WHAT IS CHROMA KEY?

Chroma-key technique is a way to put you on the planet Mars if that’s what your video script calls for.

Chroma-key is cheap and easy.
It can transport you from Mars to the American Revolution in the punch of a button.

Chroma means color.

In video, key means to put one image on top of another.
Chroma key means you remove any one color from your image and replace it with something else. Anything else you want. A still shot of the planet Mars downloaded from NASA. Or a video with gruesome battle scenes on the Potomac.

WHAT COLORS ARE USED?
Bright lime green and bright turquoise blue are the most commonly used colors for chroma-key, hence the name green screen. If you stand a person in front of a green screen, all of the green can be removed and replaced with whatever image you want.
Any color will work with chroma key. Blue and green are chosen often because they are furthest away on the color spectrum from face tones.

YOUR WEATHERCASTER USES GREEN SCREEN
One of the main uses for chroma-key in television that you’re no doubt familiar with is the weather report. Next time you watch a TV weather report, you'll know that the different maps, satellite and radar images appear behind the weathercaster because of chroma-key technique.

The image of the weathercaster is captured with one camera and in that shot, you can see the green screen. If you were to stand in the studio, you would easily see the weathercaster standing in front of the green screen, perhaps the walls and floor are painted green and he’s walking right on top of the green.

The weather radar, satellite or other maps are all separate video inputs that the director will insert to replace the solid green behind the weathercaster. The weathercaster is watching where he is by looking at a monitor that's placed slightly off camera and shows the blending of the two images. The viewer never sees this monitor.

A good weather caster will shift his head toward the green screen but his eyes toward the monitor. This way, it looks he’s looking intently at his map but he’s really checking to see if he needs to take baby steps or big daddy steps.

Lots of weathercasters doze and think they’re talking about snow in Milwaukee when the viewer is seeing them on top of a radar image of Miami. The weathercaster never knows the difference but the viewers think he flunked geography.

You'd be amazed at how often chroma key is used. When taking a tour of the ABC studios in New York with my high school students after we won our Emmy, we watched a news update go live. The anchor person was actually in a TINY green screen room, smashed up against the wall. But on the air, they looked like they were in a HUGE, elaborate studio. It was seamless. There was NO indication in the final shot that it was a chroma key. (You can tell by looking at the edges of the person against the background.) I was blown away by how realistic it looked and I'm not easily impressed.

HOW DO YOU DO GREEN SCREEN?
In a studio, most green screen is done live through a video switcher. But green screen can also be done with advanced computer editing programs. Most advanced programs have the capacity to use any color, not just green. You can use brown if that's what works for you.

Lime green and bright blue are the preferred colors because they're on the opposite side of the light spectrum from skin tones. You don't want someone's face being replaced with the weather map! But if you want to chroma key a green alien onto Mars, a brown screen instead of a green screen may be the way to go.

Whatever color you decide, videotape your subject in front of a flat wall that's solid with that color. Light your video so there are no shadows. Any variation of the color behind your subject will be difficult to work with. Any shadow creates a slightly darker color which might confuse your edit system.

Once you get a solid even colored background, your switcher or edit computer completely remove that color and you supply a substitute. The precise mechanism by which to accomplish this will vary from system to system.

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