[Tex/LaTex] How to set up beamer to grey out past items but hide future items

beamerhighlightingitemizetransparency

I know about beamer's feature \setbeamercovered{invisible} and \setbeamercovered{transparent} to hide or grey out upcoming item's . But is there some way to have past items greyed (transparent) and future items hidden (invisible), so that only the current item is black/highlighted? And the best would be if on or 1 slide after the last all the item's could be visible highlighted. Something like this:

[grey] past 1
[grey] past 2
current
[hidden] future 1
[hidden] future 2
[hidden] future 3

Is there any easy way of doing this?

Best Answer

From the beamer manual, section 17.6 (Transparency Effects), use \setbeamercovered to set the default cover behavior, and override it for old items with the again covered option.

\documentclass{beamer}
\setbeamercovered{invisible}
\setbeamercovered{%
  again covered={\opaqueness<1->{15}}}
\begin{document}
\begin{frame}
\begin{itemize}[<+>]
\item Apple
\item Peach
\item Plum
\item Orange
\end{itemize}
\end{frame}
\end{document}

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[Tex/LaTex] Piecewise highlighting in beamer presentation

\documentclass{beamer}
\begin{document}
\frame{%
\setbeamercolor{normal text}{fg=gray,bg=}
\setbeamercolor{alerted text}{fg=black,bg=}
\usebeamercolor{normal text}
\begin{itemize}
\item \alert<+>{Hallo}
\item \alert<+>{Welt}
\item \alert<+>{Foobar}
\end{itemize}
}
\end{document}

[Tex/LaTex] Re-cover Beamer overlays, while keeping a subphrase uncovered

Edit 5

Following the OP's suggestion an alternative solution is to interrupt the covering and restart it (in contrast to locally uncovering the text). This could be done with the following macro:

\newcommand<>{\myinterruptcover}[1]{%
  \expandafter\ifx%
    \csname beamer@doafter\the\beamer@coveringdepth\endcsname\relax
  #1%
  \else
  \alt#2{\beamer@endcovered #1\beamer@startcovered}{#1}%
  \fi
}

The conditional checks if we currently are in a covered region, i.e. if \beamer@doafter<N> (see below) is defined.

I still prefer the \myuncover macro, because it just locally undoes the color change of the covering.

Edit 4

@Henry DeYoung asked for an explanation of how I came up with the macro. I added a rather longish one below.

Edit 3

After some more digging in the beamer sources I came up with the following, which also keeps the structure color (by using local color definitions):

\documentclass{beamer}

\makeatletter
\newcommand{\my@endcovered}{%
  % check if \beamer@doafterN is different from \relax
  % where N is the current covering level
  \expandafter\ifx%
    \csname beamer@doafter\the\beamer@coveringdepth\endcsname\relax
  \else
  % reset the color hook, but only locally
  % -- in the 'real' \beamer@doafter the global \xdef is used 
  %    instead of \edef
  \edef\beamer@colorhook{%
    \csname beamer@oldcolorhook\the\beamer@coveringdepth\endcsname}%
  % I think the following is not needed here, though I don't really
  % know what effect it has
  % \edef\beamer@pgfextension{%
  %  \csname beamer@oldpgfextension\the\beamer@coveringdepth\endcsname}%
  % finally, set colors
  \color{.}%
  \fi%
}%
\newcommand<>{\myuncover}[1]{\alt#2{{\my@endcovered #1}}{#1}}
\makeatother

\setbeamercovered{again covered={\opaqueness<1->{25}}}

\begin{document}
\begin{frame}{Frame Title}
  \begin{itemize}[<+>]
  \item \myuncover<.->{Who}'s on {\color{structure} first}
  \item \myuncover<.->{What}'s on {\color{structure} second}
  \item \myuncover<.->{I don't know}'s on {\color{structure} third}
  \end{itemize}
  \uncover<+>{-- Abbott and Costello}
\end{frame}
\end{document}

enter image description here

I think that solves all the problems.

Explanation

The beamer command \uncover (like all commands related to overlay features) is defined in beamerbaseoverlay.sty as

\newcommand{\uncover}{\alt{\beamer@fakeinvisible}{\beamer@makecovered}}

so that \beamer@makecovered is triggered when the overlay specification matches. That macro, in turn, is defined at the same place as

\long\def\beamer@makecovered#1{\beamer@startcovered#1\beamer@endcovered}

which is why I first suggested to locally use \beamer@endcovered to end the covering. However, \beamer@endcovered has some undesired global effects, so it is necessary to go one level deeper. Looking at the macro definition

\def\beamer@endcovered{%
  \beamer@smuggle{%
  \csname beamer@doafter\the\beamer@coveringdepth\endcsname%
  \global\advance\beamer@coveringdepth by -1\relax%
  }%
}%

we see that \beamer@endcovered basically expands to \beamer@doafter<N>, where <N> is the current value of the counter \beamer@coveringdepth, which keeps track of the covering level. Finally, that counter is decreased by 1. As a comment in the sources says, \beamer@smuggle is used to "smuggle skips past", and I decided to ignore this one for the moment (if anyone reads this who knows better, please correct me).

Now it is getting a bit more complicated: every \beamer@doafter<N> has a corresponding \beamer@do. Both macros are defined by \beamer@actions

\def\beamer@actions#1#2{%
  \gdef\beamer@do{#1%
    \expandafter\gdef\csname beamer@doafter%
    \the\beamer@coveringdepth\endcsname{#2}}}

which is executed in \beamer@startcovered. As you can see, actually the doafter macro is defined when the do macro is expanded. The relevant piece of code from \beamer@starcovered is in the definition of \opaqueness:

\def\opaqueness<##1>##2{%
\only<##1>{%
  \beamer@actions{%
    \expandafter\xdef\csname beamer@oldcolorhook%
    \the\beamer@coveringdepth\endcsname{\beamer@colorhook}%
    \expandafter\xdef\csname beamer@oldpgfextension%
    \the\beamer@coveringdepth\endcsname{\beamer@pgfextension}%
    {\globalcolorstrue\colorlet{beamer@freeze\the\beamer@coveringdepth}{bg}}%
    \xdef\beamer@colorhook{!##2!beamer@freeze%
      \the\beamer@coveringdepth\beamer@colorhook}%
    \gdef\beamer@pgfextension{!##2opaque}%
    \color{.}%
  }%
  {%
    \xdef\beamer@colorhook{\csname beamer@oldcolorhook%
      \the\beamer@coveringdepth\endcsname}%
    \xdef\beamer@pgfextension{\csname beamer@oldpgfextension%
      \the\beamer@coveringdepth\endcsname}%
    \color{.}%
  }}}%

Here, the \beamer@actions is called, if the overlay specification matches. The macros \beamer@do and \beamer@doafter<N> are then set to the arguments. Now it is obvious that the \beamer@do macro is responsible for setting the color of the covered object (which depends on the user settings). The previous color hook is saved to a macro named \beamer@oldcolorhook<N>, where <N> again is the covering level. Exactly this color hook is then restored when \beamer@doafter is expanded:

    \xdef\beamer@colorhook{\csname beamer@oldcolorhook%
      \the\beamer@coveringdepth\endcsname}%
    \xdef\beamer@pgfextension{\csname beamer@oldpgfextension%
      \the\beamer@coveringdepth\endcsname}%
    \color{.}%

and those are just the lines of code we were after. I simply replaced the global \xdef by the local \gdef to limit the scope of the color redefinition.

Finally just one bit is missing: as mentioned above this definition of \beamer@doafter is only true if the overlay specification (of \only) in \opaqueness is matched. If that is not the case, the macro does not do anything and is equal to \relax. So my macro checks if \beamer@doafter is equal to \relax in which case it also does nothing.

Edit 2

Now this should really yield the desired result:

\documentclass{beamer}
\setbeamercovered{again covered={\opaqueness<1->{25}}}
\makeatletter
\newcommand*{\myuncover}[1]{%
#1\only<.->{\makebox[0pt][r]{\beamer@endcovered #1}}%
}
\makeatother
\begin{document}
\begin{frame}{Frame Title}
  \begin{itemize}[<+>]
  \item \myuncover{Who}'s on first
  \item \myuncover{What}'s on second
  \item \myuncover{I don't know}'s on third
  \end{itemize}
  \uncover<+>{-- Abbott and Costello}
\end{frame}
\end{document}

enter image description here

Best Answer

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[Tex/LaTex] Piecewise highlighting in beamer presentation

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