Metropolis
Image from Metropolis (1927)
“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 and is known as the ‘Schufftan Process.'”
— Source: Internet Movie Database
Trivia