Why do programmes that use computer graphics use a blue screen as background?

Both blue and green are regularly used these days.

Traditionally blue was used for most keying (the process of lifting the foreground from the screen) as of the three colour channels used in film and tv, there is the least blue in skin tones.

Now that keyers are better green is often preferred and although there is some green in skin tones, green is easier to light and gives less problems with the edges where the keyed foreground is joined to the background.

Red can also be used for keying, but almost exclusively on models, green/blue spaceships to be keyed over various backgrounds.

You can also key on brightness, or any difference between the foreground and background.

There is also the issue of 'spill', blue being a darker colour reflects less of the background colour on to the foreground subject, green casts more, but with the colour suppression tools available in CG systems these days it is not a problem.

There is also the issue of how much of the signal each colour takes, which favours green, and also the amount of grain (in both film and TV) associated with each channel, blue being the most grainy, this again favours green.

The colour for keying people is mostly decided by their costume, but given the choice I prefer green.

Beyond getting a good 'clean' background to key off, the other important thing is to light the subject so that they are lit in exactly the same way as the background.

I have been a visual effects supervisor for over ten years, and worked in computer graphics for around 25 years.

Did you by any chance watch 'The Apprentice' last night? My partner did, quite a fun idea, but no way of making money. They made the usual mistake of calling the system 'special effects', which is the wrong term, that applies to physical effects such as explosions on film/TV sets, and also has major health and safety implications for the crew. I point out that my visual effects are definitely not special and I have no responsibility for other members of the crew's safety.

An example of a green screen from a shot I did a few years ago.

The man with the ball on a pole is there to give the actors in the background an eyeline to follow for the position of the mermaid. Very important that all the actors look at the effect when they should, or it shows up straight away that it was not there.

A still from the finished shot:


In addition to the actress keyed off the background there were a number of computer generated elements, such as the tail, and also a number of other elements such as shots of water in a tank, bubbles, fish, etc. to add in.

Why bother adding the water as well as mermaid character, just think of the weight of water a tank that size would be, how long it would take to clear, and the thickness of glass (double layered for safety) and how little you would see through it.

Brilliant pictures and information, thanks!!

Jon