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Quick Explanation

If your background is from a C or C-like background, this next tip may come as a bit of a surprise.

#usda 1.0

def Xform "root" (
    prepend variantSets = "something"
)
{
    variantSet "something" = {
		# ...
    }
}

The "something" variantSet is not inside of </root>. It wraps </root>. This means that you can change metadata depending on the selected variant.

#usda 1.0

def Xform "root" (
    prepend variantSets = "something"
)
{
    variantSet "something" = {
        "option_a" (
            some_metadata = "foo"
        ) {

        }
        "option_b" {

        }
    }
}

Example

  • Open metadata_swap.usda in usdview to see the above example, live.
  • Open arcs.usda to see an example of how to use variants to control composition arcs.

When there's no variant selection, </root> has no "some_metadata" authored. Metadata is missing for "option_b", too. But if it's '"option_a", "some_metadata" is authored with a value of "foo".

VariantSets As Composition Arc Managers

As mentioned in the relationship_forwarding article, you can use VariantSet as multiplexer and de-multiplexer.

Since we can modify opinions inside a Prim's ()s, that also means that we can modify its composition arcs, using variant selections.

#usda 1.0
(
	startTimeCode = 0
	endTimeCode = 10
)

def Xform "transform" (
    add references = [
        @model.usda@,
        @surfacing.usda@,
    ]
    prepend variantSets = "state"
)
{
    variantSet "state" = {
        "blue" (
            add references = @surfacing_blue.usda@
        ) {
        }
        "motion" (
            add references = @animation.usda@
        ) {

        }
        "sphere" (
            add references = @sphere.usda@
        ) {

        }
    }
}

In conclusion, variantSets aren't strictly "inside" of a Prim. They are that Prim.

Limitations

From basic experimentation, it looks like you cannot

  • Delete a reference within a variantSet of the same Prim
  • Cannot dynamically change the type of a Prim using a variant
  • Cannot change the name of the Prim

Everything else though, metadata, attributes, whatever, is all up for grabs.