It's called Chromakey, or just Keying for short. Any time you hear somebody refer to "keying it out," they're talking about using color and/or light to key out or make invisible certain parts of the image, so that a second image resting underneath the first will show through. Chromakey is specifically keying out color, a lumakey is when you key out something based on light. However, most modern systems for chromakey make use of light/brightness levels as well as color.
For a long time, when shooting on actual film stock, a blue screen was the standard for several reasons. First, there is very little blue in a person's skin tones, so there's less to worry about making part of a person's skin invisible. Second, the blue shade used tended to have less grain in the image, which made it easier to isolate the background cleanly and remove it. Third (and this is why blue will sometimes still be used today), for scenes set outdoors, if you have blue spill onto the actors and/or props, you don't have to worry about trying to remove it completely, you can attribute it to natural cast from the sky. And when we say blue spill, what we were referring to is shades of blue from light reflecting off the blue screen and falling on the actor, a common problem when actors aren't well lit or are too close to the screen.
Green began being used for the screen with the rise of digital cameras. Most digital cameras compress the color information with a 4:2:2 ratio, or for lower end cameras, a 4:1:1 ratio. What essentially happens is they have 3 channels - red and blue chroma channels, and then a luma channel that carries only brightness information. The red and the blue channel get compressed to save space (hence the 2:2 part of 4:2:2, meaning those channels are compressed in half). The luma channel remains uncompressed. The green in the image is derived from looking at the blue and red channels (sometimes referred to as 'difference' channels), and calculating what's left after removing that information from the luma channel. If you see a color system labeled as YCbCr (or YPbPr for analog systems, like the red, green and blue component digital cables found on pre-HDMI TV's), it uses this sort of system of transmitting the color and brightness information.
So what does all that mean for green screen? Because the green in the image is derived from observing the difference between the luma and the blue and red channels, the result is a greater amount of green information compared to red and blue information, which translates to a better key when trying to remove the green. The more color information your computer has to work with for chromakey, the better. Also, as mentioned earlier, many chromakey systems will also utilize brightness values, in addition to hue and saturation, to narrow down the exact spectrum to key out, and the YCbCr system maintains the full range of uncompressed luma information. Lastly, like the blue screen before it, there tends to be very little green in a person's skin tones, making green the better option for chromakey compared to red, orange, etc.
Once you've seen exactly how chromakey works, you'll probably start noticing more often where it's being used. Green screens for driving scenes are extremely common, because if you shoot while in a moving car, even a car that has its engine off and is being towed by a truck off-screen, the actual sound of moving down the road (especially a poorly paved one) will drown out your actors' lines, and you'll have to re-record that dialogue later and attempt to sync the new lines up with the footage. These days, it's just easier on everybody to green screen the car shot on a nice, quiet sound stage.
So in short, the solid color screen in the background is there to present an even, smooth surface to cut out around the actor/props. It's not necessary that the screen be green, you can use any color. The trick is to use a color that isn't present on your actors and/or the props on screen. It just happens that most chromakey footage uses people, and green and blue both have very little amounts in skin tones, hence the common use of those 2 colors.
For a long time, when shooting on actual film stock, a blue screen was the standard for several reasons. First, there is very little blue in a person's skin tones, so there's less to worry about making part of a person's skin invisible. Second, the blue shade used tended to have less grain in the image, which made it easier to isolate the background cleanly and remove it. Third (and this is why blue will sometimes still be used today), for scenes set outdoors, if you have blue spill onto the actors and/or props, you don't have to worry about trying to remove it completely, you can attribute it to natural cast from the sky. And when we say blue spill, what we were referring to is shades of blue from light reflecting off the blue screen and falling on the actor, a common problem when actors aren't well lit or are too close to the screen.
Green began being used for the screen with the rise of digital cameras. Most digital cameras compress the color information with a 4:2:2 ratio, or for lower end cameras, a 4:1:1 ratio. What essentially happens is they have 3 channels - red and blue chroma channels, and then a luma channel that carries only brightness information. The red and the blue channel get compressed to save space (hence the 2:2 part of 4:2:2, meaning those channels are compressed in half). The luma channel remains uncompressed. The green in the image is derived from looking at the blue and red channels (sometimes referred to as 'difference' channels), and calculating what's left after removing that information from the luma channel. If you see a color system labeled as YCbCr (or YPbPr for analog systems, like the red, green and blue component digital cables found on pre-HDMI TV's), it uses this sort of system of transmitting the color and brightness information.
So what does all that mean for green screen? Because the green in the image is derived from observing the difference between the luma and the blue and red channels, the result is a greater amount of green information compared to red and blue information, which translates to a better key when trying to remove the green. The more color information your computer has to work with for chromakey, the better. Also, as mentioned earlier, many chromakey systems will also utilize brightness values, in addition to hue and saturation, to narrow down the exact spectrum to key out, and the YCbCr system maintains the full range of uncompressed luma information. Lastly, like the blue screen before it, there tends to be very little green in a person's skin tones, making green the better option for chromakey compared to red, orange, etc.
Once you've seen exactly how chromakey works, you'll probably start noticing more often where it's being used. Green screens for driving scenes are extremely common, because if you shoot while in a moving car, even a car that has its engine off and is being towed by a truck off-screen, the actual sound of moving down the road (especially a poorly paved one) will drown out your actors' lines, and you'll have to re-record that dialogue later and attempt to sync the new lines up with the footage. These days, it's just easier on everybody to green screen the car shot on a nice, quiet sound stage.
So in short, the solid color screen in the background is there to present an even, smooth surface to cut out around the actor/props. It's not necessary that the screen be green, you can use any color. The trick is to use a color that isn't present on your actors and/or the props on screen. It just happens that most chromakey footage uses people, and green and blue both have very little amounts in skin tones, hence the common use of those 2 colors.
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