User Manual

Using Channels in a Composition
When you connect one node’s output to another node’s input, you feed all the channels that are
output from the upstream node to the downstream node. 2D nodes, which constitute most
simple image-processing operations in Fusion, propagate all channel data from node to node,
including RGB, alpha, and auxiliary channels, regardless of whether that node actually uses or
affects a particular channel.
Incidentally, if you want to see which channels are available for a node, you can open the Color
pop-up menu in the viewer to get a list. This control also lets you view any channel on this list
so that you can examine the channel data of your composite anywhere along the node tree.
All channels available to the currently
viewed node can be isolated via the
viewer’s Color control.
2D nodes also typically operate on all channel data routed through that node. For example, if
you connect a node’s output with RGBA and XYZ Normals channels to the input of a Vortex
node, all channels are equally transformed by the Size, Center, and Angle parameters of this
operation, including the alpha and XYZ normals channels, as seen in the following screenshot.
The Normal Z channel output by a rendered torus (left), and the Normal Z channel after the output is connected
to a Vortex node (right). Note how this auxiliary channel warps along with the RGB and A channels.
This is appropriate because in most cases, you want to make sure that all channels are
transformed, warped, or adjusted together. You wouldn’t want to shrink the image without also
shrinking the alpha channel along with it, and the same is true for most other operations.
On the other hand, some nodes deliberately ignore specific channels when it makes sense.
For example, the Color Corrector and Gamut nodes, both of which are designed to alter RGB
Chapter – 69 Understanding Image Channels 1391