Apple Shake 4 - Manual (Page 701)

Chapter 24 Keying 701
6 Now, attach the outputs of the LumaKey and the Keylight nodes to a Max node, to
combine both alpha channels into one.
The above screenshots show the results of each individual key, combined into the
single alpha channel below. As you see, the holes in the LumaKey matte are filled by
the Keylight matte, and the loss of detail around the edge of the Keylight matte is
restored by that in the LumaKey matte. These keys can also be combined in many other
ways using different layering nodes and rotoshape masks to better control the
combination.
The LumaKey matte The Keylight matte
The combined LumaKey
and Keylight matte