Alembic: Wrong result when exporting and importing curves #76746

New Issue

Germano Cavalcante · 2020-05-14T14:26:03+02:00

2020-05-14 14:26:03 +02:00

System Information
Operating system: Windows-10-10.0.19041-SP0 64 Bits
Graphics card: Radeon (TM) RX 480 Graphics ATI Technologies Inc. 4.5.13586 Core Profile Context 19.50.01.05 26.20.15001.5006

Blender Version
Broken: version: 2.90 (sub 1), branch: master, commit date: 2020-05-12 22:14, hash: f9d0f59bed
Worked: never

Short description of error
I'm not familiar with the alembic exporter, but the result of exporting and importing curves doesn't seem right because the coordinates of the vertices are messy.

Exact steps for others to reproduce the error

Open attached file.
Export the curve object with Alembic
Load a new file
Import the created .abc file.
The result is messy curves.
curves_to_alembic.blend

**System Information** Operating system: Windows-10-10.0.19041-SP0 64 Bits Graphics card: Radeon (TM) RX 480 Graphics ATI Technologies Inc. 4.5.13586 Core Profile Context 19.50.01.05 26.20.15001.5006 **Blender Version** Broken: version: 2.90 (sub 1), branch: master, commit date: 2020-05-12 22:14, hash: `f9d0f59bed` Worked: never **Short description of error** I'm not familiar with the alembic exporter, but the result of exporting and importing curves doesn't seem right because the coordinates of the vertices are messy. **Exact steps for others to reproduce the error** - Open attached file. - Export the curve object with Alembic - Load a new file - Import the created .abc file. The result is messy curves. [curves_to_alembic.blend](https://archive.blender.org/developer/F8535200/curves_to_alembic.blend)

Germano Cavalcante commented

2020-05-14 14:26:03 +02:00

Added subscriber: @mano-wii

Germano Cavalcante commented

2020-05-14 14:27:45 +02:00

Added subscriber: @dr.sybren

Germano Cavalcante changed title from ~~Alembic: Problem when exporting and importing curves~~ to Alembic: Wrong result when exporting and importing curves

2020-05-14 21:40:17 +02:00

Sybren A. Stüvel commented

2020-05-18 10:37:19 +02:00

This has to do with Blender supporting multiple curve types in one Curve object, which isn't possible in Alembic. I can update the Alembic exporter to detect such situations and then simply only export the curves of one type and skip the others. Although that'll make the Alembic file incomplete, that way at least it should be valid.

Sybren A. Stüvel self-assigned this 2020-05-18 11:27:41 +02:00

Sybren A. Stüvel commented

2020-05-18 11:27:41 +02:00

I'm postponing this until after 2.83, as this requires a more thorough refactor of the existing Curves exporter. Doing this properly is easier with the upcoming refactor of the Alembic exporter, which is planned for 2.90.

Sybren A. Stüvel commented

2020-05-18 11:39:40 +02:00

Changed status from 'Needs Triage' to: 'Confirmed'

Sybren A. Stüvel commented

2020-07-27 13:32:46 +02:00

The current curves exporter is severely limited; it ignores the curve type (Nurb::type) and tries to deduce the type to write from the existence of data in either Nurb::bp or Nurb::bezt. This will only work correctly in certain specific cases, which are unfortunately not documented.

There is no clear (public) definition on how to write curves to Alembic. Writing curves properly, even when only aiming for compatibility with Blender itself, will require some design work.

In light of this, I'm changing this task's subtype from Bug to Known Issue.

The current curves exporter is severely limited; it ignores the curve type (`Nurb::type`) and tries to deduce the type to write from the existence of data in either `Nurb::bp` or `Nurb::bezt`. This will only work correctly in certain specific cases, which are unfortunately not documented. There is no clear (public) definition on how to write curves to Alembic. Writing curves properly, even when only aiming for compatibility with Blender itself, will require some design work. In light of this, I'm changing this task's subtype from Bug to Known Issue.

Philipp Oeser commented

2020-07-27 13:40:23 +02:00

Added subscriber: @lichtwerk

Sybren A. Stüvel removed their assignment 2021-01-14 18:00:12 +01:00

Michael Sherak commented

2021-08-05 19:13:04 +02:00

Added subscriber: @MSherak

Michael Sherak commented

2021-08-05 19:13:04 +02:00

Don't know if this will help, but here is a Alembic file exported out of Maya that can load into Blender and Houdini correctly. It's one Nurb curve exported. Maybe the deconstruction of the file will help with how curves are stored in Alembic so the they can be exported from Blender. MayaCurveExport.abc

Edit: Oh and set the Resolution Preview U to 3 and it will look like it does in Maya.

Don't know if this will help, but here is a Alembic file exported out of Maya that can load into Blender and Houdini correctly. It's one Nurb curve exported. Maybe the deconstruction of the file will help with how curves are stored in Alembic so the they can be exported from Blender. [MayaCurveExport.abc](https://archive.blender.org/developer/F10268346/MayaCurveExport.abc) Edit: Oh and set the Resolution Preview U to 3 and it will look like it does in Maya.

Alain De Hoe commented

2021-08-26 22:56:51 +02:00

Added subscriber: @aplus

Alain De Hoe commented

2021-08-26 22:56:51 +02:00

I ran into this a few months ago. From what I can tell a single curve will be exported fine, but as soon as multiple curves exist in same object, the exported curves in the alembic file will no longer match the original after alembic import.

I have attached blend file and alembic files of curves objects created with default settings and duplicated in EDIT mode, before export.
Hope this helps.

For nurbs, I noticed that the the points array for any curve that is not the 1st curve end up having a multiple of the number of points in splines[0]

for s in bpy.data.objects["NurbsCurveSub"].data.splines:

... print (len(s.points))
10
10

for s in bpy.data.objects["NurbsCurveSub.001"].data.splines:

... print (len(s.points))
10
20

curvesToAlembic.blend

nurbs.abc nurbsSub.abc

bezier.abc

README.txt

I ran into this a few months ago. From what I can tell a single curve will be exported fine, but as soon as multiple curves exist in same object, the exported curves in the alembic file will no longer match the original after alembic import. I have attached blend file and alembic files of curves objects created with default settings and duplicated in EDIT mode, before export. Hope this helps. For nurbs, I noticed that the the points array for any curve that is not the 1st curve end up having a multiple of the number of points in splines[0] >>> for s in bpy.data.objects["NurbsCurveSub"].data.splines: ... print (len(s.points)) 10 10 >>> for s in bpy.data.objects["NurbsCurveSub.001"].data.splines: ... print (len(s.points)) 10 20 [curvesToAlembic.blend](https://archive.blender.org/developer/F10330582/curvesToAlembic.blend) [nurbs.abc](https://archive.blender.org/developer/F10330583/nurbs.abc) [nurbsSub.abc](https://archive.blender.org/developer/F10330584/nurbsSub.abc) [bezier.abc](https://archive.blender.org/developer/F10330585/bezier.abc) [README.txt](https://archive.blender.org/developer/F10330586/README.txt)

Alain De Hoe commented

2021-08-26 23:06:54 +02:00

I would just like to add, that hair would be a classic example where export/import of alembic curves is needed. Right now the solution is to rely on another 3rd party software.

Hair curves are also most likely to be of the same type, so this may be a more feasible problem to look into and provide a solution depending on priorities.

Thank you.

I would just like to add, that hair would be a classic example where export/import of alembic curves is needed. Right now the solution is to rely on another 3rd party software. Hair curves are also most likely to be of the same type, so this may be a more feasible problem to look into and provide a solution depending on priorities. Thank you.

blender-admin commented

2022-03-31 15:01:14 +02:00

This issue was referenced by 4c1393c202

This issue was referenced by 4c1393c202d41422c16d7ccb04f90a3661525f6e

Kévin Dietrich commented

2022-03-31 15:02:44 +02:00

Changed status from 'Confirmed' to: 'Resolved'

Kévin Dietrich closed this issue

2022-03-31 15:02:44 +02:00

Kévin Dietrich self-assigned this 2022-03-31 15:02:44 +02:00

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Alembic: Wrong result when exporting and importing curves #76746