# Transparency does not work correctly with alignment?

Read chapter 23.4 of the beamer guide ("23.4 Uncovering Tagged Formulas Piecewise").

Notice: this is not an RTFM answer, this is easy to miss.

Edit: as egreg noticed, the above "answer" does not actually answer the question...

Now this is ugly, but works:

\documentclass{beamer}
\setbeamercovered{transparent}

\usepackage{mathtools}

\begin{document}

\begin{frame}
\begin{gather*}
\uncover<1->{a = b \wedge b = c \\}
\uncover<2->{\mathrlap{\Rightarrow a = c}\hphantom{a = b \wedge b = c}\\}
\end{gather*}
\end{frame}

\end{document}


(It does at the expense of manually specifying the longest line, and doing it twice. A hackery, non-LaTeX-style, ugly "solution", but hey, the pdf looks good.)

Also, one could imagine semi-automating it to something like this:

\documentclass{beamer}
\setbeamercovered{transparent}

\usepackage{mathtools}

\newlength{\longestalignlinelength}
\newsavebox{\longestlinebox}
\newcommand{\longest}[1]{\sbox{\longestlinebox}{$\displaystyle #1$}\longestalignlinelength=\wd\longestlinebox}
\newcommand{\putline}[1]{\mathrlap{#1}\hspace{\longestalignlinelength}}

\begin{document}

\begin{frame}
\longest{a = b \wedge b = c}
\begin{gather*}
\uncover<1->{\putline{a = b \wedge b = c}\\}
\uncover<2->{\putline{\Rightarrow a = c}\\}
\end{gather*}
\end{frame}

\end{document}


Still not very elegant, but a bit better.