… for Visual Effects

Tip #500: When is a Green-Screen Key Red?

The only requirement for a chroma-key is that the background color not be used in the foreground.

In this illustration, the color red is translucent, because we are using it as the key color.

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Green-screen is shorthand for a “chroma-key,” that is a key based upon a color. We remove a background by making all the pixels of a certain color transparent so we can put something else in its place. However, a “green-screen key” doesn’t, in fact, require anything green. It’s just that, when people are involved, we use green more than any other color. In the past, video used blue backgrounds, while film used green, simply due to how video and film responded to the two different colors.

Over time, we standardized on green because it is a color that is not in human skin tone and, while many of us like wearing varying shades of blue, green is much more rare in clothing.

NOTE: However, if you are creating a key to recreate a night scene, you are better off using a blue background, because moonlight is very blue and the edges of the key will fit in better with the night look.

What a chroma key does is look at the color of each pixel. If it finds one that matches the color you want to remove, it makes that pixel transparent. The key color could be green for people, red for lizards or blue for, say, an agricultural spot set in a cornfield.

There’s no magic that determines which color you use – any modern keyer can key on any color. Pick the one that works the best for your project. (Like the red backgrounds I saw used for “The Lizard King from Outer Space.” Very, very weird.)


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