Apple Shake 4 - Manual (Page 688)

688 Chapter 24 Keying
The following examples use the woman.iff and bg.jpg files in the /Tutorial_08/images
directory. Notice that there is quite a bit of blue spill on the woman’s shirt. In the
following tree, the Primatte output is set to alpha only; a holdout matte is created for
the line under her arm (with a QuickPaint node); and foreground and background
scrubs are added. The composite is done with an Over node that has preMultiply
enabled. Notice the blue spill that remains:
Although it’s tempting to think that it would look better if you switch the Primatte
output to comp and turn off the preMultiply in Over, this isn’t the case. This has the
unanticipated result of turning your blue edges into black edges that are actually more
difficult to remove. Additional techniques to help correct this are discussed below.
To avoid blue spill correction in your keyers, the following examples contain several
sample trees to help you.