Keylight

Keylight

The fi rst decision in Keylight is most important: sampling
a color for the Screen Colour setting (Keylight reveals
its UK origins at Framestore CFC with that u). Specifi c
tips to do so are up ahead in the section, “Generate the
Screen Matte.”
In the best-case scenario, you create any necessary garbage
mattes and then follow these steps:
1. Use the Screen Colour eyedropper to sample a typical
background pixel. View defaults to Final Result so you
get a matte instantly; set the background to a bright
color and solo the plate layer, or examine the alpha
channel (Alt+4/Option+4).
2. If in doubt about the Screen Colour setting, turn the
effect off and set another instance, repeating as necessary
until you have one that eliminates the maximum
unwanted background ; you can then
delete the rest.
Now, as needed, look for areas to refi ne.
3. Switch View to Status. Opaque pixels are displayed as
white, transparent pixels are black, and those containing
any amount of transparency are gray.
It’s an exaggerated view of the alpha channel matte.

4. Still in Status view, you have the option to try Screen
Balance at settings of 5.0, the default 50.0, and 95.0,
although Keylight will preselect it based on your background
color selection. This setting controls the weighting
between the primary matte color (blue, green, or
red) and each of the secondaries.
5. If the background is not solid black, you can boost
Screen Gain until the gray mostly disappears in Status
view, although the ideal is not to raise this value at all
(the next section explains how). Use this setting as
sparingly as possible.
6. Optionally, set the Despill Bias using the eyedropper.
Sample an area of the foreground that has no spill and
should remain looking as is (typically a bright and saturated
skin tone area). I very rarely do this (see Notes).
The rare perfect footage is now completely keyed. If it isn’t
perfect, this is a decision point; how can you
best divide this matte into multiple passes?

The Apple Shake-based implementation of Keylight adds
inputs for a garbage matte and a holdout matte. This
method of isolating and focusing on the edge inspired
me to devise the following workfl ow to achieve the same
result in an After Effects precomp, with as few extra steps
as possible.