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How Green Screens Transport You to a Different World

BY: Editors | Feb 23, 2016

How Green Screens Transport You to a Different World

Even though you’re in Phoenix, movies can be more than just an air-conditioned escape. From the AMC Ahwatukee to FilmBar, there are plenty of movie theaters where you can take a break and catch a blockbuster or an art-house darling. Of all the things to do in Phoenix, sometimes the best option is to experience a world outside the city thanks to the magic of the green screen. 

Technically a process known as chroma key compositing, the green screen is one of the most widely used visual effects in all of cinema, allowing filmmakers to combine drastically different images into the same frame, transporting characters to otherwise impossible worlds.

The Process:

Modern green screens follow a simple concept. 

  1. The camera shoots a scene with the subject in front of a green backdrop.  
  2. A different camera—or, more often, CGI—captures the background in a separate video. 
  3. 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. 

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Why Green?

Technically, any color could be used for a backdrop, not just green. Weather newscasts typically use blue backdrops, for instance. Green is more common, 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.)

A Spoonful of Sodium Makes the Magic Appear

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. 

Some approaches were more scientific. 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 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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