Apple Shake 4 Tutorials - Manual (Page 200)

200 Chapter 6 Using Primatte
Compositing Outside of Primatte
Primatte has a great deal of flexibility because you can composite within the node or
pass its alpha channel to other layer nodes for compositing. You have the most
flexibility when you do not composite inside of Primatte, because you can control how
and where color-correction and effects are applied to your images. In the following
process tree, the Primatte output is set to “alpha only,” and preMultiply is enabled in the
Over node. In this tree, Primatte does not process the edge with color suppression
operators and the edge stays much cleaner.
Note: Always supply the background image unless you are setting output to “alpha only.”
Although you can output Primatte with a premultiplied foreground and without a
background image, you should supply the background input if possible, as Primatte still
calculates some of this information. If you do not supply the background image, black
ringing occurs around the edges. If this is impractical, toggle replaceMode to use Color
and supply an appropriate Replace Color setting.
This has a corollary: Save yourself the trouble and set your output to alpha only. No
background image is needed. Then do your spill suppression with a HueCurves or
SpillSuppress node. This also ensures your image is unpremultiplied, making it ready to
color correct.