User guide

KeylightThe Foundry
ADVANCED KEYING
19
Biasing
Figure 23. Colour corrected image that would give a better key.
The green screen is now strongly green and distinct from the foreground
colours. Notice also the red cast on the pilots mask has been removed and
turned into a neutral grey.
This is effectively how the Keylight developers got around the problem.
They introduced the concept of a 'bias' colour, which is a colour cast that is
removed from the source image and screen colour, then a key is pulled from
this modified image, then the colour cast is put back. In essence this
automates the 'work around' described above, however it is done in a way
that does not slow Keylight down at all. For our Executive Decision shot, an
appropriate colour is the red cast on the pilot's mask in the source footage.
Setting our bias to this now gives us the far better result as shown in
Figure 24.
In this situation, the Despill Bias can help. By decreasing this value slightly,
you scale down the red component in the screen colour and the image. This
flips the screen colour of the shot from red to green, leaving the pilot
predominantly red. Figure 24 shows the new composite.
Figure 24. Result of dropping the Despill Bias to -17.
Keylight has now keyed this image correctly. We have effectively instructed
Keylight to see “less red” in both the screen colour and the foreground.