User guide

KeylightThe Foundry
ADVANCED KEYING
18
Biasing
Biasing
What’s biasing all about? Biasing in Keylight was originally developed for a
shot in the motion picture "Executive Decision". The foreground consisted of
reddish browns, but a combination of factors led to the 'green screen' being
lit so that its primary component was actually slightly red.
Figure 21. Is this the worst green screen you’ve ever seen?
So what happens when we pick the screen colour? Well because the screen
was 'red', as is the foreground, our pilot ends up being keyed out as shown
in Figure 22.
Figure 22. Default key showing the transparency of the foreground.
Not a great result, I’m sure you’ll agree, and much pressure was applied to
the lowly programmers to get around the problem.
A work around to this is to manually colour correct the image so that the
background is properly green, pull the key from this corrected image, then
'un-correct' the result of that so that the foreground colours match the
original. A corrected image would look something like the one shown in
Figure 23 on page 19.