Documentation: Inaccurate generalization, New Features Guide

From the New Features Guide, page 2-71 and 2-80:

When you composite two image layers using the Merge node, auxiliary channels will only propagate through the image that's connected to the background input. The rationale for this is that in most composites that include computer generated imagery, the background is most often the CG layer that contains auxiliary channels, while the foreground is a live-action greenscreen plate with subjects or elements that are meant to be combined against the background.

That is only occasionally true and almost never relevant. Better reasoning is this:

Auxiliary channels cannot be easily composited like color channels, so only one layer's channels can be passed down the node tree. By Fusion convention, the Background input takes precedence, so that is the Input from which auxiliary channels are taken.


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Really, there's not much reason to keep the aux channels at all once a composite with a live-action plate has been performed. Anything you could do with the channels would be invalid in the pixels with a non-CG foreground.