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I am attempting to create FBX files from a Blender project for game development. However, I have encountered an irregular behavior I cannot explain and it is causing problems. I am not clear if this is a bug or there is something I am missing.

Direct blend files:

Above are two .blend files - one titled "working" and one titled "broken". Both blend files contain only a very simple hierarchy of an empty root axis with a 90 degree X-axis rotation and 8 branches of a simple tree as children. There are no rotations on any of the 8 branch children of the empty axis in either:

enter image description here

As far as I can tell, these two .blend files are "identical" except they were made by two different people. In both, the root axis has a 90 x-rotation, but all children objects have 0 rotations.

If you export an FBX from each .blend file, you get different behavior though. Export process in both cases:

  • Right click the Empty Axis and Select Hierarchy
  • File > Export FBX
  • Object Types: Empty & Mesh
  • Apply Scalings: FBX Units
  • Forward: Y forward
  • Up: Z up
  • Uncheck Apply Unit, Uncheck Use Space Transform, Uncheck Apply Transform

If you then re-import the FBX's that are made you will then see that the "bark" object from the "broken" version now has a -90 rotation. However, in the FBX from the "working" version there is no such added -90 rotation to the "bark" object under "Transform".

Here you can see the added -90 rotation that is added to the "bark" object when the "broken" FBX is imported. The tree looks the same but now this object has -90 on its transform. The same does not occur with the "working" FBX:

enter image description here

Here is the "working" FBX when it is imported showing no rotation on this "bark" object:

enter image description here

Is this likely a bug to be reported or is there some possible explanation for the difference I am missing? Does this behavior reproduce on your end also or make sense in any way?

Thanks for any help or clarification.

EDIT: BUG REPORT MADE HERE: https://projects.blender.org/blender/blender/issues/107915

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  • $\begingroup$ They are not identical .blends. One .blend is rotated 180 degrees from the other (applied.) $\endgroup$
    – Nathan
    Commented May 13, 2023 at 22:07
  • $\begingroup$ Fair enough Nathan. That's why I said "identical." There obviously must be some difference. But I don't think that explains the issue as that rotation is applied already. It still doesn't make sense to me that exporting to FBX is adding a visible -90 x-rotation on one and not the other from the same export/import process. In theory these could be completely different projects with different objects/meshes. We still shouldn't see this different behavior that I am aware of. Unless there is some way to see why it is happening and prevent it or at least make it consistent. Thanks. $\endgroup$
    – mike
    Commented May 13, 2023 at 22:11
  • $\begingroup$ Okay, well you should try with actually identical .blends. Personally, I'm not interested, because I have significant doubts about the good faith of this question. $\endgroup$
    – Nathan
    Commented May 13, 2023 at 22:17
  • $\begingroup$ Okay Nathan. This thread is the culmination of days of trouble shooting on the Unity forum for an asset for tree importing. I did not make the "working" one - someone on the forum did. I don't know why theirs works and mine doesn't. But it is clearly abnormal that the same hierarchies would create an extra x-rotation of -90 on one and not the other, regardless of the meshes in each object. I am not sure what "good faith of the question" means. I need to import many objects into Unity and can't have random -90 unapplied rotations added in an inconsistent fashion. It breaks Unity functions. $\endgroup$
    – mike
    Commented May 13, 2023 at 22:24
  • $\begingroup$ This is the simplest I have been able to narrow the issue down in an easy comparison of two blender files, each with only 9 objects (1 empty, 8 meshes). The hierarchy and rotations of the objects before exporting/importing should be what matters. If the hierarchy is identical and every object has identical settings, the same result should happen both times. But it does not. $\endgroup$
    – mike
    Commented May 13, 2023 at 22:25

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