Apple Shake 4 - Manual (Page 432)

432 Chapter 15 Image Processing Basics
This example uses the KeyMix node, which handles unpremultiplied foreground
elements, the background, and a mask to split between the two. Enable invert in the
KeyMix parameters (you could also switch the foreground and background inputs).
If you are using a premultiplied image that was rendered straight out of a 3D
animation package, there are special tools that unpremultiply and premultiply the RGB
channels by the alpha. These tools are Color–MDiv and Color–MMult. “MDiv” is “Mask
Divide,” and “MMult” is “Mask Multiply.” Used in a node tree, MDiv unpremultiplies the
image, then you color correct, and then premultiply using MMult. When using a
premultiplied image as the foreground, the tree should look something like the
following example.
Identical tree using KeyMix
Identical tree with a premultiplied foreground