As requested my answer about using TikZ with semi transparent overlays.
Code
\documentclass[handout]{beamer}
% basic packages
\usepackage[latin1]{inputenc}
\usepackage[T1]{fontenc}
% load TikZ to draw the heighlighting areas
\usepackage{tikz}
\usetikzlibrary{fit,calc,decorations.pathreplacing}
\newcommand{\tikzref}[1]{% to define an anchor
\tikz[remember picture]{%
\coordinate (#1) at (0,0.5ex);%
}%
}
\tikzset{
transp/.style={
fill opacity=0.75,fill=white, inner sep=1.5mm,
},
reverseclip/.style={
insert path={(current page.north east) --
(current page.south east) --
(current page.south west) --
(current page.north west) --
(current page.north east)},
},
}
% use listings to show an example how to heighlight code
\usepackage{listings}
\lstloadlanguages{[LaTeX]TeX}
\lstset{%
language=[LaTeX]TeX,
basicstyle=\ttfamily,
keywordstyle={},
escapechar={§},% needed to set tikz anchors in listings
}
\begin{document}
% EXAMPLE A
\begin{frame}{Heighlighting parts of an equation}
% 1. Set the equation as usual. Add anchors with \tikzmark at the beginning and
% end of the formular, named bmath and emath in this example, and at the
% beginning and end of the part you want to heighlight, named bfrac and efrac.
\[
\tikzref{bmath}f(x) =
\left(2x^4 + \tikzref{bfrac}\frac{3x^3}{4m}\tikzref{efrac}+
7x+\pi\right)\cdot\frac{\alpha}{2\gamma\varepsilon_0}\tikzref{emath}
\]
% 2. Add the code to highlight
% 2.1 The {tikzpicture} shouldn't take space on the frame but 'overlay' it and
% we make the \tikzref anchors acessible via 'remember picture'.
\begin{tikzpicture}[overlay, remember picture]
% 2.2 Draw the frame around the part that should be heighlighted, i.e. not coverd
% by the semi-transparent overlay. This path has the 'clip' option meaning
% that all following path are clipped by that.
\path [clip] % [A, see Notes]
($(bfrac) - (0,5mm)$) -- %
($(efrac) - (0,5mm)$) -- %
($(efrac) + (0,5mm)$) -- %
($(bfrac) + (0,5mm)$) --%
cycle
[reverseclip];
% 2.3 Add the semi-transparent overlay to cover the whole equation.
\node [transp,fit={($(bmath)+(0,5mm)$) ($(emath)-(0,5mm)$)}] {};
\end{tikzpicture}
\end{frame}
% EXAMPLE B
\begin{frame}[fragile]{Heighlighting parts of a code example}
% 1. Set the code environment as usual and add \tikzref anchors. Here § is used
% to escape from vermatim back to LaTeX.
\begin{lstlisting}[basicstyle=\ttfamily\footnotesize]
§\tikzref{z1}§\documentclass[ngerman,11pt]{scrartcl}
\usepackage[T1]{fontenc}
\usepackage[latin1]{inputenc}
\usepackage{babel}
§\tikzref{z5}§\usepa§\tikzref{pkgcmd}§ckage{list§\tikzref{pkg}§ings}§\tikzref{lst}§
§\tikzref{z6}§ \lstset{basicstyle=\ttfamily}§\tikzref{lstset}§
§\tikzref{z7}§ \lstMakeShortInline|§\tikzref{svrb}§
§\tikzref{z8}§\begin{document}
Eine \emph{Auszeichnung} und
§\tikzref{bvrb}§|verb§\tikzref{verb}§atim|-Text§\tikzref{evrb}§ enthält.
\dots und eine {unbedeutende Gruppe}.
% Kommentare erscheinen nicht in der Ausgabe§\tikzref{txt}§
§\tikzref{z15}§\end{document}
\end{lstlisting}
% 2. Add the {tikzpicture} to draw the heiglighting (see example A).
\begin{tikzpicture}[overlay, remember picture]
% clipping:
\begin{scope}% [B, see Notes]
% cut out 1
\path [clip] let \p1=(z5), \p2=(z7), \p3=(lstset) in%
($(z8) + (0,1ex)$) %
-- ++(\x3-\x1,0) %
-- ++($(0,0) + (0,2ex) + (0,\y1-\y2)$)
-- ++(\x1-\x3,0) %
-- cycle [reverseclip];
% cut out 2
\path [clip] %
($(bvrb) - (0,1ex)$) %
-- ($(evrb) - (0,1ex) $) %
-- ($(evrb) + (0,1ex) $) %
-- ($(bvrb) + (0,1ex)$) %
-- cycle [reverseclip];
% clip everything without cutted areas
\node [transp,fit=(z1) (z15) (txt)] {};
\end{scope}
% 3. Add some additional text annotations using 'nodes' and the \tikzref anchors.
\def\ddx{0.15}\def\ddy{1ex}\def\dx{0.7}
\draw<2-> [<-] ($(lst) + (\ddx,0)$) -- +(\dx,0) node [right, anchor=mid west] {load};
\draw<3-> [<-] ($(lstset) + (\ddx,0)$) -- +(\dx,0) node [right, anchor=mid west] {mono font};
\draw<4-> [<-] ($(svrb) + (\ddx,0)$) -- +(\dx,0) node [right, anchor=mid west] {short verb};
\draw<5-> [<-] ($(verb) + (0,-\ddy)$) |- +(0.3,-0.5) node [right, anchor=mid west] {example};
\end{tikzpicture}
\end{frame}
\end{document}
Output

Notes
To get a semi transparent overlay with some cut out areas, I lay a node above the whole relevant part of a frame and clip this. The needed reveseclip
-Option was created by Jake at How can I invert a 'clip' selection within TikZ?.
While designing this example I’ve found out that the direction of drawing clip path [A] is relevant so try drawing path (counter) clockwise if it doesn’t work.
Note that TikZ needs two runs of (pdf)LaTEX to get the right positions.
In some cases [B] it seems to be a good idea to limit the clipping to a {scope}
to make other drawings which are not effected by it.
A problem might be that we need to contaminate the original listing code with the §\tikzref{…}§
. Maybe someone has an idea how to export adjusted listing automatically?