Beamer: Uncover underbrace

Question

I've got an underbrace (\underbrace{foo}_{bar}) which I want to uncover (i.e. foo should be there from the beginning, while the underbrace and the bar should be uncovered on the next slide. I've tried the following, which (not surprisingly) doesn't work:

\documentclass{beamer}

\begin{document}
\begin{frame}
  Without uncover (this is what it should look like on the second subframe):
  \begin{displaymath}
    \underbrace{foo}_{bar}
  \end{displaymath}
  With uncover (doesn't work):
  \begin{displaymath}
    \uncover<2>{\underbrace}{foo}\uncover<2>{_{bar}}
  \end{displaymath}
\end{frame}
\end{document}

In the second displaymath, the underbrace is displayed before foo and the bar as index to foo, and moreover the underbrace is immediately visible.

Note that \only wouldn't be a solution because on one hand in my actual document I use \setbeamercovered{transparent} so the underbrace and the text below should be lightly visible from the beginning, and on the other hand \only would also not keep the space for the underbrace.

So how can I achieve that?

Answer 1

One way to do that, is to use the definition of the transparency (15% Text foreground on the background and to avoid the “jumping” use as suggested here the overprint environment.

Though you have to define your term that's \underbraced twice and in my quick hack the transparency color is the one mentioned above, because i didn't find whether i can somehow get that by \usebeamercolor

\documentclass[transparent]{beamer}
\usepackage{amsmath}
\usepackage{cancel}
\begin{document}
    \setbeamercovered{transparent}
    \begin{frame}%
        \begin{overprint}
            \onslide<1>\begin{displaymath}
                \color{normal text.fg!15!normal text.bg}
                \underbrace{\usebeamercolor[fg]{text}foo}_{bar}
            \end{displaymath}
            \onslide<2>\begin{displaymath}
                \underbrace{foo}_{bar}
            \end{displaymath}
        \end{overprint}
    \end{frame}%
\end{document}

And (thanks to @Andrew Stacey) the even shorter version just switching colors using \only (because \color is sensitive to that) would be

\begin{frame}
    \begin{displaymath}
        \color{normal text.fg!15!normal text.bg}
        \only<2->{\color{normal text.fg}}
        \underbrace{\usebeamercolor[fg]{text}foo}_{bar}
    \end{displaymath}
\end{frame}

Answer 2

Here's a solution that just uses \onslide (so that there's no need to know the precise definition of transparency):

\documentclass{beamer}
\setbeamercovered{transparent}
\begin{document}
\begin{frame}
    \begin{displaymath}
        \onslide<2-> \underbrace{ \onslide<1->
        foo
        \onslide<2-> }_{bar} \onslide<1->
        \text{some more stuff for slide 1}
    \end{displaymath}
\end{frame}
\end{document}

Answer 3

Based on Hendrik Vogt's answer, and for my needs, I created these commands to make my life easier:

\newcommand<>{\uncoverubrace}[2]{%
  \onslide#3 \underbrace{ \onslide<1->%
  #1%
  \onslide#3 }_{#2} \onslide<1->%
}
\newcommand<>{\uncoverobrace}[2]{%
  \onslide#3 \overbrace{ \onslide<1->%
  #1%
  \onslide#3 }^{#2} \onslide<1->%
}

Now the use is very simple:

\usepackage{mathtools}
...
\begin{displaymath}
\uncoverubrace<2->{a_i}{\mathclap{\text{$N$ elements}}} =
\frac{
  \uncoverobrace<3->{b_j-b_i}{\mathclap{\text{$2N$ items needed}}}
}{2h}
\end{displaymath}

Room for improvement: allow relative overlay specifications (<+->).

Answer 4

Based on Jellby's answer, I was not only able to simplify the code, but also patch the original underbrace using the xparse package such that it works with arbitrary <overlay-specification>.

\documentclass{beamer}
\setbeamercovered{transparent}
\usepackage{amsmath, xparse, letltxmacro}
\LetLtxMacro{\oldunderbrace}{\underbrace}

\DeclareDocumentCommand{\underbrace}{d<> m e{_}}{%
  \IfValueTF{#1}{% IF <overlay-specification> given
    % using global onlside flag, cf. p82 beamer manual v3.59
    \oldunderbrace{#2\onslide<#1>}_{#3}\onslide%
  }{% ELSE
    \oldunderbrace{#2}_{#3}
  }%
}%

\begin{document}

\begin{frame}{\alt<1,3>{LINKS}{ZWO}}
\begin{block}<+->{}
\begin{align*}
\underbrace<+->{\text{\color{green} LINKS!}}_{\color{red} \text{ZWO!}}
\underbrace<+->{\text{\color{green} LINKS!}}_{\color{red} \text{DREI!}}
\underbrace<+->{\text{\color{green} LINKS!}}_{\color{red} \text{VIER!}}
\end{align*}
\end{block}
\end{frame}

\end{document}

I guess if one wanted one could even built-in an \@ifclassloaded{beamer} in order to automagically ignore the <> when other classes are loaded.

For \overbrace it is a bit different, see my other post here: Beamer: Uncover underbrace & overbrace weird behaviour

Answer 5

Ok, from the proposition of Hyperplane, here is the solution for \overbrace. As explained at Beamer: Uncover underbrace & overbrace weird behaviour \overbrace behaves differently from \underbrace, the code had thus to be adapted a bit.

\LetLtxMacro{\oldoverbrace}{\overbrace}
 \DeclareDocumentCommand{\overbrace}{d<> m e{^}}{%
   \IfValueTF{#1}{% IF <overlay-specification> given
     % using global onlside flag, cf. p82 beamer manual v3.59
     \onslide<#1>\oldoverbrace{\onslide #2}^{#3}\onslide%
   }{% ELSE
     \oldoverbrace{#2}^{#3}
   }%
}%

Answer 6

This code is a litte bit simpler, so it might be worth sharing. The idea is to combine alt with vphantom. This is how the command is defined:

\newcommmand<>{\underbraceonslide}{\alt#3{\underbrace{#1}{#2}}{\vphantom{\underbrace{#1}{#2}}#1}}

It uses beamer's overlay specifications, e.g.

\underbraceonslide<2->{ \frac{1}{n} }{\to 0}