|c|}
    \cline{2-7}
    \multicolumn{1}{c|}{} 
          & $T_1$    & $T_2$    & $T_3$    & $T_4$    & $T_5$    & $T_6$    \\ 
    \hhline{~ = = = = = =}
    \multicolumn{1}{c|}{}
          & $r_1$    & $r_2$    & $r_3$    & $r_4$    & $r_5$    & $r_6$    \\
    \hline
    $t_1$ &          & $\times$ &          & $\times$ & $\times$ &          \\
    \hline
    $t_2$ & $\times$ &          & $\times$ &          &          &          \\
    \hline
    $t_3$ &          & $\times$ &          &          &          &          \\
    \hline
    $t_4$ & $\times$ &          &          & $\times$ & $\times$ & $\times$ \\
    \hline
    $t_5$ &          & $\times$ & $\times$ &          &          & $\times$ \\
    \hline
    $t_6$ &          &          &          & $\times$ & $\times$ & $\times$ \\
    \hline
  \end{tabular}
\end{table}
\end{document}

For me, it seems like an anti aliasing issue but I'm probably doing something wrong on my side. I would be glad if someone could point me to a fix or to an alternative solution that looks the same as it does now. Thank you.

Answer 1

With {NiceTabular} of nicematrix, you will have a perfect output in all PDF viewers at all levels of zoom because the rules are drawn with the graphical layer PGF/Tikz (and not by concatenation of small characters).

\documentclass{article}

\usepackage{nicematrix}

\begin{document}

\begin{table}[h]
  \centering
  \begin{NiceTabular}{|l|c|c|c|c|c|c|}[corners]
    \Hline
          & $T_1$    & $T_2$    & $T_3$    & $T_4$    & $T_5$    & $T_6$    \\ 
    \Hline
    \Hline
          & $r_1$    & $r_2$    & $r_3$    & $r_4$    & $r_5$    & $r_6$    \\
    \Hline
    $t_1$ &          & $\times$ &          & $\times$ & $\times$ &          \\
    \Hline
    $t_2$ & $\times$ &          & $\times$ &          &          &          \\
    \Hline
    $t_3$ &          & $\times$ &          &          &          &          \\
    \Hline
    $t_4$ & $\times$ &          &          & $\times$ & $\times$ & $\times$ \\
    \Hline
    $t_5$ &          & $\times$ & $\times$ &          &          & $\times$ \\
    \Hline
    $t_6$ &          &          &          & $\times$ & $\times$ & $\times$ \\
    \Hline
  \end{NiceTabular}
\end{table}
\end{document}

You need several compilations (because nicematrix uses PGF/Tikz nodes under the hood).

As you can see, the vertical rules are not broken by the double horizontal rule (that's by design in nicematrix).

If you prefer to have the vertical rules borken (as with the standard behaviour of the tabulars in LaTeX), it's possible but a bit more complicated:

\documentclass{article}

\usepackage{nicematrix,tikz}

\begin{document}

\begin{table}[h]
  \centering
  \newcolumntype{I}{!{\vrule}}
  \begin{NiceTabular}{lIcIcIcIcIcIcI}[corners]
    \Hline
          & $T_1$    & $T_2$    & $T_3$    & $T_4$    & $T_5$    & $T_6$    \\ 
    \Hline
    \Hline
          & $r_1$    & $r_2$    & $r_3$    & $r_4$    & $r_5$    & $r_6$    \\
    \Hline
    $t_1$ &          & $\times$ &          & $\times$ & $\times$ &          \\
    \Hline
    $t_2$ & $\times$ &          & $\times$ &          &          &          \\
    \Hline
    $t_3$ &          & $\times$ &          &          &          &          \\
    \Hline
    $t_4$ & $\times$ &          &          & $\times$ & $\times$ & $\times$ \\
    \Hline
    $t_5$ &          & $\times$ & $\times$ &          &          & $\times$ \\
    \Hline
    $t_6$ &          &          &          & $\times$ & $\times$ & $\times$ \\
    \Hline
  \CodeAfter \tikz \draw (3-|1) -- (last-|1) ;
  \end{NiceTabular}
\end{table}
\end{document}