Screen Gain

Screen Gain

Before I describe Screen Gain, a disclaimer: The ideal
Screen Gain setting is 100, no change to the default. This
adjustment is compensation for a poorly lit matte, and
while raising it may make the matte channel look better,
you are also likely to see increased color grain and lost
edge detail with values above the default.
A clear symptom that the background lacks suffi cient
intensity is that areas of the background are of a consistent
hue yet fail to key easily. Similarly, a clear symptom that
your foreground is contaminated with refl ected color from
the background is that it appears semi-opaque. Screen
Gain is designed to help in these cases.
Screen Gain boosts (or reduces, although I’ve never used
a setting lower than the default) the saturation of each
pixel before comparing it to the screen color. This effectively
brings more desaturated background pixels into the
keying range.
A major point of holding out the edge using the three-pass
method described above is to avoid having to use Screen
Gain whatsoever; it can be useful to quickly refi ne a crude
one-pass key.



Screen Balance
Keylight is designed to expect one of the three RGB color
values to be the dominant background color. It is even
more effective, however, if it knows whether one of the
two remaining colors is more prevalent, and if so, which.
Screen Balance compensates for a dominant secondary
background color.
On this theory, you would employ a balance of 95% with
blue screens and leave it at 50% for green screens, and in
version 1.2 of Keylight, that is exactly how the plug-in sets
Screen Balance depending on whether you choose a blue
or green Screen Colour setting. I hardly touch this control
because Keylight added this automatic adjustment in version
1.1.
The generalized recommendation from The Foundry is to
set it “near 0, near 100, and compare” these to the default
setting of 50 to evaluate which one works best. In other
words, imagine there are three settings (instead of 100)
and try 5%, 50%, and 95%.
Bias
The Bias settings are Despill Bias and Alpha Bias. These
color correct the image in the process of keying by scaling
the primary color component up or down (enhancing or
reducing its difference from the other two components).
The Foundry recommends that in most cases you leave
Alpha Bias at the default and that you click the Despill Bias
eyedropper on a well-lit skin tone that you wish to preserve;
despill pivots around this value.
Like Screen Gain, Bias has an unpleasant side effect: it
can signifi cantly increase the graininess of keyed footage.
There are other despill methods available (later in this
chapter) that don’t add this complication.
Refinement
When you spot an area that looks like a candidate for
refi nement, save (to hold an undo point should you need

to use File > Revert), zoom in, and create a region of interest
around the area in question.
Now take a look at the tools provided by Keylight to
address some common problems.
Holes and Edges
The double-matte method (core and edge) shortcuts a lot
of the tug of war that otherwise exists between a solid foreground
and subtle edges. Even with this advantage, both
mattes may require adjustments to the Clip White or Clip
Black controls.