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blue screen, green screen and other colours


When the colour keying special effect was first introduced it invariably used a blue background (for me the term "blue screen" is synonymous with the effect). But now it always seems to be green screen - judging by the "making of..." extras on DVDs. This is probably common knowledge but why change to green and not just stay with the blue?

And even in home video editing software one can pick the colour used for keying, so have any movie effects been made using any colours other than blue and green?

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I once worked at a TV station whose news set had a blue screen for the weather folks and reporters to stand in front of. If the talent was wearing blue clothing, s/he would start "keying out" and master control would have to tweak the matte settings to keep the talent from being semi-transparent. When the station converted to a green screen, it was much easier for all concerned because the on-air talent was much less likely to wear the same shade of green as the screen.

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Yes, any color can be used now that almost all compositing is done in the digital domain. Some work better than others though.

Both the Blue and the Green that are most often used, are very very pure colors. most blues and greens you see in everyday life are a mix of colors. This is why the paints and fabrics that are custom created for effects compositing are so expensive.

Blue's advantage today is that it is the opposite of flesh tone, that is, it's the complementary color to most flesh tones. Green is closer to flesh tone, and flesh tone sometimes has little green in it. Also, if you have some blue contamination or spill in the scene, it is usually less objectionable and noticed than green spill. The software programs can remove this, but if the operator isn't careful, they may cause flesh tones to look a bit off, if green is removed too aggressively.

Most of the Green Paints and fabrics used reflect back more light overall, which is can make the compositing process somewhat easier. Also, most digital imaging cameras have most of their sensitity and the least amount of noise in their green channel, with more noise in the blue channel. So Green has become the favorite of the last decade.

The software today is so good that having other green or blue colors is normally not a problem, as long as they're not that perfect shade of green or blue.

Other colors have been used, namely red. Usually when there are no people in the foreground shot.

Steven Bradford
Seattle WA
http://www.seanet.com/Users/bradford/blue_green_screen_visual_effects_ 1.html

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Thank you for such a full answer.

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There are several reasons, as other posters have mentioned. Clothing can 'disappear' if the same colour as the background. Also it is easier using a greenscreen as it is a colour which very few people have as an eye colour.
The BBC in early 1970s experimented with a golden yellow cyclorama but it made actresses hair disappear. A good example of bad keying is at the end of episode six of Doctor Who: The Time Monster, where Katy Manning's hair and dress disappear.

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Blue and other background colors were used first in film making. I suspect that blue painted scrims made it into TV studios because they were the only available color at the time because of the film industry. However there are technological advantages to using green as the key color. Green is right in the middle of the visible light spectrum, whereas red and blue are at the fringes. In 3-color component color video, the green channel typically has the strongest signal, giving more dynamic range to use. This makes it easier for both analog and digital color video processors to make a clean key. Green is also the one color that's not subject to decimation under various analog and digital video encoding formats. That means that a video signal that was quadrature encoded into a YIQ composite signal still had more or less the same amount of green information as the original RGB from the camera, making green the most versatile key color. In digital television green has the same advantage, being a part of the luminance channel at full fidelity. That's the long and short of it.

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