Optical printing technique layering multiple film elements into one shot — predecessor to digital compositing. Exposed pass by pass on an optical printer.
Kämpfer-Schattmann process
Today, when multiple layers are composited digitally, the same logic is used that Kämpfer and Schattmann solved mechanically in the 1920s – only back then without computers, but with optical printers and precise film technology. The process worked through repeated exposure: a film strip that had already been exposed was rewound to zero and exposed a second time with a different image layer. Layer by layer – that was the principle. This allowed miniature models to be combined with live-action, matte paintings to be merged with moving elements, or multiple characters to be assembled in a single shot without them standing together in the studio.
The technical hurdle was brutal: each additional exposure added grain and reduced optical quality. Exposure metering had to be precise, otherwise the layer would become too dark or too light. Scratches and dust on the negative film became visible with each pass – so cleaning and handling were critical. In editing, they worked with contact optical printers that moved both film rolls synchronously. Timing was everything: if the synchronization deviated even by frames, a shift was immediately visible during viewing. Layers had to be planned like a game of chess – which layer on top, which below, which should remain transparent.
For larger VFX sequences, the process and its variants were standard until the 1990s. Each layer required its own print pass. A complex compositing shot with four or five elements meant five or six exposures in a row – and each development error forced a repeat. That's why previz and storyboarding were so important; you didn't want to unexpectedly start over from scratch. Major effects houses like Industrial Light & Magic built entire departments around these machines, creating layer plans like detailed architectural blueprints.
The digital revolution has made the process obsolete – today you layer elements in a compositing program, see the result in real-time, and save it as a file. But understanding the roots helps grasp why layer logic is structured so similarly in After Effects or Nuke. It's the same conceptual model, only the computer turns the crank instead of the camera.