[Maya] usdExport cannot export shading with instances

[stolyaroleh]

Description of Issue

There seems to be a fundamental difference between how Maya and USD handle instancing and material assignment.

In Maya, it's possible to override material assignment for instances: an instanced shape has an array attribute instObjGroups, the elements of which can be connected to multiple shading nodes. In this way, instances share geometry, but can have different materials.

As far as I understand, in USD, a reference to material lives on a Gprim. Since usdExport translates Maya instances into USD instances, authoring edits on instanced Gprims is forbidden, which is exactly what PxrUsdMayaShadingModeExporter tries to do.

Additionally, it is possible to crash usdExport and Maya if there is a name conflict between instanced shapes, coming from a referenced scene. I think this happens because PxrUsdMayaUtil::MDagPathToUsdPath, used by PxrUsdMayaShadingModeExporterContext::GetAssignments is not injective: it maps distinct MDagPaths foo|bar|baz and foo:bar|baz to the same SdfPath. This confuses usdExport: it attempts to write to a null prim, which raises a SIGABRT.

Steps to Reproduce

1. Open attached scene usdexport_shading_instances.ma
2. Run the following:

from maya import cmds
cmds.usdExport(
    file='~/test.usda',
    exportInstances=True,
    shadingMode='displayColor'
)

3. Observe errors in console:

Coding Error: in _ValidateEditPrimAtPath at line 1183 of /home/oleh.stolyar/src/wwfx-bin/thirdparty/USD/pxr/usd/lib/usd/stage.cpp -- Cannot create prim at path </Cube_1/Looks>; authoring to an instance proxy is not allowed.

4. Create a new scene, and add a cube to it.
5. Add a reference to usdexport_shading_instances.ma (File -> Create Reference...).
6. Rename cube transform node to usdexport_shading_instances_Cube_1 and shape node to usdexport_shading_instances_Shape.
7. Repeat step 2.
8. Observe a crash:

------------------------ 'maya.bin' is dying ------------------------
maya.bin crashed. FATAL ERROR: Used null prim
in Usd_IssueFatalPrimAccessError at line 224 of /home/oleh.stolyar/src/wwfx-bin/thirdparty/USD/pxr/usd/lib/usd/primData.cpp

System Information (OS, Hardware)

CentOS 7

Package Versions

v0.8.1

Build Flags

Mostly default.

[stolyaroleh]

What would be a good way to handle shading and instances?
I was wondering if it's possible to isolate shading in a separate, stronger layer, which would override instanceability and provide opinions about material references at locations of instance proxies. If yes, would geometry still be shared between prims?

[spiffmon]

Hi Oleh- thanks for reporting these issues. Funnily enough, we are currently working to extend the material binding system in UsdShade to allow binding "by collections ", which will allow us to assign materials to instanced prims from "outside" the instance. Look for a "Roadmap update" post on the interest forum soon for more details. The second issue is not something we've run into, and we'll look into it. We may use this issue primarily to target the #2 bug in the report.

…

--SpiffiPhone

On Nov 22, 2017, at 1:07 AM, Oleh Stolyar ***@***.***> wrote: What would be a good way to handle shading and instances? I was wondering if it's possible to isolate shading in a separate, stronger layer, which would override instanceability and provide opinions about material references at locations of instance proxies. If yes, would geometry still be shared between prims? — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub, or mute the thread.

[jtran56]

Filed as internal issue #154222.

[sunyab]

Maya plugin development has migrated to Autodesk's maya-usd project, so this is no longer relevant. Closing this out.