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The channel limiting buttons in the Settings panel of a
Transform node, so only the Green channel is affected
For example, if you wanted to use the Transform node to affect only the green channel of an
image, you can turn off the Green, Blue, and Alpha buttons. As a result, the green channel is
processed by this operation, and the red, blue, and alpha channels are copied straight from the
node’s input to the node’s output, skipping that node’s processing to remain unaffected.
Transforming only the green color channel of the image with a Transform effect
Skipping Channel Processing
Under the hood, most nodes process all channels first, but afterward copy the input image to the
output for channels that have been enabled. Modern workstations are so fast that this isn’t usually
noticeable, but there are some nodes where deselecting a channel actually causes that node to skip
processing that channel entirely. Nodes that operate this way have a linked set of Red, Green, Blue,
and Alpha buttons on another tab in the node. In these cases, the Common Control channel buttons
are instanced to the channel buttons found elsewhere in the node.
Blur, Brightness/Contrast, Erode/Dilate, and Filter are examples of nodes that all have RGBA buttons in
the main Controls tab of the Inspector, in addition to the Settings tab.
Adding Alpha Channels
Much of visual effects compositing has to do with placing a foreground subject over a background.
Possibly the most fundamental method is through the use of an alpha or matte channel. If an alpha
channel is not contained within the clip, you add one via keying or rotoscoping. While more specific
methods are covered in detail in later chapters, here’s an example of how this is handled within Fusion.
In the case of extracting an alpha matte from a green screen image, you typically connect the image’s
RGB output to the “Input” input of a Keyer node such as the Delta Keyer, and you then use the keyer’s
controls to extract the matte. The Keyer node automatically inserts the alpha channel that’s generated
alongside the RGB channels, so the output is automatically RGBA. Then, when you connect the keyer’s
output to a Merge node to composite it over another image, the Merge node automatically knows to
use the embedded alpha channel coming into the foreground input to create the desired composite,
as seen in the following screenshot.
Chapter 78Understanding Image Channels 1380
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