Apple Shake 4 Tutorials - Manual (Page 100)

100 Chapter 3 Depth Compositing
3 Connect the Copy1 node to the second input of ZCompose1.
So what’s the result? Probably different than you expected. There are several places
where the balloon images are not masked by the Z channel.
ZCompose will not work properly unless all images have an alpha channel. In this case,
you need to create an alpha channel for the city_bg.jpg image.
To create an alpha channel for city_bg:
1 Double-click the city_bg node to load it into the Viewer and into the Parameters tab.
2 Click the View Channel button in the Viewer shelf to display the alpha channel.
The screen goes black, which indicates that the city_bg image does not have an alpha
channel.