Why must the picture before the special effects of a movie or TV series be in the green set?

It is wrong to say that foreigners' eyes are blue and use green. The person who said this has not done keying or is not a professional keying!

Keying requires a solid background color, the higher the purity, the better, and then the more uniform the background, so that it can be separated by color keys later. So in theory, you can choose any color. Red, orange, yellow, green, blue, blue and purple will do! As long as there is no large area of similar color on the picked object. Of course, RGB sensor has the best color, which can ensure the highest sensitivity and high-quality signal-to-noise ratio and the lowest manic wave performance. In the era of monolithic cmos cameras represented by ARRI and red, many green screens are used because there are two green photosensitive spots on Bayer array, which has the largest amount of information.

In addition, the brightness of green background is higher than that of blue background, which is also an important factor for lighting engineers. Technically, the amount of information in the two green channels is twice as much, so in theory, the light on the green background is saved by half compared with other colors (don't underestimate this lighting, a space light with a large area of 6000W generally needs at least 8 lights, that is, 48 K-48 kWh per hour, plus other lights, at least 60- 100K, and 3 yuan can be counted as 65438 per hour. If you shoot 12 hour, your electricity bill is 2-4 thousand. Look at the dense lights on the roof of Hollywood studios (tens of thousands of electricity bills a day) and you will know that they are all money! ! ! If you lose half the lights, you will save a lot of money! Director: No! I'll use a blue background ... the producer cried himself out in the toilet) (Note: 3 yuan is not the price that the power company sells to you for one day, and there are some studio surcharges in the middle, special statement).

As for the one with black back, white back and gray back, it is not a color key, because these colors are all gray, so the brightness key is used for keying. Everyone has said the principle of color key keying, and what I want to add is other processing of keying. The core of keying is to obtain the complete alpha channel of the object, so the most important thing is actually the edge part of the object! ! ! For example, hair, translucent skirts, screens, etc., this keying requires the highest smoothness and saturation of the background. There are also translucent parts such as glass refraction. And the eyes are blue or green, which is actually very easy to handle. Generally, just do a mask tracking. The edges of the object are all processed. Are you worried that it is not easy to dig in the middle?