I'm using \includegraphic{image.pdf} to insert an image into a Beamer presentation. The image that I have is a drawn diagram that was scanned. Part of the diagram is sketched in light blue which looked alright on the piece of paper but is washed out by surrounding white paper around it once scanned.
Is there a way in beamer to darken all colors of an pdf image?
You might be able to use the decodearray feature of \includegraphics.
\documentclass{article}
\usepackage{graphicx}
\begin{document}
\includegraphics[width=3in]{warthog18}
\includegraphics[decodearray={0 .75 0 .75 0 1},width=3in]{warthog18}\par
\includegraphics[width=3in]{example-image.jpg}
\includegraphics[decodearray={0 .75 0 .75 0 .75},width=3in]{example-image.jpg}
\end{document}
You may try to draw your PDF file on a gray rectangle into a blend group (p.340, pgfmanual). The multiply blend mode is useful to darken an image.
Ex:
\documentclass[tikz]{standalone}
\begin{document}
\begin{tikzpicture}[line width=0pt]
\begin{scope}[blend group=multiply]
\foreach \prop[count=\c] in {0,10,...,100}{
\node[inner sep=0] at (0,\c) {\includegraphics[width=1cm]{tiger}};
\fill[white!\prop!black] (0,\c) ++(-.5,-.5) rectangle ++(1,1);
}
\end{scope}
\end{tikzpicture}
\end{document}
LaTeX can do everything, but in this case it is probably easiest to manipulating your scanned image with dedicated image software. Since you say it is a scan, you probably want some software good with pixel graphics. A free example for this would be gimp.
That lets you adjust a variety of parameters, among these Contrast, Brightness and Saturation. Without having seen your image, I would guess that playing around with these parameters might enhance it.
