Clip parameters, is it possible to clip from bottom of page?

[GaryReckard]

I'm trying to capture the footer section of a list of ~200 sites with Browsershot. Capturing the fullpage is unnecessary for my needs. And for some reason using select('.page-footer') doesn't reliably capture the whole footer area.
I was thinking of using clip, but cannot find any specs for exactly how the $x, $y, $width, $height arguments are used.
Is it possible to clip starting from the bottom of the page?
Thanks!