Colour Difference

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Intro

  • 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

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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.

Maths

This Technique was used with optical printers before computers. The original maths was: i) if (R < G) E = B - G ii) if (R > G) E = R - B

General Notes

One problem with a difference keyer is that it often does not pull the inside of the matte far enough into the foreground element. This happens when either of the foreground colour channels are more than the background colour, and so the maths creates a '0' value matte. The less saturated (i.e. grey) the foreground is, the less likely this is to happen, as the background value is going to be closer to the foreground values. PreTreating Plates is a solution in this situation.

Links

External Links


Created by ome. Last Modification: Friday 02 of January, 2009 09:36:32 GMT by ome.
List of attached files
ID Name desc uploaded Size Downloads Actions
100 default Hue_diffKey.comp Fusion 5 Wed 16 of Aug., 2006 08:01 GMT by ome 10.35 Kb 2465 View Download