In cinematography, bipacking, or a bipack, is the process of loading two reels of film into a camera, so that they both pass through the camera gate together. It was used both for in-camera effects (effects that are nowadays mainly achieved via optical printing) and as an early subtractive color process.
Eastman, Agfa, Gevaert, and DuPont all manufactured bipack film stocks for use in color processes from the 1920s onwards. Two strips of film, one orthochromatic and having a very thin and superficial red dye layer on its emulsion, and one panchromatic, would be exposed together with their emulsions pressed into close contact, the orthochromatic one nearest the lens. The orthochromatic negative ended up reversed from the normal handedness, but as the two negatives were often contact-printed onto one duplitized film for subsequent color-toning, as in the Prizma process, this often worked to the advantage of the laboratory.
Early color processes such as Prizmacolor, Multicolor, Cinecolor, and Trucolor all used bipack film.
The most famous version of Technicolor, the full-color three-strip Technicolor Process 4 used from 1932 to 1955, exposed two of the three strips—the blue and red images—in bipack. The green record, the highest definition record, was exposed directly.
Alas, certain early color TV transfers were exposed without respect to whether the film was wound conventionally on the reel (A-wind, i.e. emulsion facing toward the hub) or whether the wind was reversed (B-wind) rendering the resulting color image as somewhat faulty, i.e. due to the thickness of the film itself, one primary color was out-of-focus. Later transfers corrected this error.