IV.

Travelling Mattes and Compositing

Historically, one of the greatest challenges in visual effects cinematography has been to find ways of combining elements filmed at different times and locations in order to produce a single, seamless shot. For example, being able to combine footage of an actor filmed in a Hollywood studio with stock footage of the pyramids would save the time and expense of sending an entire production crew to Egypt.

Early methods of combining separately filmed images involved exposing the original camera negative to multiple images in order to create a combination, or composite. Early examples of this technique are seen in the Edwin S. Porter film The Great Train Robbery (1903). In one scene a robbery takes place inside a telegraph office while a train can be seen passing outside the window. This was achieved by filming the telegraph office in a studio and masking or “matteing” off the area of the frame occupied by the window, thus leaving that portion of the negative unexposed. The camera was then taken to a railway, a mask or “matte” applied to the already exposed area of the frame, and a passing train photographed into the unexposed window area of the frame. Two separate exposures filmed in two locations were therefore combined on one piece of film to produce a single composite shot.

Film-makers soon searched for ways of convincingly combining several images without having to apply masks to the frame during filming. Furthermore, they wanted combined elements to be able to travel around the frame, rather than stay in one fixed place, as in the window example, above.

The most effective early method was the Williams Process, patented by Hollywood cinematographer Frank Williams in 1918. Though relatively crude, this method established the basic principles that remain at the heart of all matte photography techniques to this day.

The process required actors who were to be placed into other environments to be filmed in front of a plain white (or black) background in the studio. The developed negative was then copied using high-contrast film, which turned parts of the image into either solid black or clear “white” areas. In the case of an actor walking around the screen, this would result in an image that was clear except for a solid black silhouette or “male matte” of the moving actor. The image would next be copied to produce a negative “female matte”, in which the area of the actor was clear but the surrounding frame was black. Pre-filmed footage of background scenery such as the pyramids, for example, would then be sandwiched with the male matte of the actor and re-photographed onto a new negative. This would allow the pyramid image to be copied onto the new piece of film except in the area occupied by the male matte.

The footage of the actor was combined with the female matte and also photographed onto the negative. This time the area already exposed to the pyramid scenery would be covered by the black area of the matte and be prevented from further exposure, while the image of the actor would be copied into the uncovered, previously unexposed, area of the negative. When developed the result would be a scene in which a studio-filmed actor appears to walk in front of the pyramids.

This method of male and female counter-mattes, allowing elements such as actors to move around within the frame, is known together with its subsequent optical and digital variations as travelling matte photography.

Various methods for creating travelling mattes were developed over the years, the greatest challenges coming with the development of colour photography. From the 1950s onwards most travelling mattes were created by filming actors in front of blue screens. Several methods of blue screen travelling matte photography were developed, each using a complex combination of coloured filters to separate performers, models, or animated characters from the blue background and create the male and female mattes needed to place them into new environments. When elements could not be filmed in front of a blue screen they could even be painstakingly drawn around and isolated by hand—a process called rotoscoping.

The process of combining various pieces of footage using mattes is called compositing, and until the era of digital visual effects production this was achieved using a machine called an optical printer—the single most important piece of equipment at the heart of every visual effects studio. The first sophisticated optical printers were built by Linwood Dunn in the early 1930s. They consisted of a high quality projector that beamed the image from one strip of developed film into a camera, which re-photographed it onto a new negative. By using a system of beam-splitting prisms, several projectors could beam their images into the camera simultaneously, allowing a number of elements and their mattes to be combined at once. The most sophisticated optical printers were built in the 1980s, allowing up to four images to be accurately combined in each “pass”; the negative film in the camera could then be rewound and exposed to another four images, and so on. Some of the most ambitious optically composited images are seen in Return of the Jedi (1983, George Lucas) for which the effects company Industrial Light and Magic sometimes combined over a hundred images of spaceships, scenery, and animation, and their associated travelling mattes, to produce a single brief shot.

Several alternative techniques to the laborious process of creating travelling mattes were developed from the 1930s onwards. Most popular of these was rear projection, in which actors were filmed performing in front of a translucent screen onto which pre-filmed footage was projected from behind. This technique was most frequently used when studio-bound actors needed to look as if they were driving on the open road.