Apple Shake 4 Tutorials - Manual (Page 194)

194 Chapter 6 Using Primatte
By default, Primatte replaces the alpha channel, but you can also add, subtract, or
multiply the two masks. With this technique, you can create several masks and
combine them in Primatte. Since there is no incoming mask in this example, the
arithmetic parameter has no effect.
In this example, there are three primary areas where the blue spill is apparent: The
edge of the woman’s hair, the shadows on her shirt, and possibly the little transparent
bit of her skirt at her knees. Each time you use the foreground operator, you create
more areas that you must spill suppress. On the flip side, the fewer foreground
operators you use, the more rotoscoping or paint touch up you must do. An
alternate—and common—way to handle this is to target specific areas of the image by
pulling multiple keys with separate nodes, then combine the keys into one node.
Spill Suppression in Primatte
Spill suppression is the removal of reflected light (blue or green spill) on the
foreground material that comes from the bluescreen itself. In this example, blue spill is
immediately evident on the woman’s white shirt. Although the suppression tools in
Primatte are interesting and useful, as a general principle you are encouraged to
perform the suppression outside of the Primatte node using nodes such as HueCurves,
SpillSuppress, and ColorReplace. After all, why would you want to tackle two difficult
tasks at once?
To use the spill sponge operator:
1 In the Primatte1 Parameters tab, click the spill sponge operator button.