Apple Shake 4 Tutorials - Manual (Page 101)

Chapter 3 Depth Compositing 101
3 In the city_bg parameters, click the button to enable autoAlpha.
A solid white alpha channel is created for the image.
Note: As an alternative, you can also attach a Color–SetAlpha node after the city_bg
node.
4 Click the left side of the ZCompose node to load the corrected composite into the
Viewer.
Adjusting Z Depth
When you combine the effect of two or more Z channels, as in this example, the
distance values do not always correspond to the result you want in the composite.
The Color–Mult node (not lMult) includes a depth parameter that lets you “push” or
“pull” the depth values. You don’t need to do this for the current example, but it’s
worth noting for future reference.
To adjust the Z depth, add a Color–Mult node to an image, then adjust the depth
slider. Give it a try and you’ll see the image forward or backward according to depth.