User guide

KeylightThe Foundry
ADVANCED KEYING
20
Biasing
The Bias Colours in
everyday use
It also turns out that the bias colour is actually useful for situations without
strong casts, typically where there is some colour spill around the edge of
keys. By setting the biases to the main colour that occurs near the edge of
the foreground (typically flesh tones or hair tones), you allow Keylight to
better discriminate between foreground and background.
Why are there two
Bias Colours?
Remember that Keylight does two things, calculates a transparency and
removes the screen colour from the foreground. By default one bias value,
the 'Alpha Bias', is used for both operations. This works fine in most
situations, for example, the Executive Decision shot above.
However, sometimes you can select a bias that gives a great alpha, but
performs a poor despill, and another bias that gives a great despill, but a
poor alpha. Consider the blue screen from the TV series Merlin, courtesy of
CFC Framestore shown below in Figure 25.
We pick the strong blue of the background without choosing an alpha bias,
and end up with the lovely alpha shown in Figure 26, but the despill
resulting from this key is poor as shown in Figure 27 on page 21.
Increasing the alpha bias will give us a better despill, but this destroys our
nice alpha. The way around this is to turn off the 'Lock Biases', which gives
you a separate bias factor to use solely for despill calculations.
Figure 25. Merlin blue screen. Figure 26. Nice Alpha.