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Chapter 15 Image Processing Basics 417
Many operations allow you to select which channel is used as the modifying channel.
For example, the SwitchMatte, KeyMix, and IBlur nodes give you the option to select the
R,G,B, or A channel as your control or alpha channel. This often removes the need to
swap your channels before you do many operations. Two exceptions to this are the
Inside and Outside nodes, which always depend on the second image’s alpha channel.
To convert a BW (or BWA) image into an RGB (or RGBA) image without changing its
values, use the Command Line field to specify the following operation:
shake myBlackAndWhiteImage.iff -forcergb -fo myRGBImage
For more information on channel/compositing functions, see Chapter 16, “Compositing
With Layer Nodes,” on page 451.
Compositing Basics and the Alpha Channel
The Shake compositing nodes are located in the Layer tab. The primary compositing
nodes are the Over and KeyMix nodes. For more information on how to use these
nodes, see Chapter 16, “Compositing With Layer Nodes,” on page 451.
Example 1: Compositing Using the Over Node
The following images are used in this discussion to demonstrate how the primary
compositing nodes work:
As its name implies, the Over node places a foreground image with an alpha channel
over a background image. The foreground RGB channels should be premultiplied by
the alpha channel. In a premultiplied image, the RGB channels are multiplied by the
alpha channel.
Color–Set Adds or removes
R, G, B, A, or Z.
A value set to 0 removes the specified channel. A channel
parameter set to something other than 0 adds that
channel.
Layer–
SwitchMatte
Adds A. Copies in any channel from the second input for use as
the new alpha channel for the first input.
Node Effect Operation
Foreground (with mask) Foreground alpha channel
(part of foreground image)
Background