Background coloring with overlay specification in algorithm2e + beamer package

Question

I want to highlight some portion of my pseudo codes in my slides during the course of presentation. It's easy in powerpoint and I've made a sample slide:

However, I couldn't manage to get a similar effect with beamer, tikz and algorithm2e package.

So far, I've defined the following command.

\newcommand<>{\myspot}[1]
{
\begin{tikzpicture}
        \node[draw=none,inner sep=0cm] (n1)
        {
            #1
        };
    \uncover#2{
        \draw (n1) node[rectangle,inner sep=0cm, fill=yellow] (n2)
        {
            #1
        };
    }
    \end{tikzpicture}
}

It is very useful, but can only highlight a single line in my code.

\myspot<2>{
$pseudo_code here$
}

How can I make a similar effect shown in the sample slide by using beamer?

Answer 1

Use \tikzmark to place auxiliary nodes and the fit library, together with the remember picture, overlay options to place the shadings. A little example:

\documentclass{beamer}
\usepackage[ruled,vlined,linesnumbered]{algorithm2e}
\usepackage{tikz}
\usetikzlibrary{fit,calc}

\newcommand{\tikzmark}[1]{%
  \tikz[overlay,remember picture,baseline] \node [anchor=base] (#1) {};}

\definecolor{mybrown}{RGB}{255,218,195}
\definecolor{myframe}{RGB}{197,122,195}

\begin{document}

\begin{frame}

\begin{columns}
\column{.4\textwidth}
\onslide<2->{
\setlength\fboxsep{6pt}%
\framebox{\begin{tikzpicture}[x=0.8cm,init/.style={circle,fill=green!60!black,inner sep=2pt},
tran/.style={circle,fill=magenta!60!black,inner sep=3pt}]
\node<1,2,3>[init] at (0,0) (p) {};
\node<4>[tran] at (0,0) (p) {};
\node[init] at (1,-1) (q) {};
\node[init] at (3,-1) (r) {};
\node<1,2,4>[init] at (2.5,-2) (s) {};
\node<3>[tran] at (2.5,-2) (s) {};
\node[init] at (3.5,-3) (t) {};
\draw[blue] (p) -- (q) -- (r)  -- (s);
\draw[blue] (r) -- (t);
\end{tikzpicture}}
}

\column{.6\textwidth}
\begin{algorithm}[H]
\SetKwData{Left}{left}\SetKwData{This}{this}\SetKwData{Up}{up}
\SetKwFunction{Union}{Union}\SetKwFunction{FindCompress}{Find}
\SetKwInOut{Input}{input}\SetKwInOut{Output}{output}

\For{$i\leftarrow 2$ \KwTo $l$}
{
\emph{special treatment of $i$}\;
\For{$j\leftarrow 2$ \KwTo $w$}{\label{forins}
\Left$\leftarrow$ \FindCompress{$Im[i,j-1]$}\;
\tikzmark{a}\If(\tcp*[h]{}){\Left compatible \This}{\label{lt}
\lIf{\Left $<3$}{\Union{\Left}}\;
\lElse{\Union{\This,\Left}\tikzmark{b}\;}
}
\tikzmark{c}\If(\tcp*[h]{}){\Left compatible \This}{\label{lt1}
\lIf{\Left $<3$}{\Union{\Left}}\;
\lElse{\Union{\This,\Left}\tikzmark{d}\;}
}
}
\lForEach{element $e$ of the line $i$}{\FindCompress{p}}
}
\caption{disjoint decomposition}\label{algo_disjdecomp}
\end{algorithm}
\end{columns}

\begin{tikzpicture}[remember picture,overlay]
\coordinate (aa) at ($(a)+(0,0.2)$);
\coordinate (bb) at ($(b)+(1.2,0)$);
\node<3>[draw=myframe,line width=1pt,fill=mybrown,opacity=0.4,rectangle,rounded corners,fit=(aa) (bb)] {};
\coordinate (cc) at ($(c)+(0,0.2)$);
\coordinate (dd) at ($(d)+(1.2,0)$);
\node<4>[draw=myframe,line width=1pt,fill=mybrown,opacity=0.4,rectangle,rounded corners,fit=(cc) (dd)] {};
\end{tikzpicture}
\end{frame}

\end{document}

The four resulting slides:

---

Added by Andrew Stacey

Here's a version using the slightly improved tikzmark which makes it possible to put the boxes behind the text. While I was at it, I also added in overlay-aware styles for those two nodes, and non-jumping pictures because the small jump when the node size changes annoyed me (you need to compile the code to see the effect of this, try commenting out the stop jumping key on the tikzpicture environment). I found that \resetcounteronoverlays{algocf} did work - unless I missed something. On the plus side, I got rid of the dependency on the fit and calc libraries. Here's the full code:

\documentclass{beamer}
% \url{https://tex.stackexchange.com/q/51582/86}
%
% Using code from:
%  https://tex.stackexchange.com/a/6155/86 for overlay-aware styles
%  https://tex.stackexchange.com/a/50054/86 for advanced tikzmark
%  https://tex.stackexchange.com/q/18704/86 for the non-jumping pictures
\usepackage[ruled,vlined,linesnumbered]{algorithm2e}
\usepackage{tikz}
\makeatletter

\newcounter{jumping}
\resetcounteronoverlays{jumping}

\def\jump@setbb#1#2#3{%
  \@ifundefined{jump@#1@maxbb}{%
    \expandafter\gdef\csname jump@#1@maxbb\endcsname{#3}%
  }{%
    \csname jump@#1@maxbb\endcsname
    \pgf@xa=\pgf@x
    \pgf@ya=\pgf@y
    #3
    \pgfmathsetlength\pgf@x{max(\pgf@x,\pgf@xa)}%
    \pgfmathsetlength\pgf@y{max(\pgf@y,\pgf@ya)}%
    \expandafter\xdef\csname jump@#1@maxbb\endcsname{\noexpand\pgfpoint{\the\pgf@x}{\the\pgf@y}}%
  }
  \@ifundefined{jump@#1@minbb}{%
    \expandafter\gdef\csname jump@#1@minbb\endcsname{#2}%
  }{%
    \csname jump@#1@minbb\endcsname
    \pgf@xa=\pgf@x
    \pgf@ya=\pgf@y
    #2
    \pgfmathsetlength\pgf@x{min(\pgf@x,\pgf@xa)}%
    \pgfmathsetlength\pgf@y{min(\pgf@y,\pgf@ya)}%
    \expandafter\xdef\csname jump@#1@minbb\endcsname{\noexpand\pgfpoint{\the\pgf@x}{\the\pgf@y}}%
  }
}

\tikzset{%
  remember picture with id/.style={%
    remember picture,
    overlay,
    save picture id=#1,
  },
  save picture id/.code={%
    \edef\pgf@temp{#1}%
    \immediate\write\pgfutil@auxout{%
      \noexpand\savepointas{\pgf@temp}{\pgfpictureid}}%
  },
  if picture id/.code args={#1#2#3}{%
    \@ifundefined{save@pt@#1}{%
      \pgfkeysalso{#3}%
    }{
      \pgfkeysalso{#2}%
    }
  },
  onslide/.code args={<#1>#2}{%
    \only<#1>{\pgfkeysalso{#2}}%
  },
  alt/.code args={<#1>#2#3}{%
    \alt<#1>{\pgfkeysalso{#2}}{\pgfkeysalso{#3}}%
  },
  stop jumping/.style={
    execute at end picture={%
      \stepcounter{jumping}%
      \immediate\write\pgfutil@auxout{%
        \noexpand\jump@setbb{\the\value{jumping}}{\noexpand\pgfpoint{\the\pgf@picminx}{\the\pgf@picminy}}{\noexpand\pgfpoint{\the\pgf@picmaxx}{\the\pgf@picmaxy}}
      },
      \csname jump@\the\value{jumping}@maxbb\endcsname
      \path (\the\pgf@x,\the\pgf@y);
      \csname jump@\the\value{jumping}@minbb\endcsname
      \path (\the\pgf@x,\the\pgf@y);
    },
  }
}


\def\savepointas#1#2{%
  \expandafter\gdef\csname save@pt@#1\endcsname{#2}%
}

\def\tmk@labeldef#1,#2\@nil{%
  \def\tmk@label{#1}%
  \def\tmk@def{#2}%
}

\tikzdeclarecoordinatesystem{pic}{%
  \pgfutil@in@,{#1}%
  \ifpgfutil@in@%
    \tmk@labeldef#1\@nil
  \else
    \tmk@labeldef#1,\pgfpointorigin\@nil
  \fi
  \@ifundefined{save@pt@\tmk@label}{%
    \tikz@scan@one@point\pgfutil@firstofone\tmk@def
  }{%
  \pgfsys@getposition{\csname save@pt@\tmk@label\endcsname}\save@orig@pic%
  \pgfsys@getposition{\pgfpictureid}\save@this@pic%
  \pgf@process{\pgfpointorigin\save@this@pic}%
  \pgf@xa=\pgf@x
  \pgf@ya=\pgf@y
  \pgf@process{\pgfpointorigin\save@orig@pic}%
  \advance\pgf@x by -\pgf@xa
  \advance\pgf@y by -\pgf@ya
  }%
}
\newcommand\tikzmark[2][]{%
\tikz[remember picture with id=#2] #1;}
\makeatother

\definecolor{mybrown}{RGB}{255,218,195}
\definecolor{myframe}{RGB}{197,122,195}

\resetcounteronoverlays{algocf}

\newcommand<>{\boxto}[1]{%
\only#2{\tikz[remember picture,overlay]
\draw[myframe,line width=1pt,fill=mybrown,,rectangle,rounded corners]
(pic cs:#1) ++(1.4,-.25) rectangle (-.2,0.4)
;}%
}


\begin{document}

\begin{frame}
\begin{columns}
\column{.4\textwidth}
\onslide<2->{
\setlength\fboxsep{6pt}%
\framebox{\begin{tikzpicture}[
      x=0.8cm,
      init/.style={circle,fill=green!60!black,inner sep=2pt},
      tran/.style={circle,fill=magenta!60!black,inner sep=3pt},
      stop jumping
    ]
\node[alt=<4>{tran}{init}] at (0,0) (p) {};
\node[init] at (1,-1) (q) {};
\node[init] at (3,-1) (r) {};
\node[alt=<3>{tran}{init}] at (2.5,-2) (s) {};
\node[init] at (3.5,-3) (t) {};
\draw[blue] (p) -- (q) -- (r)  -- (s);
\draw[blue] (r) -- (t);
\end{tikzpicture}}
}

\column{.6\textwidth}
\begin{algorithm}[H]
\SetKwData{Left}{left}\SetKwData{This}{this}\SetKwData{Up}{up}
\SetKwFunction{Union}{Union}\SetKwFunction{FindCompress}{Find}
\SetKwInOut{Input}{input}\SetKwInOut{Output}{output}

\For{$i\leftarrow 2$ \KwTo $l$}
{
\emph{special treatment of $i$}\;
\For{$j\leftarrow 2$ \KwTo $w$}{\label{forins}
\Left$\leftarrow$ \FindCompress{$Im[i,j-1]$}\;
\boxto<3>{b}\If(\tcp*[h]{}){\Left compatible \This}{\label{lt}
\lIf{\Left $<3$}{\Union{\Left}}\;
\lElse{\Union{\This,\Left}\tikzmark{b}\;}
}
\boxto<4>{d}\If(\tcp*[h]{}){\Left compatible \This}{\label{lt1}
\lIf{\Left $<3$}{\Union{\Left}}\;
\lElse{\Union{\This,\Left}\tikzmark{d}\;}
}
}
\lForEach{element $e$ of the line $i$}{\FindCompress{p}}
}
\caption{disjoint decomposition}\label{algo_disjdecomp}
\end{algorithm}
\end{columns}

\end{frame}

\end{document}