Apple Shake 4 - Manual (Page 686)

686 Chapter 24 Keying
Another way to combine keys applies only to the Primatte node, which features a
useful arithmetic parameter. Normally, when you pull a key in Primatte, the alpha mask
is replaced in the foreground image. When the arithmetic parameter is switched from
replace to add, multiply, or subtract, you can combine the mattes within Primatte. In
the following node tree, an initial key is pulled with ColorReplace, with the affectAlpha
button turned on. The resulting alpha channel is then inverted, without inverting the R,
G, and B channels, using an Invert node. The inverted alpha channel is then combined
with the Primatte-generated alpha channel by setting the Primatte arithmetic
parameter to Add.
The final way to combine keys is to use layer nodes. In the following tree, keys are
pulled using two different nodes and then combined with a Max node, which takes the
greatest pixel value. The elements are combined with a KeyMix node. Note that the
KeyMix does not get an alpha channel, since neither clouds1 nor blonde has an alpha
channel. KeyMix only mixes the two images through the mask you have pulled.