beamer: using a spotlight to highlight a part of an image

Question

Using beamer , I would like to highlight a part of an image using a spotlight (circular, or rectangular) as shown below:

How can I achieve that in beamer?

Answer 1

You could reuse a variation of my answer here based on Jake's solution there.

\documentclass{beamer}
\usepackage{tikz}

% From https://tex.stackexchange.com/a/445311/141947
\tikzset{
    use page relative coordinates/.style={
        shift={(current page.south west)},
        x={(current page.south east)},
        y={(current page.north west)}
    },
}

\begin{document}

\begin{frame}{title}{subtitle}
    \begin{block}{Block title}
        Some content
    \end{block}
    \includegraphics[width=0.5\textwidth]{example-image-a}
    \begin{tikzpicture}[remember picture,overlay,use page relative coordinates]
        \fill[opacity=0.5,black] (0,0) rectangle (1,1) (0.25,0.5) circle (2cm);
    \end{tikzpicture}
\end{frame}

\end{document}