I don't understand the 'Prim cannot be added as a child to another Prim'

If I go to Omniverse, and try to add a cube or cylinder to another cylinder, you show me a message: Nested GPrims are not supported.

However, if I go to Blender, and create the exact same object, export it as USD, and then import the object into Omniverse, It can be assigned to a prim.

This shows it is possible, you just have to improve your workflow. Would it be that hard for you to do the ‘What I just did in Blender’ behind the scenes?

I think the ability to attach Prims to other Prims is an important part of how to build composite items.

Thanks, if you can explain to me the ‘Why’ you did this, since I can show you in 5 minutes it is really pointless if importing from Blender accomplishes the same thing.

Thanks for sharing your feedback/suggestion! I’ve shared with the devs.

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I understand that Create is not allowing you to parent a Mesh as a child of a Mesh. Mesh is one type of GPrim for which we do not allow that sort of parenting. Here’s a section from the USD docs where you’ll see that they consider it a invalid setup and why: USD Terms and Concepts — Universal Scene Description 22.08 documentation. The short of it is that you’ll run into issues with controlling visibility with that setup.

USD has not made that setup impossible (yet?) and it’s the reason why you’ll find that Blender was able to export it that way. On the other hand, Maya doesn’t support nested meshes in its native format so you would run into problems trying to roundtrip with Maya if you were using nested meshes.

To summarize, it’s not that Omniverse has chosen this limitation arbitrarily, but it is correctly preventing users from creating invalid USD. I hope this helps clarify it.

Were you using the vanilla Blender USD export or the Omniverse exporter in Blender? Thanks!

I am using the 3.4 Alpha Omniverse Blender.

I only use Blender and Omniverse or IClone, (Maya is way out of my price range last time I checked).

Thanks for the explanation.

No problem!

One more thing I forgot to mention. The reason I want to add a prim to a child prim, is if you look at the above picture, the Cylinder is the parent, so my catapult works and send the objects in the cage to another zip code if I enable Kinematic on the Rigid Body for the cage and rotate the cylinder real fast.

Is there another way to keep the pivot being the cylinder but attach the other items to a group? If I group them, your pivot tool wants to center the pivot among all the items.


You mean this Pivot Tool?

This tool lets you set the pivot for a prim to wherever you want. One approach would be to group the prims like you said and use Pick Source and Set Pivot to pick the cylinder as the source and set the pivot of the group Xform to match the cylinder.

I must have been using Pick Source all wrong. I used Pick Source to set the orientation of the prop by selecting another cube or ground plane.

Thanks, I will try using a grouped prop as the prim source.

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