Apple Shake 4 - Manual (Page 527)

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Using Masks
This chapter describes how you can use masks in Shake
to create transparency and to limit the effects of other
functions within your node tree.
About Masks
Masking is the process of using one image to limit another. This typically takes the form
of assigning one image to be used as an alpha channel by another. Masking in Shake
can also take the form of using one image to limit the effect of a particular node in the
node tree.
Masking is closely related to keying. Keying is a process for creating (pulling) a matte,
typically using color (green or blue) or brightness (whites or blacks) information from
the image to mask that image. Masking is even simpler—it’s simply assigning one
image to be used as a matte to another image or operation. (For more information on
keying, see Chapter 24, “
Keying ,” on page 681.)
Masks in Shake are extremely flexible, and can be combined in any number of different
ways. You can create masks that add to, or subtract from, the existing alpha channel of
images anywhere within your node tree.