 No optical printing system existed at the time, so to create a matte  effect, a large mirror was placed at an angle to reflect a piece of  artwork while live footage was projected onto the reverse. To expose  the projected footage, the silvering on the back of the mirror had  to be scraped off in strategically appropriate places. One mistake  would ruin the whole mirror. This was done for each separate shot  that had to be composited in this manner. This procedure was  developed by Eugen Schüfftan (qv) and is known as the "Schufftan  Process."