GR translation: downward, dark and ruinous behind her the ground which, for the frames’ passage, defines her

[Mike Jing]

V94.2-15, P95.38-96.11   She has posed before the mirrors too often
today, knows her hair and make-up are perfect, admires the frock they
have brought her from Harvey Nicholls, a sheer crepe that flows in
from padded shoulders down to a deep point between her breasts, a rich
cocoa shade known as “nigger” in this country, yards and yards of this
delicious silk spun and thrown, tied loosely at the waist, soft pleats
falling to her knees. The cameraman is pleased at the unexpected
effect of so much flowing crepe, particularly when Katje passes before
a window and the rainlight coming through changes it for a few brief
unshutterings to murky glass, charcoal-saturated, antique and
weather-worn, frock, face, hair, hands, slender calves all gone to
glass and glazing, for the celluloid instant poised—the translucent
guardian of a rainfall shaken through all day by rocket blasts near
and far, downward, dark and ruinous behind her the ground which, for
the frames’ passage, defines her.

What does the word "downward" describe?  And what's the meaning of the
word "define" here?
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Pynchon-l / http://www.waste.org/mail/?list=pynchon-l