Apple Shake 4 Tutorials - Manual (Page 94)

94 Chapter 3 Depth Compositing
As shown in the illustration above, Shake orders these pixels and outputs the higher
values when it layers the images for the composite. Z channels are usually rendered
from 3D animation software, where each pixel has a known distance from the camera.
Note: You may need to make adjustments—inverting the grayscale image, for
example—to make the Z channel from your animation software work in a Shake
composite. This is because there are different methods for processing depth
information and your software may not match the method Shake uses to handle Z
channel data.
A Z channel can be rendered into the same file as the RGBA channels. You can also
generate and save a Z channel file—either with 3D animation software or a paint
program—and then use the Shake Reorder and Copy nodes to incorporate the Z
channel into an image. This latter method is useful when working with live-action
plates and other elements that do not have a rendered Z channel.
These images show the Z channels for the images in the first example of this tutorial:
The Z channel for balloons.iff was rendered from the original 3D scene file that created
that image, and is stored as one of the channels in the file. The Z channel for the city
background was hand-painted in an image editor. In this tutorial, you’ll incorporate this
separate image as the Z channel for the background.
city_bg.jpg include three channels: RGB. The hand-painted city_bg_depth.jpg will be added
to city_bg.jpg as a Z channel.
balloons.iff includes five channels: RGBA (left) and Z (right).