Keyed Out, Colored In: Chroma Chromatic Dreams

We think of green screens as a unique tool that can transport us into deep space or out to sea with a few edits, but they can be so much goofier than that. The true magic of the green screen is that it can become whatever you can dream up.
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By Kristen Bialik

The magic of the green screen is, at this point, ubiquitous. You can find it in some of the lowest-budget videos and in almost all of Hollywood's biggest blockbusters. You see it every night on your local weather report as the weatherman stands in front of a blue or green matte that appears to be a map of sweeping cold fronts or looming precipitation. We think of green screens as a unique tool that can transport us into deep space or out to sea with a few edits, but they can be so much goofier than that. The true magic of the green screen is that it can become whatever you can dream up. Sometimes, that dream is apparently a jiggly rainbow pyramid shifting in front of a fake cityscape or neon keyboard keys dancing with snowflakes and bubbles while mustachioed drummers do their thing.

First in a series of video collections exploring the video-magic of the chroma key

Who would have thought such weirdness was possible back in 1933 when the musical Flying Down to Rio used traveling matte, an early precursor to the green screen technique. The use of mattes in filmmaking to combine two or more images into a combined image goes all the way back to the Lumiere brothers, where it was used in The Great Train Robbery to create a moving background outside a baggage car on the train set. But Linwood Dunn traveling matte process for RKO Radio Pictures pioneered a more believable use of mattes in the 1930s by projecting the background image onto a counter matte and then optically re-photographing the result.

The advent of color film unlocked a whole new world of hues and previously unconsidered destinations. In 1940, the Thief of Baghdad was released as the first film to feature Technicolor and then mind-blowing special effects. With the help of special effects artist Larry Butler, the Thief of Baghdad was also the first to use the chroma key process with bluescreen technology. By using three separate strips of film (one each for red, green, and blue), Butler could arrange the original negative and newly printed positive strips so that the blue negative and green positive strips would create a solid matte. This would be composited with new footage shot against a blue screen. All this was run through an optical printer so that the final film print would include the composited foreground and background. The process is often called "keying," "keying out," or simply a "key." This same concept was used in color film compositing for decades of filmmaking, all the way through the advent of digital technology.

Equipment limitations hindered full-on suspension of disbelief, but the main issue was that the process was unbearably time consuming. For each scene using chroma key, the film had to be combined one frame a time. The chroma key process wasn't perfected until 1980 with -- what else? -- The Empire Strikes Back! By using a quad optical printer, the slow and choppy analog process was made easier and faster with better results. Multiple computers were used to carefully control the quad optical printer so the shots wouldn't be riddled with black lines.

The switch from the magic of the blue screen to the magical green screen was largely reflective of the shift to digital. Blue screens are actually more complementary to the red hues in human skin tone and better with film because of the blue emulsion layer contained with filmstrips. But green became the media darling because of its low lighting requirements, ability to retain more detail, and high luminance value. The ability to shoot outdoors without risking background bleeding into the sky also helped propel the use of the now ever-present green screen.

Even though the chroma key technique seems like a cheap and easy way to avoid expensive, unpredictable on-location shoots, chroma key has developed into a artful process that requires a tremendous amount of knowledge and control. For the best effects, the lighting has to be perfect. The background and subject each have to be lit differently, but with exact evenness to avoid any shadows and create the illusion of separation. Wardrobes must be carefully selected, noise removed so that the illusion doesn't break through in blouses or buttons. Like any act of magic, the magician needs control of the cards.

Video has long offered a look into strange places, an escape into the unknown. Chroma Key gave filmmakers a way to "key out" the mundane and color in their dreams -- of battling among the stars, of tap-dancing on the wings of airplanes, or in this collection, doing gospel aerobic marches to a swirly pastel background. What matter is that video makers keep coming up with wild places for us to go and unlock the key to get us there.

More on green screen technology:

The history of green screen technology by Video Maker magazine

An explanation of blue screen imaging by a Director of Photography and teacher at the Seattle Film Institute

Chroma key overview at Wikipedia

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