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USD import breaks even the simplest hierarchy #124102

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opened 2024-07-03 13:06:55 +02:00 by Philipp Oeser · 8 comments
Philipp Oeser commented 2024-07-03 13:06:55 +02:00
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System Information
Operating system: Linux-6.9.4-200.fc40.x86_64-x86_64-with-glibc2.39 64 Bits, X11 UI
Graphics card: NVIDIA GeForce RTX 3080 Laptop GPU/PCIe/SSE2 NVIDIA Corporation 4.6.0 NVIDIA 550.90.07

Blender Version
Broken: version: 4.2.0 Beta, branch: blender-v4.2-release, commit date: 2024-06-24 20:57, hash: 2cc6de8f7eef
Worked: (newest version of Blender that worked as expected)

Short description of error
USD import breaks even the simplest hierarchy

Exact steps for others to reproduce the error

image

  • export as .usda

  • check the hierarchy is still present in the exported file

  • import the resulting .usda

  • note Cube.001 is not parented to Cube anymore... (next to the naming issues that !124041 aims to solve)

image

Spotted while working on !124041 (but the behavior here probably needs fixing first... or can go hand-in-hand with !124041)

**System Information** Operating system: Linux-6.9.4-200.fc40.x86_64-x86_64-with-glibc2.39 64 Bits, X11 UI Graphics card: NVIDIA GeForce RTX 3080 Laptop GPU/PCIe/SSE2 NVIDIA Corporation 4.6.0 NVIDIA 550.90.07 **Blender Version** Broken: version: 4.2.0 Beta, branch: blender-v4.2-release, commit date: 2024-06-24 20:57, hash: `2cc6de8f7eef` Worked: (newest version of Blender that worked as expected) **Short description of error** USD import breaks even the simplest hierarchy **Exact steps for others to reproduce the error** - open the attached [USD_import_hierarchy_bug.blend](/attachments/0aa9647c-d4a8-4206-921a-351236e358df) - note `Cube.001` is parented to `Cube` ![image](/attachments/6135965d-e337-4dc8-a7a9-d962539c8d97) - export as .usda - check the hierarchy is still present in the exported file - import the resulting .usda - note `Cube.001` is **not** parented to `Cube` anymore... (next to the naming issues that !124041 aims to solve) ![image](/attachments/48a9eb7d-a061-4950-af18-09cce4729bf9) Spotted while working on !124041 (but the behavior here probably needs fixing first... or can go hand-in-hand with !124041)
image.png
19 KiB
USD_import_hierarchy_bug.blend
858 KiB
image.png
16 KiB
Philipp Oeser commented 2024-07-03 13:10:00 +02:00
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CC @deadpin
CC @makowalski

CC @deadpin CC @makowalski
Michael Kowalski commented 2024-07-03 18:11:42 +02:00
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Thanks for pointing this out. I'll take a look today.

Thanks for pointing this out. I'll take a look today.
Michael Kowalski commented 2024-07-04 06:09:48 +02:00
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@deadpin for visibility.

I have an idea of what is going on and will look more into this on Friday, as I will be out Thursday.

However, in my humble opinion, the fix started in !124041, which specifically addresses the problem where the object name is changed on re-import from USD, can and should proceed independently, as it's addressing a separate issue.

I.e., in !124041 the problem is how do we name objects which we know represent a merged USD Xform and its child. The issue noted here has to do with how we decide which which USD Xforms and their children can be merged in the first place. This gets complicated when an Xform has multiple children, because we have to decide which child gets merged with its parent and then must appropriately reparent the merged child's siblings.

I have some thoughts on how to improve this. Probably the best way would be to include additional metadata in the USD to specify how the source Blender objects were parented. We could also improve the heuristics currently used for merging even without using such metadata. In any case, such changes aren't trivial and are probably not good candidates for the 4.2 release.

BTW, for what it's worth, I believe the actual transformations are still correct even with this behavior, it's just that the hierarchy is different.

@deadpin for visibility. I have an idea of what is going on and will look more into this on Friday, as I will be out Thursday. However, in my humble opinion, the fix started in !124041, which specifically addresses the problem where the object name is changed on re-import from USD, can and should proceed independently, as it's addressing a separate issue. I.e., in !124041 the problem is how do we name objects which we know represent a merged USD Xform and its child. The issue noted here has to do with how we decide which which USD Xforms and their children can be merged in the first place. This gets complicated when an Xform has multiple children, because we have to decide which child gets merged with its parent and then must appropriately reparent the merged child's siblings. I have some thoughts on how to improve this. Probably the best way would be to include additional metadata in the USD to specify how the source Blender objects were parented. We could also improve the heuristics currently used for merging even without using such metadata. In any case, such changes aren't trivial and are probably not good candidates for the 4.2 release. BTW, for what it's worth, I believe the actual transformations are still correct even with this behavior, it's just that the hierarchy is different.
Michael Kowalski commented 2024-07-04 06:10:27 +02:00
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@mont29 for visibility.

@mont29 for visibility.
Philipp Oeser commented 2024-07-04 09:14:40 +02:00
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We could also improve the heuristics currently used for merging even without using such metadata

This is probably the way to go (since mentioned metadata wont be available on exports from other software)

>We could also improve the heuristics currently used for merging even without using such metadata This is probably the way to go (since mentioned metadata wont be available on exports from other software)
Philipp Oeser commented 2024-07-04 10:41:32 +02:00
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Was experimenting with this a bit, and current logic turns out to be hairy... (at least for me)

Since Alembic is very similar in concept (and the hierarchy is flawless on the importer side) I am now looking at static std::pair<bool, AbcObjectReader *> visit_object in comparison to USDPrimReader *USDStageReader::collect_readers to see what is done better there.

Was experimenting with this a bit, and current logic turns out to be hairy... (at least for me) Since Alembic is very similar in concept (and the hierarchy is flawless on the importer side) I am now looking at `static std::pair<bool, AbcObjectReader *> visit_object` in comparison to `USDPrimReader *USDStageReader::collect_readers` to see what is done better there.
Bastien Montagne added this to the USD project 2024-07-04 10:44:20 +02:00
Michael Kowalski commented 2024-07-04 11:12:16 +02:00
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Arm also taking a quick look into making a tweak to collect_readers to handle this case. I will report what I find.

Arm also taking a quick look into making a tweak to `collect_readers` to handle this case. I will report what I find.
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Michael Kowalski commented 2024-07-04 13:36:26 +02:00
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@lichtwerk I have WIP progress for a proposed fix: #124153

However, I personally would not recommend making this change for the 4.2 release at this late stage, but am open to discussing it.

@lichtwerk I have WIP progress for a proposed fix: https://projects.blender.org/blender/blender/pulls/124153 However, I personally would not recommend making this change for the 4.2 release at this late stage, but am open to discussing it.
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