Apple Shake 4 - Manual (Page 534)

534 Chapter 19 Using Masks
A better way to mask a series of concatenating nodes:
1 Disconnect the masks from the previous example.
2 Select the Mult node and add a Layer–KeyMix node.
3 Connect the car_bg node output to the KeyMix node’s Foreground input (the second
input).
4 Connect the CornerPin node to the KeyMix node’s Key input (the third input).
This eliminates the above problems. The following images compare the two resulting
renders. In the right image that used the KeyMix node, there is no blue ring around the
soft part of the mask area.
Masking Transform Nodes
You can also use masks to isolate transforms.
To mask a pan to create depth between the street and the hood of the car:
1 Select the CornerPin node, and apply a Transform–Pan node.
2 Select the CornerPin node again, and Shift-click Layer–KeyMix.
A KeyMix2 node is added to the CornerPin node as a separate branch.
3 Connect the Pan node to the Foreground input of the KeyMix2 node.
4 Connect the car_mask node to the Key input of the KeyMix2 node.