Apple Shake 4 Tutorials - Manual (Page 184)

184 Chapter 6 Using Primatte
3 Add a Key–Primatte node, then connect the bg node to the second input (background)
of the Primatte node.
In Primatte, you select a “center” value that is the average color of the key you want to
pull. (The concept of center is explained in “
Inner Mechanics of Primatte ” on page 186.)
Unlike Keylight, Primatte is ready to scrub your background color as soon as you add
the node.
4 In the Primatte1 Parameters tab, click the center color control to select it, if it’s not
already selected (a yellow border appears around the center box).
5 In the Viewer, drag across the bluescreen in the background.
When you drag (or scrub) over the background, Primatte calculates an average (or
center sampling) of the scrubbed pixels. Note that in the Viewer, nothing appears to
change. This is because, by default, only the alpha channel is modified, as indicated in
the Primatte output parameter.
6 To view the alpha channel, position the pointer in the Viewer and press A.