Package graphicx provides a useful keyword viewport which
allows to show just a part of an image. However, one needs to
put there the actual coordinates of the viewport window.
Sometimes it is useful to have relative coordinates as
fractions of natural size. For example, one may want to print a
large image on a spread, putting a half on a verso page, and
another half on the next recto page. For this one would need a
viewport occupying exactly one half of the file's bounding box,
whatever the actual width of the image may be. This package
adds a new keyword rviewport to the graphicx package
specifiying Relative Viewport for graphics inclusion: a window
defined by the given fractions of the natural width and height
of the image.
