Chroma keying is the technology of “keying” or cutting a specific colour out of your image. The colours most commonly associated with chromakeying are green and blue and is more commonly recognised when referred to as greenscreen or bluescreen. This allows you to replace elements within a shot or even use virtual studio. Green is probably the most common colour for any keyed element as it does not match any natural skin tones or hair colour. When there may be a green element, such as a prop or costume within a scene, blue screens are then used. This creates a separation between the colour you are keying and the elements within your scene.

Chroma keying relies mainly on two components, the coloured screen or object that will be keyed out and the chroma keyer itself. Whilst many software applications allow you to key out a colour in post production there are also a range of hardware devices that help you to achieve this with live video signals.

The most important part of any chromakey is the lighting. Uneven lighting makes keying much more challenging as you have to increase the range of the key to cut everything. When increasing the range of the key, you can end up cutting out other parts of your scene. For the same reason, if you are using a cloth background, you want to make sure there are no wrinkles or folds in the backdrop.

You also want to have a good separation between your foreground and background as standing too close to the background can cast a shadow, again making the keying process much more difficult.

You would generally want to shoot with a camera that can record in a 10 bit format as this allows for greater colour information. 10 bit files can store over billion colours whilst 8 bit files store roughly 16 million. When we want to accurately define a specific hue and shade for keying, the more information we have to work with the better.

Now that we have our video containing our subject in front of the green or blue background we need to key that colour out. We can do this in two ways, either with a hardware keyer, mostly used in live productions like weather segments on the news, or in software, where we might be adding VFX to a production.

Hardware keyers are very powerful devices that can take an output from your camera and combine them with another video signal, such as a virtual studio. Some hardware keyers are so sophisticated they can even key through translucent objects like glass, which is notorious for reflecting spill from the background.

Software keyers can work in different ways often allowing you to pull a key based on different factors like Hue, saturation and luminance, also known as an HSL keyer, or on specific Red, Green and Blue pixel values, known as an RGB keyer. Once we have keyed the background we have created an alpha, or transparency, channel in the image. We can then overlay this on top of any background we wish.