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Green Screens: Two Images in One
Shooting in front of a green screen is just one of many movie magic tricks. Read on to learn more about the technology that makes it possible.
One of the most widely used visual effects in all of cinema, the green screen—technically, the process known as chroma key compositing—lets filmmakers combine drastically different images into the same frame, transporting characters to otherwise impossible worlds. Modern green screens follow a simple concept. First, the camera shoots a scene with the subject in front of a green backdrop. Next, a different camera—or, more often, CGI—captures the background in a separate video. In the editing room, a computer homes in on all the green pixels in the original scene and swaps them for the pixels in the background image. Presto! Your home movies now take place in Hawaii.
Technically, any color could be used for a backdrop, not just green. Weather newscasts typically use blue backdrops, for example. Green is usually preferred, however, chiefly because it’s so strikingly different from human skin tones. Otherwise, any similar color between the screen and the subject’s skin or clothing would show up on film as the background instead. (For this reason, the producers of 2002’s Spider-Man had to swap the green screen for blue to film the Green Goblin’s scenes.)
Today’s green-screen technology is almost entirely digital, but the process in some form has been around since at least the 1920s. Back then, filmmakers raced to invent new ways to isolate subjects into black-and-white silhouettes, called mattes, which they could then superimpose over the background. One of the most successful processes involved casting a white backdrop in sodium-vapor light, and then filming subjects with a camera fitted with a special prism that isolated sodium vapor’s specific wavelength. Using this matting technique, Mary Poppins won an Oscar for visual effects in 1964, but few other films could hope to reap such benefits—Walt Disney Studios owned the only sodium-vapor camera in the world.
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