Absolute positioning in Beamer, with overlays, without TikZ

Question

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Update: I have written a package to handle the issue: abspos.

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There are many similar questions to this here already (e.g., How can I make textpos respect beamer overlays? and beamer: how to use \pause with textblock* environment, and lots of questions on absolute positioning more generally), so let me try to clarify what I'm looking for, more specifically:

1. I'd like to to place content at an absolute position, and this content may contain tikzpictures, etc., so I would rather not place it in a TikZ node (to avoid having nested TikZ pictures – and, for that matter, to avoid an extra compilation round). I may use coffins, etc., to do much of the placement, but I need some absolute starting-point (which I guess coffins don't supply?)

2. I would like to be able to use beamer overlay commands like \pause, etc., without restrictions. In particular, I would like to avoid the need for wrapping their arguments in braces (which does solve the problem in some scenarios) – partly because I'll be using pseudo.sty, which has rather straightforward support for inserting \pause commands between lines. (Similar things hold for itemize and enumerate, with <+->.)

3. I would rather not require transparent frame backgrounds.

I don't think any of the previous answers address these two requirements (though if there is a solution here already, that would be great – if so, sorry for the noise).

The obvious solution is to use textpos, but as has been pointed out previously (e.g., in the two questions referenced above), it doesn't play nice with \pause and the like.

A simple MWE illustrates the problem:

\documentclass{beamer}
\usepackage[absolute,overlay]{textpos}
\begin{document}
\begin{frame}
\begin{textblock}{0pt}(0pt,0pt)
A\pause B\pause C
\end{textblock}
\end{frame}
\end{document}

This will give you three slides; the first says "BC", the second says "C" and the last says "ABC".

The problem is with the overlay option. If this is removed, and we use a transparent background, things work, so this is technically a solution, but as mentioned, I would rather not require this in my solution.

\documentclass{beamer}
\usepackage[absolute]{textpos}
\setbeamercolor{background canvas}{bg=}
\begin{document}
\begin{frame}
\begin{textblock}{0pt}(0pt,0pt)
A\pause B\pause C
\onslide<1->
\end{textblock}
\end{frame}
\end{document}

(Here I've also added \onslide<1-> so the rest of whatever is on the page, such as page numbers or whatever, isn't paused.)

This is almost what I want, except I would like the option of having (or letting others have) colored backgrounds. (I could of course draw those myself, from within the absolutely positioned box, but…)

So … any ideas of emulating or fixing the overlay option, or some other way of handling the issue of transparent backgrounds – or perhaps even some entirely different mechanism for absolute positioning, for that matter? (I tried leaving a bug report in the textpos repo, but that repo has since been deleted…)

Answer 1

I think maybe I have a solution, after all. It seems that simply using atbegshi with xcoffins (or l3coffins) is sufficient, and does the job correctly. In my code, I work with two coffins: One that is placed by atbegshi, at an absolute position, and another that is used to add contents incrementally to the first, possibly scaled, rotated, etc., using \ScaleCoffin, \RotateCoffin, etc., with configurable offsets and whatnot.

To just illustrate the basic functionality, however, here's a simple example using just a single coffin, placed at the center of the frame.

\documentclass{beamer}
\beamertemplatenavigationsymbolsempty
% xcoffins may be useful, beyond l3coffins, if you need \TotalHeight, etc.
\usepackage{xcoffins}
%\usepackage{atbegshi} % imported by beamer
\NewCoffin \CenteredCoffin
% A couple of global commands that aren't available in xcoffins:
\ExplSyntaxOn
\NewDocumentCommand \GlobalSetHorizontalCoffin { m +m } {
    \hcoffin_gset:Nn{#1}{#2}
}
\NewDocumentCommand \GlobalClearCoffin { m } {
    \coffin_gclear:N{#1}
}
\ExplSyntaxOff
% Place the coffin at a given offset from the upper left of the shipout
% box, and then clear its contents (ready for its next definition/use):
\AtBeginShipout{%
    \AtBeginShipoutUpperLeftForeground{
        \TypesetCoffin\CenteredCoffinhc,vc
        \GlobalClearCoffin\CenteredCoffin
    }
}
\begin{document}
\begin{frame}
\GlobalSetHorizontalCoffin \CenteredCoffin {%
    \scalebox{10}{1 \pause 2 \pause 3}%
    \onslide<1->% Reset things, so the following content isn't paused
}
\color{black!10}
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\end{frame}
\end{document}

The three resulting slides are as follows: