Chromakey

Chromakeying is a technique where a particular colour in video footage is removed and replaced with another video or image. Examples include weather broadcasts where a plain background behind the presenter is replaced with a weather map. The colour being removed can be any hue, but typically it is blue (blue screen) or more commonly green (green screen). The reason for blue and green is simple – skin doesn’t contain any blue or green tones and so remains unaffected by the colour removal. Of course, it’s important to ensure any presenters or actors are not wearing any garments the same colour as the background or these will magically disappear too! There are special blue or green suits that can be worn by actors who need to be removed from the scene – typically they are used to secretly direct animals or hold props in mid air for magical effects.

a dog against blue screen

blue removed and a new background added

Blue and green screens work very effectively but green tends to be a better choice. There are several reasons for this. Firstly, green is a much brighter colour and software finds it easier to find and remove, particularly when it’s not lit very well. Secondly, cameras tend to be more sensitive to green and have more room set aside to record green information. And thirdly, people tend to wear more blue clothing than green, so it’s a little more practical. The Media & Innovation Lab (MILL) has recently upgraded its studio chromakey with a very pure blue hue, which does work much better than the old paint, which was a standard shade from a hardware store. The reason blue was chosen was quite simple – green is quite garish and as the studio walls are often used just as a plane background, it seemed sensible to stick with the blue.

Most video software has some kind of keying filter for removing blue/green backgrounds and these are tending to get better and better. Final Cut Pro X has a particularly effective one that does a remarkable job, even on default settings. The general rule for good chromakey is to make the blue or green as bright and as consistently lit as possible and to keep the foreground objects sharp and well defined. Blurred images or fast moving objects (which cause motion blur) can give poor results because of the blurring – the software finds it difficult to decide what exactly is the blue/green background and what exactly is foreground. Fine hair also gives similar issues.

blurring problem: what’s foreground and what’s background?

Another issue is the video recording itself – most cameras record the brightness and colour information at different qualities, so although there may be lots of detail in the picture, the colour information is much cruder and this affects the ability to isolate foregrounds from coloured backgrounds. High-end cameras record brightness and colour at similar qualities that leads to finer quality removals.

Chromakeying is used extensively for special effects in movies and television, where coloured backgrounds can be replaced with computer generated cities or worlds. TV news use it also – news readers occupy small blue or green rooms which are replaced with much grander computer generated studios. High-end chromakey software and hardware is much better at dealing with blurring and shadows so that the foreground fits seamlessly into their new artificial background. Here’s a few examples – some are quite astonishing.

The MILL recently shot several films against blue screen for use in the Shareville project at the School of Health Sciences. Shareville is a web-based virtual town developed by Birmingham City University with a college, care home and range of colourful characters. These characters are played by actors filmed against blue/green screen, which is removed and replaced with (quite elaborate) computer generated rooms and environments. When lit properly, the characters do look as if they’re actually in the locations rather than against a coloured wall. This was the first big project using the MILL’s new blue screen walls and flooring and so far, the blue seems to be coming off really well. The only problems coming up are caused by the relatively low quality colour detail in the recording, causing a little roughness on foreground elements, especially hair, and dark shadows under feet, which are staying as black blobs.

Filming Shareville against bluescreen in the MILL’s TV studio

Other uses for chromakey at City have included presentations where slides display behind the presenter and interviews shot against blue and then replaced with more appealing backgrounds.

The alternative to chromakeying is to draw around every foreground element for every frame of video, and cut it out from the background – a process known as rotoscoping. This gives fantastic results but is a very laborious process. However, this is sometimes the only solution in features, especially when the character steps outside the coloured background or the blue/green screen isn’t clear enough because of dust, smoke or mist.