Chroma Key: Feeling Blue?

greenscreen_vs_bluescreen

Green screen and blue screen technology uses a process called “Chroma Keying”. Shoot your actor or object on a different coloured background and then use a keying program in an edit suite to remove that colour. When it is removed, you will be left with a moving cardboard cut-out called an ”alpha channel”. Then you can add in any background you wish behind this alpha channel. Of course you wouldn’t want to film a green plant against a green screen or you would remove all green colours in the image. You would have any empty plant pot as all of the leaves would be removed in this process, for example. The editing program would remove the green background and a lot of the plant as well. For this occurrance, and others, you would use a blue screen instead but more on that later in this article.

Colour Channels

Green screen, over blue screen, has emerged as the more dominant chromakey colour over the past 15 years because of the rise of digital cameras and the internal methods and processes used in them. In most digital cameras, the green channel is the most luminous with better definition from other colours. Green screen generally out performs blue screen extraction in many cases. There is also a lot more blue, naturally occurring in real life than you would expect. Clothing, dark circles under eyes and shadows often carry a lot of blue too. Edit programs work by trying to key out differences in chrominance (Which is the tone of colour of the background) but more modern programs also work, and take reference from the luminance (the difference in brightness)

Colour Spill and Reflections

Lighting, colouring, skin tones and reflections are huge factors in digital video film making and production in general. The reflections and ‘bounce’ from blue and green can affect the time needed in post production. Green screens downfall is that it makes warmer skin tones look washed out. There are several ways to counter act this such as minus green or magenta gel over low level lighting to lift the skin colour. Also current edit programs have some great colour and spill corrections now, which can help massively. Depending on the type of shoot and the background colour then correction can be simple to do. Keying all green tones from an image can make any colour with green in it look odd, just as removing all the blue can change the look too.

A New Age of Green Screen

Black, WHITE AND GREENSCREEN

Filmed to be projected onto Croydon’s buildings

Digital cameras are taking over from film. Digital lighting sensitivity and image processing of colour channels (RGB) have increased dramatically. Lighting sensitivity affects noise levels and grain in the image and the image processing affects the actual resolution of that colour channel. Both are vital elements in getting workable, and swift, keying results without too much pre-keying corrections (increasing brightness, luminance, tone etc).

Of the RGB channels that make up any digital image .The green channel is the cleanest channel generally. It offers the greatest luminance and sends the least film grain, or noise, over that channel.

A process that occurs within the cameras body is something called Bayer Patterning . The Bayer Pattern on most new camera sensors records double the amount of green pixels compared to blue or red. Recording formats are factors to think about. Recording uncompressed or “raw” data at 4:4:4 then you’ll see the most optimal quality image.

With these uncompressed file types offering un-paralleled definition and detail You’ll see more pixels which will give the edit program an easier time as it works to define what’s the subject and what is the background. Post production, VFX and CGI has opened up a world of digitally imagined possibilities and using chromakey green continues to open up possibilities for film makers.

Written by Sam Parkinson, Studio Manager

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