Control/creating composition arcs on export?

[dveight]

Hi,

I am looking for a way to specify a composition arc to be written instead of writing geometry etc. Our workflows currently convert USD data into maya nodes on import and we store information as attributes on group / transform nodes such as its location in our database.

What I am looking for is a way on export to take a string attribute on a transform group:
/environment/treesDessing/treeA01/main/

Where main has a string attribute which defines its inherit location:
string inherit_path = /_ASSETS/instanceSources/trees_default/_class_treeA_main

Below this point is where the meshes etc live which are part of the cache which was loaded.

When this is written out as a usd file i'd expect a structure like this:

def Xform "environment" (
    kind = "assembly"
)
{

    def Xform "treeDressing" (
        kind = "assembly"
    )
    {
        def Xform "treeA" (
            kind = "assembly"
        )
        {

            def Xform "main" (
                append inherits = < /_ASSETS/instanceSources/trees_default/_class_treeA_main>
                kind = "assembly"
            )
            {
            }
        }
    }
}

I suspect that the only way to do this is with some kind of export chaser? I do not want to encode the actual geometry meshes etc under the /environment/treesDessing/treeA01/main/ location.

Ideally this would work with the usdStage proxy shape (anything with custome attributes which map to composition arcs)

[wallworm]

We are actively working on USD component and variant authoring now and this can possibly fit into the overall workflow of authoring components and then assembling them. I'll bring it back to the team.

For the time being your best bet may be a PrimWriter instead of (or alongside a Chaser). You can create a custom PrimWriter that can exclude writing any mesh data at all and instead simply insert your inherits arc.

[dveight]

Hey, that sounds great. I will have a look at that. I've not seen much online for this functionality. Is this the best place to start:
https://github.com/Autodesk/maya-usd/blob/dev/test/lib/mayaUsd/fileio/testPrimWriter.py

[wallworm]

Yes, that shows a Prim Writer. Take note of CanExport(). When the exporter runs, it passes each of your objects through all available primwriters and the first on it finds that returns mayaUsdLib.PrimWriter.ContextSupport.Supported will be used for writing the prims. If none are found, then the default fallbacks that we ship will be used. See https://github.com/Autodesk/maya-usd/blob/dev/lib/mayaUsd/fileio/primWriter.h for the ContextSupport values.