Why blue background are used for special effects and What is difference between the blue and the green backgrounds? [duplicate]

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Green/blue screens are based on the idea of chroma-keying. The idea is that a specific color is chosen and replaced by a different color/background. So, a green colored background can be switched out with a background of, let's say, a weather map.

Technically speaking, the colors can be gray, white, black, or purple, but green and blue are used most often because they "stand out" the best. I say "stand out" in the sense that since chroma-keying is picking up a color and replacing it with a different image, if a weatherman was wearing a red tie while filming on a red screen, the tie would be replaced as well as the background. In the movie/weather/gaming industry, green and blue screens are used.

Blue screens are used in place of green screens because it's generally easier to chroma-key. The color is softer than in a green screen. However, green screens are still more commonly used because of multiple reasons. One of them being the fact that some people like to wear blue clothing. Green screens are also more commonly used because of the color green being the highest luminance of all the color channels (RGB). Green screens are very bright, but because of the high luminance, there is also a high level of light reflection from a green screen, resulting in "green spill".

Green spill is when there is so much light reflection that some color reflects onto your subject as well. Green spill is very problematic, and is usually best fixed before shooting by watching your lighting, and chroma-key background.

But, in the end the thing that determines the use of a blue screen versus a green screen is foreground objects. It's not a good idea to shoot Kermit the frog in front of a green screen, and shooting an "Avatar" character in front of a blue screen isn't a very bright idea either. If you're shooting video or with a single-chip camera, be aware of the component resolution of your camera. Green is used with video because traditionally it is stored at the highest resolution. Many of today's compression formats store blue at the lowest resolution. If your camera is recording 4:2:0, that means blue is 1/8 the resolution of green, which can lead to matte edge issues if you try to key with blue. If you're using a single-chip camera with a bayer filter (probably all current single-chip cameras), green is recorded at twice the resolution of blue or red.

Single-chip cameras use a bayer pattern to record red, green, and blue using a single sensor. A 4k single-chip camera records green at 2k, red at 1k, and blue at 1k.