Apple Shake 4 - Manual (Page 538)

538 Chapter 19 Using Masks
A slight problem occurs when you try this using the Over node. A matte line appears
between the two masks.
Fortunately, in Shake, there’s always a different method you can try. This problem is
easily solved by substituting a different node for the Over.
A better way to combine the sign and car masks:
1 Select the Over node, and Control-click IAdd.
The Over node is replaced with the IAdd node, and the line disappears.
Next, you can put the woman outside of the new mask IAdd1 using the Outside node.
Since this technique was used in the previous example, try a different approach. Use
the Atop node—similar to Over, except the foreground only appears where there is an
alpha channel on the background image.
2 Add a SwitchMatte node, and connect the KeyMix1 node to the Foreground input of the
SwitchMatte node.
3 Connect the IAdd1 node to the Background input of the SwitchMatte node.
The alpha channel is copied from IAdd1 to KeyMix1.
4 In the SwitchMatte node parameters, enable invertMatte to invert the mask (since Atop
only composites in the white areas in the background alpha mask).
5 In the SwitchMatte node parameters, disable matteMult.
6 Select the woman_pre node and apply a Layer–Atop node.