Apple Shake 4 - Manual (Page 685)

Chapter 24 Keying 685
Next, filters are attached to the Primatte node. A DilateErode is added, and the xPixels
parameter set to 1 (this closes up any holes in the alpha channel). You can also use a
Median filter to do the same thing. A second DilateErode is applied, with the xPixels set
to -5. This eats away at the matte. This filtering process cleans up the matte, making it
more solid. A Blur softens it, and is fed into the HoldOutMatte input (the third input) of
the Keylight node. The result is a solid matte with soft edges.
An Alternative for Making Hard Mattes
Copy your Keylight node and boost the screenRange to .3 to harden the matte.
A Matte Touch-Up Tool: The KeyChew macro, covered in Chapter 32, “ The Cookbook ,”
is also a good tool for duplicating the DilateErode chain in the above example.