How to determine head height automatically

Question

I'm creating three documents with three different headers, containing images or large banners. Is there a way to determine the head height automatically or even better a way to change the head height automatically with reference to the content?

I know LaTeX is telling me the head height is too small and I should change it to xx pt, but I would like to not have to do it manually.

A little excerpt of the beginning of one of my documents:

\documentclass[pdftex,12pt,oneside]{report}

\usepackage[pdftex]{graphicx}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{color}

\usepackage[paper = a4paper, tmargin = 2cm, bmargin = 2cm, lmargin = 2.5cm, rmargin = 2cm]{geometry}

\usepackage{fancyhdr}
\fancypagestyle{titlepage}
{
    \fancyhf{}
    \fancyhead[L]{~\\[0.5cm]\includegraphics[height=2cm]{./dhbw_logo}}
    \fancyhead[R]{~\\[0.5cm]\includegraphics[width=2cm]{./cz_logo}}
    \renewcommand{\headrulewidth}{0pt}
    \renewcommand{\footrulewidth}{0pt}
}

Answer 1

I do not think, it makes much sense to use an overlarge height of the titlepage's header for the whole document. Nonetheless the question is interesting in general.

Package fancyhdr already updates \headheight or \footskip, if the header or footer is too small. The following example stores the latest values of these dimension registers at the end of the document in an auxiliary file \jobname.heights. If the file exists, it is read in the preamble, before package geometry is loaded that might use the updated values for its calculations. Therefore the .aux file cannot be used for this purpose, because it is read a little too late (in \begin{document}).

\documentclass[12pt]{report}

\usepackage{atveryend}
\makeatletter
\AtVeryEndDocument{%
  \if@filesw % respect \nofiles
    \begingroup
      % same write register as environment `filecontents` uses
      \chardef\reserved@c=15 %
      \immediate\openout\reserved@c=\jobname.heights\relax
      \immediate\write\reserved@c{%
        \string\setlength{\string\headheight}{\the\headheight}%
      }%
      \immediate\write\reserved@c{%
        \string\setlength{\string\footskip}{\the\footskip}%
      }%
      \immediate\closeout\reserved@c
    \endgroup
  \fi
}
\makeatother
\InputIfFileExists{\jobname.heights}{}{}

\usepackage[
  includehead,
  includefoot,
  a6paper,
  landscape,
  showframe,
]{geometry}

\usepackage{fancyhdr}

\pagestyle{fancy}
\fancyhf{}
\chead{\Huge Header}
\cfoot{\Huge \begin{tabular}{c}Footer\\\thepage\end{tabular}}
\renewcommand{\headrulewidth}{0pt}
\renewcommand{\footrulewidth}{0pt}

\begin{document}
Hello World
\end{document}

First run

Package fancyhdr warns:

Package Fancyhdr Warning: \headheight is too small (12.0pt): 
 Make it at least 30.0pt.
 We now make it that large for the rest of the document.
 This may cause the page layout to be inconsistent, however.

Package Fancyhdr Warning: \footskip is too small (30.0pt): 
 Make it at least 50.99991pt.
 We now make it that large for the rest of the document.
 This may cause the page layout to be inconsistent, however.

And the contents of the page is displaced:

Second run

The previously written \jobname.heights is used:

\setlength{\headheight}{30.0pt}
\setlength{\footskip}{50.99991pt}

There are no warnings and the page is:

Workaround for package calc

Package fancyhdr has bugs, e.g. \global\setlength that does not work, if package calc is loaded. The following code patches the macro \@fancyvbox.

\makeatletter
\def\reserved@a#1#2{\setbox0\vbox{#2}\ifdim\ht0>#1\@fancywarning
  {\string#1 is too small (\the#1): ^^J Make it at least \the\ht0.^^J
    We now make it that large for the rest of the document.^^J
    This may cause the page layout to be inconsistent, however\@gobble}%
  \dimen0=#1\global\setlength{#1}{\ht0}\ht0=\dimen0\fi
  \box0}
\ifx\reserved@a\@fancyvbox
  \typeout{Patching \noexpand\@fancyvbox from package fancyhdr.}
  \def\@fancyvbox#1#2{%
    \setbox0\vbox{#2}%
    \ifdim\ht0>#1\relax
      \@fancywarning{%
        \string#1 is too small (\the#1): ^^J%
        Make it at least \the\ht0.^^J%
        We now make it that large for the rest of the document.^^J%
        This may cause the page layout to be inconsistent, however\@gobble%
      }%
      \dimen0=#1\relax
      \global#1=\ht0\relax
      \ht0=\dimen0 %
    \fi
    \box0 %
  }
\fi
\makeatother

The code should be inserted in the preamble after package fancyhdr is loaded.

Answer 2

Package fancyhdr already updates \headheight or \footskip

Not entirely true: for accentuated letters, fancyhdr ignore the accents and sets the heights to that of the selected fontsize.

Example:

\documentclass[a4paper, 12pt, twoside]{report}
\usepackage{fancyhdr}
  \pagestyle{fancy}
  \fancyhead[RO, LE] {}
\begin{document}
  \chapter{nothing, just setting a weird title: l\`ik\'e s\^o}
    \newpage
\end{document}

In this example, fancyhdr puts \headheight to the size of 12pt's \Large for the title of the chapter in the head. However, a title with accents is bigger than without, and LaTeX adjusts the head height at the first title with such accents, leaving the first chapters with a different head height.

This should be fancyhdr default setting, depending on language. Instead, one can correct fancyhdr's oversight, if there are chapters with accentuated letters, using calc's heightof. For french language, worst case scenario is circumflex accent, thus:

\setlength{\headheight}{\heightof{{\Large \^{E}}}}

fixes the headheight.