User Manual

Table Of Contents
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
The main reason for compositing is to take a foreground image that lacks an alpha channel, add
one via keying or rotoscoping, and then composite it over a background image to make a new
image. While more specific methods are covered in detail in later chapters, here’s an overview
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 DeltaKeyer, 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.
A simple node tree for keying; note that only one connection links the DeltaKeyer to the Merge node.
One way of rotoscoping with a Polygon node is to connect the image being rotoscoped to the
background input of a MatteControl node. The Polygon node then connects to the Garbage
Matte input (which you invert in the Inspector.) This lets you view the image while drawing, using
the controls of the Polygon node. The resulting alpha channel is merged together with the RGB
channels so the Merge Alpha node’s output is RGBA, which can be connected to a Merge node
to composite the rotoscoped subject over another image.
A simple rotoscoping node tree.
In both cases, you can see how the node tree’s ability to carry multiple channels of image data
over a single connection line simplifies the compositing process.
Chapter – 69 Understanding Image Channels 1393