Apple Shake 4 Tutorials - Manual (Page 110)

110 Chapter 3 Depth Compositing
This is one example of the node tree you could create for this solution:
The KeyMix1 node layers a copy of city_bg, which is masked with RotoShape2, over the
finished composite from ZCompose1. KeyMix has a mask input, like all other nodes, but
also has a unique input for the mask you want to combine with the other elements.
Adding Haze to the Entire Image
When you applied the Fade node, the result was a “fade-to-distance” effect. This works
for the balloons because there’s a background for them to fade into. Not true for the city
background; it will just fade to black because there’s nothing behind the city_bg image.
To create a simliar fading effect for the city horizon, you add a Compress node
combined with a DepthKey node, as shown below, to simulate haze: