Adding shadow over image

Question

I'm making a beamer presentation and I'd like to first show an image in its true form:

and in the next slide have it shown with a shadow over a part of it like so:

Is there a way to do this?

Answer 1

Here is a very robust way of doing it:

\documentclass{beamer}% http://ctan.org/pkg/beamer
\newsavebox{\myimage}
\savebox{\myimage}{\quad\fbox{\includegraphics[width=150pt]{tiger}}\quad}% Store image with padding
\begin{document}
\begin{frame}
  \frametitle{A frame title}
  \onslide<2>{%
    \smash{% Remove vertical height
      \rlap{% Right overLAP (zero-width, left-aligned)
        \color{black!30}% Background colour
          \rule[\dimexpr-\ht\myimage-\baselineskip]{\wd\myimage}{.5\ht\myimage}% Box
          }}}%
  \usebox{\myimage}
\end{frame}
\end{document}

The idea is to print a block (\rule[<depth>]{<width>}{<height>}) of specific colour (\color{black!30} = 30% black) in the background of the image (stored in a box \myimage) that has zero width (\rlap) and height (\smash) and position it properly to only highlight the area you're after. The padding is used in \myimage (a \quad or \hspace*{1em} on both sides).

The overlay specification \onslide<.> prints the block only on slide 2. You would have to play with the measurements, since I don't have your image/chart.

Note: This requires your image (or chart) to be transparent so the block can be visible where nothing is drawn.

---

If you're after the reverse, and using some transparency in an overlay, then the following might be what you're after. Similar techniques are followed, only using the transparent package's \transparent{<num>} to specify the transparency of the colour used (0 = fully transparent; 1 = fully opaque):

\documentclass{beamer}% http://ctan.org/pkg/beamer
\usepackage{transparent}% http://ctan.org/pkg/transparent
\newsavebox{\myimage}
\savebox{\myimage}{\quad\fbox{\includegraphics[width=150pt]{tiger}}\quad}% Store image with padding
\begin{document}
\begin{frame}
  \frametitle{A frame title}
  \rlap{\usebox{\myimage}}%
  \onslide<2>{%
    \smash{% Remove vertical height
      \color{black}\transparent{0.8}% Overlay colour = black + 80% opaque
        \rule{\wd\myimage}{.5\ht\myimage}% Box
    }}%
\end{frame}
\end{document}

Answer 2

There is the always present tikz solution.

You can put your image into a tikz node and then use the node anchors as reference points for the rectangular cover. I defined a macro to ease the repeated use. The first argument is the slide in which that cover will appear, the second argument is the amount lf the image to be covered (1 covers the full image, 0.5 only the bottom half, and so on). An example:

\documentclass{beamer}
\usepackage{tikz,graphicx}
\usetikzlibrary{calc}
\def\coverfig#1#2{
    \coordinate (zone) at ($(fig.south)!#2!(fig.north)$);
    \fill<#1>[black, opacity=0.3] (fig.south west) rectangle(fig.east|-zone);
}

\begin{document}
\begin{frame}
  \begin{center}
  \begin{tikzpicture}
    \node[inner sep=1em] (fig) {\includegraphics[width=0.6\linewidth]{tiger}};
    \coverfig{1}{.8}
    \coverfig{2}{.5}
    \coverfig{3}{.2}
  \end{tikzpicture}
\end{center}
\end{frame}
\end{document}

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