hf-tikz in the background

Question

I use hf-tikz quite often in my beamer presentations. One thing that I find quite annoying each time that I use it is that, if the rectangle providing the highlighting happens to be over some text preceding the one enclosed by the \tizmarkin and \tikzmarkend commands, such text is hidden.

See, for instance, this MWE

\documentclass{beamer}
\usepackage{tikz}
\usepackage[beamer]{hf-tikz}    
\begin{document}
    \begin{frame}
        \begin{align*}
            \mathbf{\hat{f}}(\tikzmarkin<2>[above left offset={-1.1,0.35},below right offset={1.0,-1.35}]{myhl}t+R\tikzmarkend{myhl}) &=  a + b + c
        \end{align*}
    \end{frame}
\end{document}

yielding this (the misalignment is due to bad screenshots)

Is there a way to avoid this? Or (as I fear) is it part of the way hf-tikz works and cannot, thus, be somehow bypassed?

Answer 1

The hf-tikz is almost ten years old and doesn't offer much what you couldn't just do with tikzmark directly which is basically what I've done here but with the original interface of hf-tikz.

This is: mark the begin and the end of the to highlighted part but \tikzmarkdraw it as early as possible so that it gets drawn before \mathbf{\hat{f}} gets put onto the page but it needs to be on the same page. With beamer this is very easy because you know where the pagebreaks are.

You could just put all \tikzmarkdraws at the start of the frame.

Code

\documentclass{beamer}
\usepackage{tikz}
\usepackage[beamer]{hf-tikz}
\NewDocumentCommand{\tikzmarkbegin}{m}{\tikzmarkend{begin-#1}}
\NewDocumentCommand{\tikzmarkdraw}{D<>{.-} O{} m}{%
  \only<#1>{%
    \tikz[remember picture,overlay]\draw[hf/rect,#2]
        ([shift={(\aboveleftoff)}]pic cs:begin-#3) rectangle
%        node [anchor=base] (node-#3) {}
        ([shift={(\belowrightoff)}]pic cs:#3);%
  }%
}
\tikzset{hf/rect/.style={line width=+1pt, fill=\fcol, draw=\bcol, rounded corners}}
\begin{document}
\begin{frame}
\begin{align*}
\tikzmarkdraw<2>[above left offset={-1.1,0.35},below right offset={1.0,-1.35}]{myhl}
\mathbf{\hat{f}}(\tikzmarkbegin{myhl} t + R\tikzmarkend{myhl}) &=  a + b + c
\end{align*}
\end{frame}
\end{document}

Output

Answer 2

It is not necessary to move the hf-tikz annotation to the background. You can change the blend mode instead:

\documentclass{beamer}
\usepackage{tikz}
\usepackage[beamer]{hf-tikz}    
\begin{document}
    \begin{frame}
        \begin{align*}
            \mathbf{\hat{f}}(\tikzmarkin<2>[above left offset={-1.1,0.35},below right offset={1.0,-1.35},blend mode=multiply]{myhl}t+R\tikzmarkend{myhl}) &=  a + b + c
        \end{align*}
    \end{frame}
\end{document}

Answer 3

As you are showing the box only on the second overlay, you could use the tikzmark library and draw the background before the equation:

\documentclass{beamer}
\setbeamertemplate{background canvas}{\includegraphics[scale=2]{example-image-duck}}
\usepackage{tikz}
\usetikzlibrary{tikzmark,fit}
\begin{document}
\begin{frame}
  \begin{tikzpicture}[overlay, remember picture]
  \node<2> [draw=pink!50!violet,thick,fill=orange!30!white, rounded corners, inner xsep=7pt, fit=(myhl)] {};
  \end{tikzpicture}
    \begin{align}
        \mathbf{\hat{f}}(\tikzmarknode{myhl}{t+R}) &=  a + b + c
    \end{align}
    \pause
\end{frame}
\end{document}

If you would like to show it on all overlays, you could add it to the background layer:

\documentclass{beamer}
\setbeamertemplate{background canvas}{\includegraphics[scale=2]{example-image-duck}}
\usepackage{tikz}
\usetikzlibrary{tikzmark,fit}
\begin{document}
{
\addtobeamertemplate{background canvas}{}{  \begin{tikzpicture}[overlay, remember picture]
  \node [draw=pink!50!violet,thick,fill=orange!30!white, rounded corners, inner xsep=7pt, fit=(myhl)] {};
  \end{tikzpicture}
}
\begin{frame}
    \begin{align}
        \mathbf{\hat{f}}(\tikzmarknode{myhl}{t+R}) &=  a + b + c
    \end{align}
\end{frame}
}
\end{document}