Vertically centering in Latex Beamer slightly too much up

Question

One MWE of many (w and w/o the plain frame option, w and w/o the vfill, w and w/o the c frame option etc.):

\documentclass[10pt]{beamer}
\setbeamertemplate{navigation symbols}{}
\setbeamertemplate{footline}{}
\begin{document}
\begin{frame}[c]{}
\vfill
\rule{\textwidth}{1pt}
\vfill
\end{frame}
\end{document}

This produces a horizontal line that is not exactly centered (using text instead does not work either). The ratio of the space above and below the line is approximately 4.5/5.5 in this case. Why?

PS: centering via TikZ as in question 208633 works fine.

Is \begin{frame}[c]{} \vskip0pt plus 1filll \rule{\linewidth}{1pt} \vskip0pt plus 1filll \end{frame} centred for you? — LaRiFaRi, Aug 26 '15 at 11:59
That improves the situation but moves the line a bit too much down, i.e. barely below the center. — stefanct, Aug 26 '15 at 12:21
One issue may be that \rule is on the baseline. Editing LaRiFaRi's solution to \rule[.5ex]{\linewidth}{1pt} looks slightly more centred. Even better is \begin{frame}[c]{}\null\vfill\null\rule[.5ex]{\textwidth}{1pt}\vfill\end{frame}, though I'm not sure as why it works. — campa, Aug 26 '15 at 13:47
Yes, the second one with the nulls seems to be perfectly centered. If someone could write a real answer (including an explanation why the nulls change the behavior) I'd be happy to mark it as the correct answer. — stefanct, Sep 12 '16 at 01:22
@samcarter Now I remember why I had written only a comment instead of an answer: It strongly depends on where I put the \nulls, and I have no idea why exactly. — campa, Feb 18 '17 at 15:20

score 3 · Accepted Answer · edited Apr 13 '17 at 12:35

Addition to the answer of campa:

The same result as campa's third frame can also be achieved by a symmetrical definition of the frametopskip and framebottomskip:

\documentclass[10pt]{beamer}

\setbeamertemplate{navigation symbols}{}
\setbeamertemplate{footline}{}

\begin{document}

% from https://tex.stackexchange.com/a/354503/36296
\begin{frame}[c]{} % this works
\null\vfill\null
ace\rule[.5ex]{.5\textwidth}{.5pt}
\vfill
\end{frame}

\makeatletter
\define@key{beamerframe}{c}[true]{% centered
  \beamer@frametopskip=0pt plus 1fill\relax%
% \beamer@framebottomskip=0pt plus 1.5fill\relax%
  \beamer@framebottomskip=0pt plus 1fill\relax%
}
\makeatother
\begin{frame}[c]
aba\rule[.5ex]{.5\textwidth}{.5pt}abc
\end{frame}

\end{document}

score 2 · Answer 2 · edited Feb 18 '17 at 15:43

2

This is only a half-cocked answer, as I do not know exactly the behaviour behind it. One (minor) issue is that \rule gives a line on the baseline, though this is not the real problem. Consider the following example

\documentclass[10pt]{beamer}

\setbeamertemplate{navigation symbols}{}
\setbeamertemplate{footline}{}

\begin{document}

\begin{frame}[c]{} % the problem
\vfill
ace\rule[.5ex]{.5\textwidth}{.5pt}
\vfill
\end{frame}

\begin{frame}[c]{} % slightly better
\null\vfill
ace\rule[.5ex]{.5\textwidth}{.5pt}
\vfill
\end{frame}

\begin{frame}[c]{} % this works
\null\vfill\null
ace\rule[.5ex]{.5\textwidth}{.5pt}
\vfill
\end{frame}

\end{document}

The first frame is the not centered one. From experience I knew that just calling \vfill somewhere may be give unexpected results (TeX modes) so I put en empty box at the beginning of the frame. This gives the second frame in the example above, which is slightly better but not yet really centered. To get what we want I had to put another empty box after the \vfill. And I must admit I'm quite befuddled as to why...

edited Feb 18 '17 at 15:43

samcarter_is_at_topanswers.xyz

158,329

answered Feb 18 '17 at 15:39

campa

31,130

Sorry it took so long... and thanks for adding the pictures. Do you by any means have any idea about what is going on? I'm going through the beamer code in the hope to find how the centred frames are handled. – campa Feb 18 '17 at 15:50
I guess the "problem" is that the frametopskip for centred frames is more stretchy? But that's just a guess, I did not look in the code. – samcarter_is_at_topanswers.xyz Feb 18 '17 at 15:55
For a centred frame: \beamer@frametopskip=0pt plus 1fill\relax% and \beamer@framebottomskip=0pt plus 1.5fill\relax%. Now if ones adjusts the bottomskip to, say, 0pt plus 1.1fill, the rule looks to be centred. – samcarter_is_at_topanswers.xyz Feb 18 '17 at 16:04
And with a symmetrical definition of 1fill in both, I get the same results as your third frame. – samcarter_is_at_topanswers.xyz Feb 18 '17 at 16:08

Vertically centering in Latex Beamer slightly too much up

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