How does a Green Screen Work?

It’s one of the most entertaining things to watch, people acting in front of a blank green screen and suddenly the greenscreen.jpgworld behind them is blowing up in all directions. But how does it work? 

Well, the green screen is an integral part of the special effects process known as Chromakey. Chromakey allows television producers and movie makers to use advanced technology to superimpose their subjects onto an virtual background. With digital processing, it gives the editor a solid background that's easily removed by software.

Chromakeying, sometimes known as colour keying, is the process of singling out a particular colour in an image and then using computer software to make that colour transparent. This allows another image, such as an animated weather map on the weather forecast, to show through.

Why green? Green is often used because it's easy for most modern cameras to pick up due to the colour contrast, giving the editor the cleanest possible image. It's also a less common clothing colour, and is therefore less likely to cause any embarrassing disappearing outfits and floating heads. Blue was once common but, as with white which is never used, it's common in wardrobes in eye colours.

Green screens are more prevalent at the moment, largely thanks to the fact that the sensors on today's digital cameras are more sensitive to green than any other colour. Green also requires less light to properly illuminate. That said, blue screens still get a lot of usage, especially if the subject being shot needs to wear a green costume!
To select a good colour, pick one that's not going to be found on anything else in the scene. Consumer video software with chromakey capability however, (Adobe Premiere for example) will allow you an option to select any colour to be removed.

greenscree.jpgTips for recording with a green screen

1. Avoid shadows. Shadows change the perceived colour by the camera, which means the green screen won't be properly replaced in editing. When you are shooting a full body action shot, ensure the screen and subject are perfectly lit, to keep the shadows to a minimum. 

2. Ensure the screen surface is flat and smooth. You can use a wide number of surfaces, from paint to fabric to paper, but no matter which you opt for, you really need to make sure there are no wrinkles or folds that could impact the appearance of a seamless single colour background.

3. If your camera automatically sharpens your image, turn that feature off. This artificial sharpening may help your subject look better in natural shots, but in chroma key instances it actually makes it harder for software to separate the subject from the background.

4. Watch out for reflections, both on your subject and on props. Shiny things like glasses or metal that reflect the light could pick up the colours of the green screen, making it more difficult to key correctly. Also watch for blonde hair, which can struggle on a green screen – either use a blue screen, or throw a magenta gel on the backlight to counteract the green.

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