- Colour Difference keys create a matte based upon the difference between colour channels
- Colour Difference keyers are very good at defining fine detail without creating too much noise (Good for hair and transparent objects like glass).
- They are limited in the range of selection (i.e. variations of colour or luminance in the screen).
- They create low contrast mattes that must often have the black & white points moved (in levels or curves or clamp) to create pure blacks or whites for the matte.
RGB Difference Keyer
This ia a RGB difference key against green, the most common difference key. Although good results can be created using simpler between two different channels by subtracting one channel from another. This method is explained in detail in Steve Wright book. Ultimate was the first digital colour difference plugin keyer. Although the maths has be now been improved, and the current Ultimatte/AvantEdge plugin appears to be a hybrid keyer.