What is Chroma Keying and How Do Green Screens Work?
20th August 2010 by Simon Malone
A few months ago I was explaining the concept of VirtualStudio.TV to a prospective client when I was taken off guard by a very good question. “What is Chroma Keying and how do green screens work?” It's a great question that I will try to answer in detail in this post.
A green screen is just that, a green screen, but without “Chroma Keying” it is not much use. Chroma Keying is the process by which a specific colour element (chroma) is removed from a video scene and replaced (keyed) with a different element. Essentially it’s the way video producers remove one background and replace it with another. You will have seen hundreds of examples of this in films, such as the kids cycling across the sky in ET, and in TV, as it’s the process used when you see someone presenting the weather in front of a moving map. For us at VirtualStudio.TV this is how we place our presenters within our 3D virtual studios to create Internet Videos and Virtual Events.
Green or blue screens have become the industry colour standard for Chroma Keying since it was invented in the 1940’s. Unlike other bright colours such as Yellow and Red, neither are found within any skin tone and this is very important. For effective Chroma Keying the distinction between what you want to keep (the presenter) and what you want to remove and replace (the green background) has to be made. Therefore using a green or blue screen means there is no chance of the background mixing with the skin tone of the subject. If they did mix the subject's skin would be partly covered by a background as the computer could not distinguish clearly between what to keep and what to remove. There are some more advanced technical reasons why people choose either green or blue screens in their productions, but another simple reason is eye colour. Chroma Keying a close up shot of a person with green eyes using a green screen background would remove the green colour from their iris and replace it with the content you wanted to place behind them – not an ideal result!
That explanation gives an overview of why a green screen is used in the process, but has not explained how computers actually key out and replace elements within video frames. I call it “computer magic” as I am happy to accept that it can happen and at the moment that is good enough for me!
From my experience the main skill in the art of achieving a perfect Chroma Key effect is lighting. Historically green or blue screens have required a great deal of it in order to be effective. Powerful lights are needed to increase the intensity of the backdrop to give a strong consistent colour to work with. Those lights however can make the subject look very dark in comparison to the background, so soft lighting aimed towards the subject is needed to overcome this. Further lights are then required to soften the hard edges created by the earlier lighting effets. This level of lighting is expensive, very complicated to get right and often has a similar effect to a sun bed if stood in front of it for too long!
It is for this reason that the screen in our studio is not green, or blue, but grey! We use brand new Chroma Key technology from a great UK based company called Reflecmedia. Their advanced system uses a grey cloth that is made up of thousands of tiny glass beads that are almost invisible to the naked eye. Green LED light rings placed on the camera lens focus light onto the cloth that is then reflected back into the lens. This instantly produces a very bright green colour in the camera that requires a tiny level of lighting compared to conventional methods. This is just one of the great developments that have been seen in this production method over recent years.
What we do with this intense green reflection is at the heart of our offering at VirtualStudio.TV. Using our technology we are able to remove the green and replace it with our 3D virtual studios. These studios have screens that contain video and PowerPoint Presentations just like on the TV news. Like a “real” studio you can see reflections and shadows. If you move, a shadow moves, and when the screen plays a video you can see a reflection on the floor and on the walls of the virtual studio. We can also zoom in and out without ever touching the real cameras in the studio. How our processing equipment achieves this really is “computer magic”!
I love showing people the technology in our green screen studio and as much as I have tried to explain it here, seeing it in the flesh and getting up in front of the cameras yourself is the only way to fully understand it! Anyone is welcome to come and visit us so please do contact us and we will happily give you the guided tour.
That’s one tricky question answered for the day. Another one popped into my head earlier today after I set our security alarm off for the third time this week. Why is it that we always say “the alarm has gone off” when it has actually gone “on”?!
You may also like these posts and pages:
- Why We Use Reflecmedia Chromatte Green Screen at VirtualStudio.TV
- What to Wear for a Green Screen Video Shoot - 10 Tips
- Have a look at our range of Virtual Studios
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