Beamer - `uncover` within `only`

Question

I've nest an uncover inside an only and it's not working as expected. The header of my document is :

\documentclass[8pt]{beamer}

\mode<presentation>{\usetheme{Warsaw}}
\setbeamertemplate{navigation symbols}{}

\usepackage{empheq}
\usepackage{tikz}
\usetikzlibrary{shapes,arrows,backgrounds}

And the frame in question is the following :

\begin{frame}[label=review]
\frametitle{A review of the steps}
\setbeamercovered{transparent}
\begin{columns}
\column{0.4\linewidth}
\begin{enumerate}[<+->]
\item \alert<1>{Supernova detection.}
\item \alert<2>{Spectroscopic confirmation.}
\item \alert<3,6>{Multi-band photometry.}
\item \alert<4,6>{Parameter extraction.}
\item \alert<5,6>{Hubble diagram fit.}
\end{enumerate}
\column{0.7\linewidth}
\begin{center}
  \begin{tikzpicture}%[show background grid]
    \node[anchor=south west,inner sep=0] (step1) at (0,0) {\includegraphics[width=0.18\linewidth]{figures/pedagogy/03D4ag-zoom1.png}};
    \begin{scope}[x={(step1.south east)},y={(step1.north west)}]
      \draw<1>[red] (0,0) rectangle (1,1);
    \end{scope}
  \end{tikzpicture}
  \begin{tikzpicture}%[show background grid]
    \node[anchor=south west,inner sep=0] (step2) at (0,0) {\includegraphics[width=0.18\linewidth]{figures/guypres/1994D_spectrum.pdf}};
    \begin{scope}[x={(step2.south east)},y={(step2.north west)}]
      \draw<2>[red] (0,0) rectangle (1,1);
    \end{scope}
  \end{tikzpicture}
  \begin{tikzpicture}%[show background grid]
    \node[anchor=south west,inner sep=0] (step3) at (0,0) {\includegraphics[width=0.18\linewidth]{figures/03D4ag-lc.png}};
    \begin{scope}[x={(step3.south east)},y={(step3.north west)}]
      \draw<3>[red] (0,0) rectangle (1,1);
    \end{scope}
  \end{tikzpicture}
  \begin{tikzpicture}%[show background grid]
    \node[anchor=south west,inner sep=0] (step4) at (0,0) {\includegraphics[width=0.18\linewidth]{figures/03D4ag-lc-model.png}};
    \begin{scope}[x={(step4.south east)},y={(step4.north west)}]
      \draw<4>[red] (0,0) rectangle (1,1);
    \end{scope}
  \end{tikzpicture}
  \begin{tikzpicture}%[show background grid]
    \node[anchor=south west,inner sep=0] (step5) at (0,0) {\includegraphics[width=0.18\linewidth]{figures/pedagogy/hubblediagram.png}};
    \begin{scope}[x={(step5.south east)},y={(step5.north west)}]
      \draw<5>[red] (0,0) rectangle (1,1);
    \end{scope}
  \end{tikzpicture}
\end{center}
\end{columns}

\begin{overlayarea}{\textwidth}{\textheight}
\only<1>{
  \begin{center}
    \includegraphics[width=0.4\textwidth]{figures/pedagogy/03D4ag-zoom1.png}
    \hspace{0.2cm}
    \includegraphics[width=0.4\textwidth]{figures/pedagogy/03D4ag-zoom3.png}
  \end{center}
}
\only<2>{
  \begin{center}
    \includegraphics[width=0.7\textwidth]{figures/guypres/1994D_spectrum.pdf}
  \end{center}
}
\only<3-4>{
  \begin{center}
    \includegraphics[width=0.4\textwidth]{figures/03D4ag-lc.png}
    \hspace{0.2cm}
    \uncover<4>{
      \includegraphics[width=0.4\textwidth]{figures/03D4ag-lc-model.png}
    }
  \end{center}
}
\only<5>{
  \begin{center}
    \includegraphics[width=0.7\textwidth]{figures/pedagogy/hubblediagram.png}
  \end{center}
}
\end{overlayarea}

\end{frame}

The behavior I'm expecting is that 03D4ag-lc.png will be displayed on slide 3, and 03D4ag-lc-model.png will appear on slide 4, without displacing 03D4ag-lc.png. Instead, 03D4ag-lc-model.png seems to completely ignore the uncover command, and is present on both slides. Is this normal ? If so what is the proper way of accomplishing what I'm aiming for here ?

Answer 1

In your code you are setting the covered effect to transparent via the \setbeamercovered{transparent} macro.

As @JosephWright notes, the transparency does not work for imported images, so you got no covered effect at all when you tried to uncover an imported image with:

\uncover<4>{
  \includegraphics[width=0.4\textwidth]{figures/03D4ag-lc-model.png}
}

To hide images you need to re-set the transparency effect using

\setbeamercovered{invisible}

I suggested to put the spec inside your overlayarea so that it does not override the first setting in the rest of the frame.

You may also want to chack section 17.6 of the Beamer manual.