Eastman, Agfa, Gevaert, and DuPont all manufactured bipack film stock for use as a colour process from 1920s onwards. Two strips of film, one orthochromatic with a red dyed base, and one panchromatic, would be exposed with the emulsion layers in contact with each other, resulting in one of the two negatives being reversed. As these negatives were frequently contact printed onto duplitized film for toning processes such as those commencing with Prizma, this worked to the advantage of the laboratory.
The most famous version of Technicolor, the full-colour three-strip Technicolor Process 4 used from 1934 to 1954, exposed two of the three strips—the blue and red images—in bipack.
To achieve the in-camera effect, a reel would be made up of pre-exposed and developed film, and unexposed raw film, which would then be loaded into the camera. The exposed film would sit in front of the unexposed film, with the emulsion of both films touching each other, causing the images on the exposed film to be contact-printed onto the unexposed stock, along with the image from the camera lens. This method, in conjunction with a static matte placed in front of the camera, could be used to print angry storm clouds into a background on a studio set. The process differs from Optical Printing in that no optical elements (lenses, field lenses, etc) separate the two films. Both films are sandwiched together in the same camera and make use of a phenomenon known as contact printing.
The process had its beginnings in providing a repeatable method of compositing live action and matte paintings, allowing the painted section of the final image to be completed later, and not tying up the set/sound-stage whilst the artist matched the painting to the set. It also alleviated the considerable difficulties caused by matching shadows on the painting to the set on an open-air set. The process worked equally well for matting-in real water to a model, or a model skyline to live action. The process was also referred to as the Held Take process. Perhaps the most famous example of a held take is the long shot of astronauts clambering down into a lunar excavation in 2001: A Space Odyssey.
The technique, if used with a camera not specially designed for contact printing, runs the risk of jamming the camera, due to the double thickness of film in the gate, and damaging both the exposed and unexposed stock. On the other hand, because both strips of film are in contact and are handled by the same film transport mechanism at the same time, registration is kept very precise. Special cameras designed for the process were manufactured by Acme and Oxberry, amongst others, and these usually featured an extremely precise registration mechanism specially designed for the process. These process cameras are usually recognisable by their special film magazines, which look like two standard film magazines on top of each other. The magazines allow the separate loading of exposed and unexposed stock, as opposed to winding the two films onto the same reel.
The bipack process, which is a competing method to optical printing, was used until digital methods of compositing became predominant in the industry. Industrial Light and Magic used a specially-built rig, built for The Empire Strikes Back that utilised the method to create matte painting composites.