Specifying part of an image to be used in cm

Question

I have a multipage pdf document and would like to include only the upper part (say 3cm down and 2cm right from upper left corner) of a given page. Including part of a single page can be done with \includegraphics,

\documentclass[twoside,a4paper,12pt]{report}

\usepackage{graphicx}

\begin{document}
\includegraphics*[page=7,width=\textwidth,viewport=30 30 120 120]{file.pdf}

\end{document}

but how do change the units to something like cm or inches?

Answer 1

I'd measure the graphics (original size) and then trim it. I assume that you want to enlarge that zone to the text width

\begin{figure}
\centering
\sbox{0}{\includegraphics{filename}}
\setlength{\dimen6}{\wd0}\setlength{\dimen8}{\ht0}
\includegraphics[%
  width=\textwidth,
  trim=0 {\dimexpr\dimen8-3cm\relax} {\dimexpr\dimen6-2cm\relax} 0,
  clip]{filename}
\caption{Particular of the upper left corner of the page}
\end{figure}

The four parameters for trim= are the amount to be trimmed on the left, below, on the right and above respectively. With clip you are telling pdftex to not show the trimmed part.

Note that this doesn't require measuring manually the graphics. However, you find the bounding box information in the log file.

The scratch registers used are \dimen6 and \dimen8 that are not used by graphicx.sty; one must, however, check that the result is what's expected. The safest way would be to define two lengths for the purpose.