Apple Shake 4 - Manual (Page 425)

Chapter 15 Image Processing Basics 425
The result is identical to munch_premult—the RGB is multiplied by the mask.
Next, invert the foreground alpha channel ( 1-A ).
3 To do this, select the munch_mask node, then Shift-click the Color–Invert node.
The Invert node is attached to the munch_mask node as a separate branch.
The next step in the formula ( (1-A)*Bg ) calls for you to multiply the background by
the inverted alpha.
4 In the Node View, select the Invert node, then click Layer–IMult.
An IMult node is added to the Invert node.
5 Connect the bg node to the Background input of the IMult2 node.
Next, the crucial step—add the two results together.
6 In the Node View, select IMult1, then click Layer–IAdd. Connect the IMult2 node to the
background input of the IAdd node.
Comp = (Fg * A) + ((1-A)*Bg)
Tree IMult2