About Chroma Keying

The terms 'Blue Screen', 'Chroma Key' and 'CSO (Colour Separation Overlay)' are commonly used to describe the same process although the techniques differ slightly depending on their origin. The basic principle behind the technique is this - an actor, or an object, is filmed in front of a plain colour background, then using a process described as keying the solid colour is removed from the image and replaced with an alternative image.

Television weather reports offer perhaps the most common chroma key example - the presenter is seen to stand in front of a computer generated map showing moving clouds, sunshine and wind icons.

Here the presenter is stood, not in front of the computer generated images but, against a solid colour screen. A composite image is then produced from the two sources to generate what we see on television.

Blue and green are the most used colours as they are furthest away from flesh tones. The choice depends on skin colour and the colour of any objects to determine the cleanest key. Issues such as costumes and even eye colour need to be carefully observed!

To achieve the desirable effect has traditionally taken significant time and resources. To work properly the coloured background has to be evenly lit - to achieve consistent colosur across the area. The problem with this is that conventional fabrics, particularly blue, absorb a lot of light and therefore it takes significant amounts of light to achieve the required effect. The time it takes to rig such lighting (not to mention the purchase and electricity costs) is significant - a typical medium sized studio may require 40 kilowatts of light. Perhaps more of a problem can be the effect that this light has on the actor or object in the foreground.

The light reflected from the background acts as a backlight to the subject - and for example arms, legs and particularly hair pick up this light and are tinged with the same colour as the background. Keying devices have difficulty in removing this colour and as a result there is fuzziness on the borders between the inserted background and the real foreground - often referred to as fringing or spill.

Another common difficulty with the process is the problem of shadows - any shadows generated by the foreground subject or other people and devices will result in the background colour changing to a darker shade. The keying device will therefore have to remove a greater spectrum of the colour to separate the foreground from the background - this often starts to cause problems in the definition of the foreground subject.