Xuezhi Dong, if you follow this video https://www.youtube.com/watch?v=sKz16-W7vpY&t=127s there was a sphere with clean topology, and it was correctly converted into nurbs. In your case, there were geometry breaks and you got a lot of nurbs. You can leave your geometry as polygonal mesh and make it live object, but draw curves with another tool, this is what I always do. Pencil curve tool can be use only on nurbs(
Artemisia Campagna, it's hard why it's black without more information. USD export will be added in the future. There's Alembic USD Plugin and Alembic files can be used in place of USD files.
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