printing in

from The Collaborative International Dictionary of English v.0.48

Printing in \Print"ing in\ (Photog.)
   A process by which cloud effects or other features not in the
   original negative are introduced into a photograph. Portions,
   such as the sky, are covered while printing and the blank
   space thus reserved is filled in by printing from another
   negative.
   [Webster 1913 Suppl.]