Many nodes output anonymous attributes (e.g. the top selection in the
Cylinder node). The actual data is only contained in the geometry output
though. The field output just contains a reference to this data (essentially
just the generated name of an attribute). This data can be output even
without computing the geometry itself.
As of right now, this only simplifies the code a bit, but does not have a
bigger impact, because to use the anonymous attribute, you always need
the geometry anyway. However, with the upcoming simulation nodes,
it's possible to create the anonymous attribute in one frame and to access
it in another frame. In the other frame we only need the anonymous
attribute reference and don't have to create the actual geometry again.
Skipping creating the actual attribute on the geometry can have a
significant performance benefit.
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