Geek Trivia

Chroma Key Special FX Backgrounds Are More Frequently Green Than Blue Thanks To?

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Answer Digital Movie Cameras

For decades, film makers have shot actors in front of bright blue, green and, less frequently, other single color backgrounds to create footage for the special effects artists to use chroma key compositing with to add a new background and other objects into the scene. It is this technique that allows for the filming of actors inside a sound studio in Hollywood, but to appear on screen as if they were standing on a distant mountain or even a distant planet.

Historically, the most widely used color in the chroma key process was blue because early film-based compositing techniques relied on shooting heavily filtered high-contrast film that was more sensitive to blue-channel light. Other color backgrounds simply didn’t cut it when the film used in the process was most sensitive to blue.

The advent of digital film making led to a shift toward green chroma key backgrounds for the same reason film cameras used blue: digital cameras are more sensitive to green. Thanks to a slight green-channel bias introduced by the nature of the bayer pattern used to create the pixels in the camera sensor, digital images contain the least noise in the green channel and actors shot against a green background can be more cleanly extracted from the scene and composited onto new backgrounds.

While green chroma key might be the most common now, don’t think that the use of blue screens is extinct. Not only are blue screens still in use (albeit less so), in many cases they are absolutely essential. Take, for example, the case of the contrasting costumes in the 2002 film Spider-Man. When Spider-Man and his enemy the Green Goblin were fighting each other, the scenes had to be shot separately and double composited. Spider-Man couldn’t be shot against a blue screen or his costume would wash out and the effect would be ruined, and the Green Goblin couldn’t be shot against a green screen for the exact same reason.

Image courtesy of Walt Disney Studios.