Bytefield colors and baseline align

Question

I just found out about the bytefield package and read the documentation to customize the default layout a bit.

1. I followed the documentation to make sure the baselines of all fields match:

   \newcommand{\baselinealign}[1]{%
     \centering
     \small
     \raisebox{0pt}[\bitboxmaxheight][0pt]{#1}%
   }
   

2. I want background colors for some fields so I copied the code for \colorbitbox from the documentation.

   \newcommand{\colorbitbox}[3]{%
     \rlap{\bitbox{#2}{\color{#1}\rule{\width}{\height}}}%
     \bitbox{#2}{#3}
   }
   

However, combining them both, prints the colored background too high.

\newlength{\bitboxmaxheight}
\setlength{\bitboxmaxheight}{\heightof{W}}
\newcommand{\baselinealign}[1]{%
  \centering
  \small
  \raisebox{0pt}[\bitboxmaxheight][0pt]{#1}%
}
\newcommand{\colorbitbox}[3]{%
  \rlap{\bitbox{#2}{\color{#1}\rule{\width}{\height}}}%
  \bitbox{#2}{#3}
}
\begin{bytefield}[boxformatting={\baselinealign}]{32}
  \bitheader{0,3,4,7,8,15,16,31} \\
  \bitbox{4}{Version} & \bitbox{4}{Type} & \colorbitbox{lightgray}{8}{Unused} & \bitbox{16}{Checksum} \\
  \bitbox{32}{Group Address}
\end{bytefield}

How can I solve this issue?

Answer 1

Instead of guessing the value of the amount of raising, measure it:

\documentclass{article}
\usepackage{xcolor,bytefield}

\newcommand{\baselinealign}[1]{%
  \centering
  \strut\small#1%
}
\newcommand{\colorbitbox}[3]{%
  \sbox0{\bitbox{#2}{#3}}%
  \makebox[0pt][l]{\textcolor{#1}{\rule[-\dp0]{\wd0}{\ht0}}}%
  \bitbox{#2}{#3}%
}

\begin{document}

\begin{bytefield}[boxformatting={\baselinealign}]{32}
  \bitheader{0,3,4,7,8,15,16,31} \\
  \bitbox{4}{Version} & \bitbox{4}{Type} & \colorbitbox{lightgray}{8}{Unused} & \bitbox{16}{Checksum} \\
  \bitbox{32}{Group Address}
\end{bytefield}

\end{document}

Instead of the trick with \heightof, I just insert a strut in normal size, before issuing \small.

Answer 2

After a bit of playing around, the following code seems to do the job. However, I would love to see a better way to use the \raisebox instead of determining the vertical "skip" by trial-and-error.

\newcommand{\colorbitbox}[3]{%
  \rlap{\bitbox{#2}{\raisebox{-1.3ex}{\color{#1}\rule{\width}{\height}}}}%
  \bitbox{#2}{#3}
}

Answer 3

Thanks to @egreg's answer, I was able to come up with the following:

\newcommand{\baselineboxformatting}[1]{
  % Measure size of contents
  \sbox0{\small#1}
  % Use the difference between the contents' height and the bitbox's height,
  % clamped to [-.3\baselineskip, 0], as our minimum depth.
  \setlength{\skip0}{\ht0 - \height}
  \ifdim\skip0>0pt
    \setlength{\skip0}{0}
  \else
    \ifdim\skip0<-.3\baselineskip
      \setlength{\skip0}{-.3\baselineskip}
    \fi
  \fi
  \centering\rule[\skip0]{0pt}{\baselineskip}\small#1
}

This avoids needing to mess with \colorbitbox to compensate; instead, the formatting itself should automatically handle anything too large to fit in with a full .3\baselineskip of depth, which extends to anything, not just \rule.