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Understanding Auxiliary Channels
Auxiliary channels describe a family of special-purpose image data that typically describes 3D
position, orientation, and object information for use in 2D composites. For example, Z-Depth
channels describe the depth of each region of an image along a Z-axis (XYZ), while an XYZ
Normals channel describes the orientation (facing up, facing down, facing to the left or right) of
each pixel in an image. Auxiliary channel data is generated by rendering 3D images, so it is
often generated by 3D applications like Blender or Maxon Cinema 4D. It 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.
One of the most common reasons to use auxiliary data is that 3D rendering is computationally
expensive and time-consuming, so outputting descriptive information about a 3D image allows
sophisticated alterations to occur in 2D compositing, which is faster to perform and adjust.
There are two ways of obtaining auxiliary channel data:
First, auxiliary data may be embedded within a clip exported from a 3D application
that’s in a format capable of containing auxiliary channels. In this case, it’s best to
consult your 3D application’s documentation to determine which auxiliary channels can
be generated and output.
Second, you may also obtain auxiliary channel data by generating it within Fusion via
3D operations output by the Renderer 3D node, using the Optical Flow node, or using
the Disparity node.
An RGBA 3D rendered scene that can also generate auxiliary channels.
TIP: When trying to locate information about auxiliary channels in other applications,
some 3D applications refer to auxiliary channels as Arbitrary Output Variables (AOVs),
render elements, or secondaries.
Image Formats That Support Auxiliary Channels
Fusion supports auxiliary channel information contained in a variety of image formats. The
number of channels and methods used are different for each format.
OpenEXR (*.exr)
The OpenEXR file format is the primary format used to contain an arbitrary number of additional
image channels. Many renderers that write to the OpenEXR format allow the creation of
channels that contain entirely arbitrary data. For example, a channel with specular highlights
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