[BugFix] fix bug of joint relationship process in USDAsset._as_trimesh_scene()

[Kobayashi-z]

Why

Previously, joints were being processed twice due to USD's bidirectional nature, causing incorrect hierarchy issues. This fix ensures consistent scene graph construction. So I skip processing when encountering the reversed entry
USD joints are bidirectional (A-B is same as B-A).

                joints[(body_0, body_1)] = joint_prim

                joints[(body_1, body_0)] = "reversed"

here

[clemense]

Do you have an example USD file that fails, and for which this change improves things? I'm getting errors with some test file imports due to this change.

[clemense]

Just to be clear joints are not being processed twice currently.

[Kobayashi-z]

Thanks for your response. You can test with the USD file in the attachment (note: you'll need to rename the file extension to .usd). I encountered an error when running the following code.

import scene_synthesizer
from scene_synthesizer.assets import USDAsset
scene = scene_synthesizer.Scene()
asset = USDAsset("FoodMixer.usd")
scene.add_object(asset, "foodmixer")

The traceback is below

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/xiaolin.zhou/code/scene_synthesizer/src/scene_synthesizer/scene.py", line 1388, in add_object
    scene_to_add = asset.as_trimesh_scene(
  File "/home/xiaolin.zhou/code/scene_synthesizer/src/scene_synthesizer/assets.py", line 648, in as_trimesh_scene
    trimesh_scene = self._as_trimesh_scene(
  File "/home/xiaolin.zhou/code/scene_synthesizer/src/scene_synthesizer/assets.py", line 1195, in _as_trimesh_scene
    raise ValueError("Can't parse USD. Tree structure incorrect.")
ValueError: Can't parse USD. Tree structure incorrect.

FoodMixer.txt

[clemense]

Thanks! The USD contains a loop, that's why it can't be parsed into the scene_synth scene graph (which is a DAG).

Two immediate options:

1. If you don't care about the articulation of the object, you can use ignore_articulation=True during import. This will ignore all joints in the USD.
2. You can load the asset as a reference via reference_only=True which will not parse the USD but keep a pointer to it. During export to USD you can do write_usd_object_files=False which will include a reference to the original file.

If that doesn't fit your use case we can look into adding an option to break the loop by ignoring a joint during parsing.

[clemense]

Let me revise my explanation: There doesn't seem to be a kinematic loop. Instead the problem is that the PhysicsArticulationRootAPI is applied twice, to the following prims: /world/object/_body_0 and /world/object/worldBody.

[clemense]

Ok. The latest version (v1.14.13) has two additional changes:

• The USD import logic now breaks loops (by preferring to not ignore joint constraints) which enables it to load more files (such as the FoodMixer.usd)
• The visibility attribute is now respected, i.e. meshes/prims that are invisible are not loaded (old behavior can be reproduced by setting ignore_visibility=True)

I will now also close the MR, since the issue (that this MR tried to address) is resolved.