The Chroma Key Process
History and Use
Larry Butler invented the chroma key process for the 1940 movie "Thief of Baghdad." He was acknowledged for his achievement with an Academy Award in special effects. The process has been constantly improved and refined in the following decades.
The technique is commonly used in movies and television. If you've ever watched your local weatherman give the forecast, you've seen the chroma key process in action. Likewise, chroma keying is used heavily in movies that have a lot of special effects. For example, many of the locales in the "Star Wars" prequel trilogy were computer rendered and then combined with green screen shots of the actors. Many sketch comedy and late night comedy shows use chroma keying in sketches. For example, Conan O'Brien has used the technique to insert a moving background behind his desk to make it appear as though he is "driving" his desk around.
Shooting the Subject
In the first part of the chroma key process, the camera films a subject in front of a blue or green screen. This is why chroma keying is often referred to as "green screen" or "blue screen." The green or blue screen acts as a blank space that is filled in later. The subject cannot wear any clothing that matches the color of the screen. If so, the clothing will be keyed out with the rest of the background.
Lighting
When shooting for chroma key, the production crew uses even lighting. It's important to avoid shadows and variations in brightness on the background in order to preserve an even color for the keying process. If parts of the background become too light or dark, those parts may move outside of the color range for the chroma key.
Adding the Second Video Layer
To complete the chroma key process, the editor inserts alternate video in place of the keyed-out background. This can be done in real time or completed in post production using digital-compositing software. In a real-time situation such as a weather broadcast, the production crew combines the shot of the weatherman with the background image using two separate video channels. They combine these channels together in real time using a digital video switching system to obtain the completed shot. In post-production, the editor uses digital editing software to create a matte out of the green screen shot and then insert the background image as a separate layer.
Additionally, the chroma key process may be used to create an entire shot or only part of the shot. For example, a filmmaker may shoot an actor with a couple of actual props, but also use the chroma key process to fill in the rest of the shot. Sometimes, the process includes the props as well. For example, an actor may hold a green ball that is transformed into a glowing orb using chroma key.
Increasing Sophistication
Due to technological advancements, chroma key technology is constantly becoming more versatile. Now, computers can process camera motion in a chroma key shot. This makes it possible for the keyed-in background to realistically match the movement of the camera, creating a realistic effect.
Tags: chroma process, green screen, chroma keying, background image, blue screen, process camera
