Capturing and processing VFX elements on actual film stock rather than digital — preserves film grain and negative's color signature. Industry standard before DI workflow.
Before we had digital compositing, effect elements actually had to be exposed onto film. The intermediate film process was the standard method for creating VFX shots that optically and color-wise seamlessly matched the original negative. The idea: You shoot your live-action on 35mm or 16mm, expose illuminated material — mattes, composites, animations — onto actual film stock, and this exposed negative is then integrated into the edit as a practical element. No scans, no grading suite, no reverse engineering. The film itself was the storage medium and simultaneously the quality guarantee.
The practical mechanics: Optical printers, rephotography workstations, and rostrum camera stands were your tools. You had a 35mm camera that photographed an illuminated ground glass or a slide positive original — that was your composition. Each layer (live-action background, animated character, lighting effect, matte painting) was exposed separately or optically superimposed. The result: a true negative that required no additional color correction because the film material itself already defined the overall curve of the production. Grain, color space, contrast behavior — everything came from the stock and was therefore authentic to the rest of the image.
The downside was time and cost. Every mistake forced you to re-print. Changes in color or composition required new exposures. Digital Intermediate has de facto replaced this process — today we scan the original negative, composite everything in DCI or Rec.709, and print back to film at the end, if necessary. But the intermediate film process had an optical elegance: film grain occurred naturally, colors were linked to the material itself, not to lookup tables. Old VFX shots from the 1980s and 90s made this way often show a coherence in the image that digitally composited work can hardly replicate. This is not due to the technique, but because the film itself was the unification of all layers.