It doesn’t matter whether it’s the Marvel Cinematic Universe, Forrest Gump, Jurassic World or productions at the other end of the spectrum like weather forecasts. Green screens are used a lot more than you probably think they are.
It’s more than likely you understand the existence of green screens. They’re used to create stunning visual effects, not just in the backdrop, but often on the actual actors or presenters in a shot.
Have you ever wondered, though, why are green screens green?
Well that’s exactly what we’re going to discuss in the following post. First, though, it is worth explaining what a green screen is and looking at how do greens screens work, before we look at why they are green.
What is Chroma Key?
Chroma Keying is a special visual effects process used in the post-production stage that involves the isolation of a specific level of brightness or colour in a digital image and then making it completely transparent.
This then allows for another piece of footage or image to be placed underneath the colour that’s been removed. Although it’s most commonly referred to, at least in this modern age, as green screen, chroma keying can be achieved with any colour, not just green.
In fact, in the past, blue screens were used more commonly. Although it seems like something extremely comprehensive and modern, the first use of blue screen/chroma keying in Hollywood was for a film called The Thief of Bagdad, made in 1940.
Now that we’ve looked at how chroma keying works, let’s look at why green of all the possible colours is used for those magical screens.
Reasons Why Green Screens are Green
So, following on nicely from the above, the next question you are probably asking yourself is do green screens need to be green for a reason? Well, in theory, the screens used in chroma keying could be any colour.
It really doesn’t make much difference when it comes to the actual effect or technique. So why are green screens green? This is where it all gets fascinating and quite technical.
The Main Subjects in a Scene or Piece of Footage
The first and foremost reason why green screens are used in chroma keying rather than any other colour has to do with the main subjects being filmed for a specific scene or piece of footage. In other words, human beings.
One thing that most humans have in common is they don’t have very much green in either skin, hair or even clothing (unless they are wearing green clothes, obviously, more on that in a little bit though).
This means that the colour green is far easier than others that we do have in our skin, hair and clothing more commonly to remove and replace without replacing some of the subject.
The Technical Side
Another, more technical reason why green is used in the screens used for chroma keying concerns how well digital sensors work. You see, as good as they are, digital sensors don’t record red, greens and blues with the same amount of detail as each other.
The pixels in a digital sensor are only able to pick up the total intensity of the light. Therefore, each pixel is equipped with a filter that allows only a specific colour to get through. So that you have blue, red and green pixels.
The resulting full colour image, where all the pixels have the fullest range of colours, is created based on the limited information provided. Interestingly, our vision is more suspectable to green than the other colours.
So, they take the approach of using a sensor that’s divided into separate blocks of 2×2 pixels and equipping two with green filters and two with blue and red. This is more commonly known as a Bayer filter.
As a result of using this kind of filter, in the raw data of the image there is more green than blue or red, allowing the green features to have a much more detailed look, or higher resolution.
Chroma keying is affected by how accurately the edges between the background screen and the foreground subjects are recorded. This means that the higher resolution of green enables precise background cut-outs.
Before digital film was invented and analogue film was used, blue screens were used in chroma keying because the chemical makeup of analogue film produced higher levels of detail with reduced grain for anything that was blue.
When a Green Screen is Not Used
As noted earlier, green screens are used specifically because as humans we have fewer green tones in our skin, hair and even clothes. However, there are times when blue screens are still used. Take the movie Spider-Man from 2002, whose main villain was the Green Goblin.
Perhaps you can see where we’re going with this one.
The filmmakers had to switch to blue screen for all special effects’ shots involving Green Goblin. However, anyone who knows a little about Spiderman will know that a pretty large part of his costume involves one colour – blue.
This meant that they had to switch back to green for shots involving the web-slinger. As you’d imagine in a film where your friendly neighbourhood web crawler is fighting the evil Green Goblin, both are going to be in the same shot.
Hopefully you now understand a bit better how green screens work and why the colour green is used. At From The Hip, we have our very own fully equipped and specially designed green screen studio that can be used to add great visual effects to your video footage.