Illustration of maya-usd adaptors not working when exporting skeletons

maya-usd is used to import and export USD files within Maya. It comes with a framework that allows you to modify the USD import and export behaviour. Primarily, this is done by writing custom SchemaApiAdaptors or import and export chasers. For the most part this works well, however, it fails when exporting skeleton data with API schemas applied to them. When a custom adaptor is registered for these API schemas, they are not triggered at all, and the default roundtrip behaviour seems absent as well. This seems to be a bug. The purpose of this repository is to provide a simple set of steps to reproduce it.

Prerequisites

In order to run the reproduction steps the following is necessary:

• VS Code
• Maya 2022
• maya-usd 0.19 or 0.20.

Reproduction steps

1. Clone this repository
2. Open this folder within VS Code
3. Start the Maya 2022 task in VS Code (this will ensure the env is set correctly)
4. Run the run.py script within Maya through the script editor

Expected behaviour

Correctly exported exported_data.usda

When we import sample_data.usda we load all data associated with the WithAdaptorAPI and WithoutAdaptorAPI. When we export this data again, we would expect the following (updated) data to be available in exported_data.usda:

• arm Mesh prim:
  • WithAdaptorAPI applied
  • WithoutAdaptorAPI applied
  • example:data: updated to "this-was-exported"
  • example:default_data: "this-will-be-kept-the-same" same as in sample_data.usda
• joint1 Skeleton prim:
  • WithAdaptorAPI applied
  • WithoutAdaptorAPI applied
  • example:data: updated to "this-was-exported"
  • example:default_data: "this-will-be-kept-the-same" same as in sample_data.usda

This would look like this in the .usd file:

    def Mesh "arm" (
        prepend apiSchemas = ["WithoutAdaptorAPI", "WithAdaptorAPI", "MaterialBindingAPI"]
    )
    {
        ...
        string example:data = "this-was-exported"
        string example:default_data = "this-will-be-kept-the-same"
        ...
    }

    def Skeleton "skeleton" (
        prepend apiSchemas = ["WithoutAdaptorAPI", "WithAdaptorAPI", "SkelBindingAPI"]
    )
        ...
        string example:data = "this-was-exported"
        string example:default_data = "this-will-be-kept-the-same"
        ...
    {

Execution of the JointAdaptor

We expect all of the adaptor logic in joint_example.py to be executed for both the arm Mesh prim and joint1 Skeleton prim. This would mean we should see the following output in Maya's script window:

# Importing '<...>\usd_data\sample_data.usda' # 
CanAdaptForImport: arm
ApplySchemaForImport: arm
CopyFromPrim: mayaObject: arm | prim: arm | value: this-will-be-altered
CanAdaptForImport: skeleton
ApplySchemaForImport: skeleton
CopyFromPrim: mayaObject: skeleton | prim: skeleton | value: this-will-be-altered
# Opening layer '<...>/maya-usd-skeleton-issue-example/usd_data/exported_data.usda' for writing # 
CanAdaptForExport: arm
CanAdaptForExport: arm
CopyToPrim: mayaObject: arm | prim: arm
CanAdaptForExport: skeleton
CopyToPrim: mayaObject: skeleton | prim: skeleton 
# Saving stage # 

Actual behaviour

The Mesh exports correctly, however the skeleton does not. This can be seen in the actual output when running run.py:

# Importing '<...>\usd_data\sample_data.usda' # 
CanAdaptForImport: arm
ApplySchemaForImport: arm
CopyFromPrim: mayaObject: arm | prim: arm | value: this-will-be-altered
CanAdaptForImport: skeleton
ApplySchemaForImport: skeleton
CopyFromPrim: mayaObject: skeleton | prim: skeleton | value: this-will-be-altered
# Opening layer '<...>/usd_data/exported_data.usda' for writing # 
CanAdaptForExport: arm
CanAdaptForExport: arm
CopyToPrim: mayaObject: arm | prim: arm
# Saving stage # 

Note that the we do have the same messages for the arm prim, but that the export messages are absent for the joint1 skeleton prim. This can be seen by inspecting the exported_data.usda as well:

    def Mesh "arm" (
        prepend apiSchemas = ["WithoutAdaptorAPI", "WithAdaptorAPI", "MaterialBindingAPI"]
    )
    {
        ...
        string example:data = "this-was-exported"
        string example:default_data = "this-will-be-kept-the-same"
        ...
    }

    def Skeleton "skeleton" (
        prepend apiSchemas = ["SkelBindingAPI"]
    )
    {
        uniform matrix4d[] bindTransforms = ...
        uniform token[] joints = ["joint1", "joint1/joint2", "joint1/joint2/joint3"]
        uniform matrix4d[] restTransforms = ...
        rel skel:animationSource = </group1/skeleton/Animation>
        matrix4d xformOp:transform = ( (1, 0, 0, 0), (0, 1, 0, 0), (0, 0, 1, 0), (0, 0, 0, 1) )
        uniform token[] xformOpOrder = ["xformOp:transform"]

        def SkelAnimation "Animation"
        {
            uniform token[] joints = ["joint1"]
            quatf[] rotations = [(0.99960554, 0, 0.028086055, 0)]
            half3[] scales = [(1, 1, 1)]
            float3[] translations = [(0.16728887, 0, 0.03475939)]
        }
    }

note that the properties on the arm and the values on skeleton were removed for brevity/readability

The API schemas are missing, and no data has been copied. To me this behaviour is either a bug, a missing feature, or an oversight.