（Beamer）调暗方程块并在其上书写文本

Question

我想使幻灯片中的方程式块变暗，并在其上放置一个骷髅符号（以表明这些步骤是错误的）。我尝试使用alt，但效果不佳，相反，幻灯片上出现了单词“alt”和一些奇怪的符号。

\documentclass{beamer}
\mode<presentation>
{
  \setbeamertemplate{navigation symbols}{}
  \setbeamertemplate{caption}[numbered]
}
%%%%%%%%%



\begin{document}
\section{Proofs}         


\begin{frame}
\frametitle{Generalizing from one case to another}
Some stuff goes here: $g(x)= g_1(x) = g_2(x) \leq g_3(x)$

\pause
Analogous attempt: 
\begin{align*}
f(x) &= f_1(x)\\
        &= f_2(x)\\
        &\leq f_3(x)
\end{align*}
\pause
The above stuff is wrong, since the final inequality doesn't hold unless some conditions are met. 
\end{frame}

\end{document}

具体来说，我想在align幻灯片的最后一句话出现之后，使上面的块变暗，并在其上面放置一个骷髅。 （如果不可能出现骷髅，则显示“错误”的文字）也可以。

Answer 1

这是获得此类东西的一种方法。

\documentclass{beamer}
\mode<presentation>
{
  \setbeamertemplate{navigation symbols}{}
  \setbeamertemplate{caption}[numbered]
}
%%%%%%%%%
\usepackage{tikz}
\usetikzlibrary{fit,overlay-beamer-styles,tikzmark}
\usepackage{skull}


\begin{document}
\section{Proofs}         


\begin{frame}
\frametitle{Generalizing from one case to another}
Some stuff goes here: $g(x)= g_1(x) = g_2(x) \leq g_3(x)$

\pause
Analogous attempt: 
\begin{align*}
\tikzmarknode{f}{f}(x) &= f_1(x)\\
        &= f_2(x)\\
        &\leq \tikzmarknode{f3}{f_3(x)}
        \begin{tikzpicture}[overlay,remember picture]
        \only<3->{\node[fill=white,fill opacity=0.5,text opacity=0.8,
        fit=(f) (f3),align=center]      {~\\\scalebox{3}{$\skull$}};}
\end{tikzpicture}
\end{align*}
\pause
The above stuff is wrong, since the final inequality doesn't hold unless some conditions are met. 
\end{frame}
\end{document}