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How/why do green screens work? What's so special about the color green that lets us seamlessly replace the background with another image and keep the human intact?

Are there other colors that work similarly?

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closed as off-topic by tpg2114, Jon Custer, Kyle Kanos, ZeroTheHero, stafusa Dec 8 '18 at 0:00

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    $\begingroup$ @Pieter Interesting - so if I wanted to place my pet chameleon on a fighter jet in World War 2, I'd need a screen whose color dynamically changes to the one least present on the chameleon at that point in time and then go through the footage frame by frame to replace the screen color with the backdrop [probably could be automated...]. I'm guessing this is why I haven't seen many chameleons in CGI-heavy movies. $\endgroup$ – pushkin Dec 4 '18 at 20:06
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    $\begingroup$ @pushkin Chameleons don't really continually perfectly mimic the colour around them. In fact, most of their colour changes are to communicate with other chameleons. $\endgroup$ – J.G. Dec 4 '18 at 21:18
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    $\begingroup$ It used to be blue, before it was green. $\endgroup$ – nl-x Dec 5 '18 at 8:43
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    $\begingroup$ Reading just the title, I would have expected at least a comment saying "otherwise it would have been called red-screen or blue-screen" $\endgroup$ – frarugi87 Dec 5 '18 at 12:27
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    $\begingroup$ Not only did green screens used to be blue screens, but sometimes the weather reporter would forget and wear a blue tie, with humorous consequences. $\endgroup$ – cobaltduck Dec 5 '18 at 19:45
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It's partly about how human colour vision works, partly about avoiding colours you want to keep, such as those of the actors.

Colour cameras record concentrations of red, green and blue light to mimic human colour vision. Before digital techniques, blue screens were preferred because, of the three primary colours, that's the one rarest in human skintones.

When digital cameras were invented, they were given greater sensitivity to green light to mimic a bias in human vision. Green screen doesn't require as much illumination of the screen as blue screen does, which prevents the risk of chroma spill onto the foreground subject's edge, which can cause a special effects failure called a chroma halo.

In the pre-digital era, when the foreground-background distinction had to be much larger than is required today (because of the complicated optical process involved in achieving chroma key), it was almost impossible to get away with any colour beyond blue. Nowadays both colours are very common, with green almost the new default; but, unlike the blue-only era of the past, typically both colours are now on standby.

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    $\begingroup$ "When digital cameras were invented, they were given greater sensitivity to green light to mimic a bias in human vision." That's pretty cool. I never would have connected that to the use of green screens. $\endgroup$ – JMac Dec 4 '18 at 20:05
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    $\begingroup$ @JMac Specifically, many digital cameras use a Bayer filter for their colors which provides two green pixels for every one red and blue. That’s a lot of extra sensitivity! $\endgroup$ – MTCoster Dec 4 '18 at 22:13
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    $\begingroup$ I'd have to re-check my sources, but I remember one of the major reasons for blue screens back in the day is that the crystals in the blue sensitive part of the film were smaller, so you got a higher fidelity edge if you used blue rather than other colors. $\endgroup$ – Cort Ammon Dec 5 '18 at 1:08
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    $\begingroup$ Seems like that, late in the analog era, Disney managed to use yellow light: youtube.com/watch?v=msPCQgRPPjI $\endgroup$ – Ismael Miguel Dec 5 '18 at 16:49
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    $\begingroup$ @IsmaelMiguel Ah, the old sodium vapour lamp trick. Too bad they could never work out the kinks. Other neither-blue-nor-green examples are found when filming or photographing models, including the use of red, orange or even ultraviolet light. $\endgroup$ – J.G. Dec 5 '18 at 18:19
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I may misunderstand the question, but the method of selecting the background based on colour you are asking for is called chroma keying.

In digital post-processing, all pixels which are sufficiently green are considered background and hence treated as transparent. What is "green" is configurable, often in HSV colorspace.

J.G.'s answer elaborates why green usually works best. Blue screens are common, too.


From Wikipedia:

Chroma key compositing, or chroma keying, is a visual effects/post-production technique for compositing (layering) two images or video streams together based on color hues (chroma range)... to remove a background from the subject of a photo or video... A color range in the foreground footage is made transparent, allowing separately filmed background footage or a static image to be inserted into the scene. [...] This technique is also referred to as color keying, colour-separation overlay (CSO; primarily by the BBC), or by various terms for specific color-related variants such as green screen, and blue screen – chroma keying can be done with backgrounds of any color that are uniform and distinct, but green and blue backgrounds are more commonly used because they differ most distinctly in hue from most human skin colors. No part of the subject being filmed or photographed may duplicate the color used as the backing.

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  • $\begingroup$ +1 Welcome to physics SE. The question seems about technique and has less physical concept. $\endgroup$ – Stefan Bischof Dec 5 '18 at 3:24
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    $\begingroup$ @StefanBischof, the OP asked two questions, "How?" and "Why?" This answer addresses the how part. J.G. did a pretty good job answering the why. $\endgroup$ – Solomon Slow Dec 5 '18 at 15:10
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    $\begingroup$ Note that the BBC often used yellow chroma-keying for Jon Pertwee-era Doctor Who because green and blue conflicted with the Doctor's outfit and the TARDIS, respectively. You can often see a slight yellow aura around characters in Doctor Who from that period. $\endgroup$ – Eric Lippert Dec 5 '18 at 23:31

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