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Exporting curves to Alembic crashes other software when reading
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Description

System Information
windows 8 64bit

Blender Version
Broken: 2.77.3 1fe9d67
Worked: (optional)

The curve import to others software will be report error, even cause crash

Exact steps for others to reproduce the error
Download the test file and import to maya, or houdini, the some curve type will make report error or cause crash.

blender to blender:

Nurbs into Maya:

Poly or Bezier into Maya:

Have test with maya 16, maya 17 and houdini 15

Event Timeline

Kévin Dietrich (kevindietrich) lowered the priority of this task from 90 to Low.

Setting to low priority since this will be a bit hard to solve without knowing how different software expect this data to be written. Alembic uses the Renderman conventions, and I already know that curves and NURBS are not as straightforward to export as meshes because of that (NURBS import is disable in Blender for this very reason). From the Maya plugin, control points need to be duplicated, but how this is determined is not documented, therefore I am afraid that a lot of trial and error needs to happen.

Kévin Dietrich (kevindietrich) raised the priority of this task from Low to 50.Aug 30 2016, 11:30 AM
Kévin Dietrich (kevindietrich) lowered the priority of this task from 50 to Low.Sep 5 2016, 3:51 AM
Sybren A. Stüvel (sybren) raised the priority of this task from Low to 30.Aug 22 2019, 4:54 PM

I see crashing of other applications as a problem with those applications, and not something Blender can fix.

@Leroy (Leroy) Do you have a clear specification of how NURBS curves and surfaces should be written, in order to be compatible with other software? And since this report is rather old already, can you test what happens with a current version of Blender and whether this is still an issue?

Sybren A. Stüvel (sybren) renamed this task from The curve problem in Alembic to Exporting curves to Alembic crashes other software when reading.Aug 22 2019, 4:54 PM
Bastien Montagne (mont29) changed the task status from Unknown Status to Unknown Status.Sep 2 2019, 7:05 PM

Time to archive this imho...

I also have this problem. Blender 2.83

@Andrey (andruxa696) In that case I have the same question for you as I had for @Leroy (Leroy): Do you have a clear specification of how NURBS curves and surfaces should be written, in order to be compatible with other software?

Unfortunately I do not have a clear specification and I do not understand the technical nuances. For my self I found solution to use only blender NURBS and create more vertices in it- then it works fine. But never export Poly or Bezier curves - then maya(version 2018 and 2019 ) crashes. Houdini 18 not crashes, but does not see Poly or Bezier curves correct. For Houdini the same solution - create blender Nurbs and add more vertices to it. Hope it will help somebody.
@Sybren A. Stüvel (sybren) I can only say that maybe we can convert and exporty Poly or Bezier curves the same way as NURBS curves, or convert it to NURBS before export.
For some of this bugs even no need to check it in other software - just export from blender and import it back.


last example where few Nurbs curves is part of one object - also exports incorect .
If I create few curves in one object in Maya - it exports to Houdini correctly , so it is not the limitation of Alembic.
I hope it will help you at least a little. Good luck and thank you @Sybren A. Stüvel (sybren) for you hard work.


@Sybren A. Stüvel (sybren) here is blend file with example - just export it in alembic format and import back. you will see what I mean. Thanks.

@Andrey (andruxa696) In that case I have the same question for you as I had for @Leroy (Leroy): Do you have a clear specification of how NURBS curves and surfaces should be written, in order to be compatible with other software? And since this report is rather old already, can you test what happens with a current version of Blender and whether this is still an issue?

@Sybren A. Stüvel (sybren) seems you missed one of my previous message( message before I sent a file), I have answered on your question already. Thanks. If it is still unclear , I do not have a clear specification of how NURBS curves and surfaces should be written.
But please reread my long answer if you missed it.
I have tested it in last 2.83 and 2.9 (hash: rB83304e4c221b) builds . It still have this problem.
Check file I have sent before.

  1. run 2.9 version. 2) select curves 3) export it in alembic format. 4) import that alembic file back to blender . Some of the curves types are broken.(nurbs curves are fine)

For me it is not big error, I almost never use curve exports. Just want to help somehow . I tried to google some about nurbs curves and their technical specification in alembic , nothing found. maybe need to ask help in http://opensource.imageworks.com/ . I have downloaded source files on github, it seems they have example files and tests only in maya and houdini. Maybe we can ask them to help with specification. I can not help more, I am not technical enough for it.
Thanks, you did great job with alembic. Good luck.

The curves exporter certainly makes some assumptions that I don't agree with, and that I'll address at some point. This will be tracked in T76746 (which doesn't involve 3rd party proprietary software, so it's easier to tackle there).