[texhax] pdfLaTeX graphicx viewport

[Adrian F. Clark]

I'm sorry if I seem dim but I have googled for the answer to this and
looked in both my normal LaTeX reference books before asking.

I want to include part of a JPEG image in a pdfLaTeX document.  Rather
than cropping the region out the image, I thought I'd use the viewport
option to graphicx's \includegraphics:

\includegraphics[viewport=0 0 1536 2048,clip]{2005-01
-12-045}

But what are meaning of the 4 values one specifies in the context of a
JPEG image?  (The question is also made more interesting by the fact
that I need to rotate the image through 90 degrees, but I'll
investigate that more thoroughly after I'm able to crop it properly.)

MTIA.

..Adrian
-- 
Dr Adrian F Clark           WEB: http://privatewww.essex.ac.uk/~alien/
EMAIL: alien at essex.ac.uk   TEL: +44 1206 872432   FAX: +44 1206 872900
PAPER: VASE Lab, Dept ESE, University of Essex, Colchester, CO4 3SQ,UK
"The great tragedy of science: the slaying of a beautiful hypothesis
by an ugly fact"  --  Huxley