A media "How Does It Work"

Have you ever wondered why the TV meteorologist doesn't always gesture at quite the right area during his broadcast?  Actually, it's more remarkable that he gets close at all: there is no map behind him, just a blank colored screen.

Chroma key, also called green screen or blue screen, is a technology used to layer two images together, in this case, a human and a background.  Besides weather reports, this techonology is used in special effects for TV and movies.  Backgrounds can be changed, and people can even be dressed in blue or green suits and be rendered invisible.  But how does it work?

The subject, usually a human, is filmed against a solid blue or green background.  These colors were chosen because they are considered to be the furthest away from human skin tones.  Color cameras read and output different proprotions of three different colors of light: red, green and blue (RGB).  Do you remember, as a kid, getting verrrry close to the TV screen and realizing it was made up of tiny rectangles with red, green and blue?  (No?  Maybe that was just me.)  Try it - turn on your TV and go to a channel that doesn't receive and gives you a blue screen.  Get closer and you'll see that it isn't blue, but rather, tiny, tiny blue rectangles amid black.  Now, turn the TV to an actual picture.  You'll see the tiny red, green and blue bars.  RGB.

The blue or green background can be filtered out during filming, as in a live weather broadcast; or during postproduction in a movie.  This was originally done by camera settings, but is now done by computer software.  Chromakey software is set to detect and erase a certain color, in this case blue or green.  Blue was initially favored as it is farther from human skin tone on the color spectrum, but it posed difficulties with blue clothing and even blue eyes.  Bright green is now the color of choice.

Once the color background has been removed, it can be replaced with anything: a weather map, the background of a tornado, or in the case of a movie, our hero hanging off a cliff!

This charismatic kid explains how green screen can be used. Make sure to click on some of his other movies...he gets a bit of an attitude if you don't!