How to have a rectangular spotlight using TikZ in beamer?

Question

I am trying to change the shape of the highlighted area in the MWE below (based on this @BambOo's solution).

\documentclass{beamer}
\usepackage{tikz}
\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}

Neither

\begin{tikzpicture}[remember picture,overlay,use page relative coordinates]
    \fill[opacity=0.5,black] (0,0) rectangle (1,1) (0.15,0.15) rectangle (0.2,0.2);
    \end{tikzpicture}

nor

\begin{tikzpicture}[remember picture,overlay,use page relative coordinates]
    \fill[opacity=0.5,black] (0,0) rectangle (1,1); 
    \fill[opacity=0,black] (0.15,0.15) rectangle (0.2,0.2);
    \end{tikzpicture}

does the job.

How can I get it working?

Answer 1

If you add even odd rule option, it works. Why? I don't know, it seems that combining a rectangle with a circle doesn't need to explicitely fix the rule

\documentclass{beamer}
\usepackage{tikz}
\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]
\filldraw[opacity=0.5, black, even odd rule] (0,0) rectangle (1,1) (0.2,0.2) rectangle (0.38,0.6);
\end{tikzpicture}

\end{frame}
\end{document}