User Manual

Channels in Fusion
Fusion introduces some innovative ways of working with the many different channels of image
data that modern compositing workflows encompass. This chapter’s introduction to color
channels and how they’re affected by different nodes and operations is a valuable way to begin
the process of learning to paint, composite, and apply effects in Fusion.
If you’re new to compositing, or you’re new to the Fusion workflow, you ignore this chapter at
your peril, as it provides a solid foundation to understanding how to predictably control image
data as you work in this powerful environment.
Types of Channels Supported by Fusion
Digital images are divided into separate channels, each of which carries a specific kind of
image data. Nodes that perform different image-processing operations typically expect specific
channels to provide predictable results. This section describes the different kinds of channels
that Fusion supports.
RGB Color Channels
The red, green, and blue (RGB) channels of any still image or movie clip combine additively to
represent everything we can see via visible light. Each of these three channels is a grayscale
image when seen by itself. When combined additively, these channels represent a full-
color image.
Alpha Channels
An alpha channel is a grayscale channel that represents different levels of transparency in an
image. In Fusion, white denotes solid areas, while black denotes transparent areas. Grayscale
values range from more opaque (lighter) to more transparent (darker).
If you’re working with an imported alpha channel from another application for which these
conventions are reversed, never fear. Every node capable of using an alpha channel is also
capable of inverting it.
Single-Channel Masks
These channels are created by Fusion whenever you create a Mask node. Mask nodes are
unique in that they propagate single-channel image data that often serves a similar function as
an alpha channel, defining which areas of an image should be solid and which should be
transparent. However, masks can also define which parts of an image should be affected by a
particular operation, and which should not. Mask channels are designed to be connected to
specific mask inputs of nodes used for keying and compositing, such as the Merge node, the
DeltaKeyer node, and the Matte Control node.
Auxiliary Channels
Auxiliary channels (covered in more detail later in this chapter) describe a family of special-
purpose image data that typically expose 3D data in a way that can be used in 2D composites.
For example, Z-Depth channels describe the depth of each pixel in an image along a Z axis,
while an XYZ Normals channel describes the orientation (facing up, facing down, or facing to
the left or right) of each pixel in an image. Auxiliary channel data is generated by rendering 3D
images and animation, so it usually accompanies images generated by 3D applications like
Blender or Maxon Cinema 4D. These channels can also be generated from within Fusion via the
Renderer 3D node, which outputs a 3D scene that you’ve assembled and lit as 2D RGBA
channels, with optionally accompanying auxiliary channels.
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