the gs command provides options `-dUseCropBox` and `-dUseTrimBox` to
only export the content of the crop- and trimbox, respectively, with the
default export area being the mediabox. there are two more boxes defined
in pdf, bleedbox and artbox; it would be useful to have `-dUseBleedBox`
and `-dUseArtBox` too.

use cases include automated testing of whether there are white spots in
the bleed box. generally, the bleedbox is no less important than the
cropbox.