pgfpages add page number

Question

I'm creating a handout for my beamer presentation using the 4 on 1 layout for pgfpages. I'd like each physical page (which contains four "pages" of the handout) to have a page number bottom centre of the page. Adding \pagestyle{plain} puts a page number on every logical page (thus, four page numbers per physical page).

Is there a simple solution to this?

It hardly seems worth providing an MWE, but here goes:

\documentclass{beamer}
\usepackage{pgfpages}
\pgfuselayout{4 on 1}[landscape]
\begin{document}
\frame{foo}
\frame{bar}
\frame{baz}
\frame{bazbarfoo}
\end{document}

I'd like a page number bottom centre of the page.

Answer 1

Here is a solution .\rule[-.5cm]{0pt}{0pt} to add some extra vertical space below the page number.

\documentclass{beamer}
\usepackage{pgfpages}
\pgfpagesuselayout{4 on 1}[a4paper,landscape]
\makeatletter \def\pgfsys@endpicture{%
\makebox[\pgfphysicalwidth]{\the\numexpr\value{page}/4\relax}\rule[-.5cm]{0pt}{0pt}}
\makeatother
\begin{document}
\frame{foo}
\frame{bar}
\frame{baz}
\frame{bazbarfoo}
\end{document}

Answer 2

The problem with redefining \pgfsys@endpicture is that this is called at the end of each PGF graphic you use (including the six tiny navigation symbols on each beamer slide). It is also used to hold special instructions when you use certain drivers; so if you use XeLaTeX you will find the slides are not positioned correctly on the second and later pages. Best to leave that alone.

To get the fewest nasty surprises, you need to inject your code for writing the page numbers directly into the pgfpages output routine. The \patchcmd command from etoolbox works for this.

While using a \makebox directly works, it also takes up vertical space, so the tops of your slides on the first row may get pushed off the page. The code below uses the same technique as pgfpages for positioning the number on the page without taking up room. To move it around, change the distances in the \pgfpoint (measuring from the bottom left corner):

\documentclass{beamer}
\usepackage{etoolbox,pgfpages}
\pgfpagesuselayout{4 on 1}[a4paper,landscape]
\newcounter{physicalpage}
\makeatletter
\patchcmd{\pgfpages@buildshipoutbox}{%
  \pgfsys@endpicture
}{%
  \stepcounter{physicalpage}%
  \setbox0\vbox{\makebox[0pt][c]{\arabic{physicalpage}}}%
  \pgfsys@beginscope
  \pgflowlevel{\pgftransformshift{\pgfpoint{.5\pgfphysicalwidth}{5mm}}}%
  \wd0=0pt%
  \dp0=-\ht0%
  \pgfsys@hbox0%
  \pgfsys@endscope
  \pgfsys@endpicture
}{}{}
\makeatother
\begin{document}
\frame{foo}
\frame{bar}
\frame{baz}
\frame{bazbarfoo}
\end{document}