**Problem:**
I am encountering an issue with a character exported from Mixamo and processed in Houdini using the kinefx tools (`FBX Character Import` and `FBX Character ROP` nodes). Without making any modifications to the data in Houdini, when I re-upload the exported FBX back to Mixamo, the skinning weights are messed up, causing strange deformations. Re-uploading the original Mixamo-exported FBX works fine.

**Steps Taken:**
1. Imported character into Houdini using `FBX Character Import` with the unit conversion option enabled.
2. Exported the character using `FBX Character ROP`, tested with and without the unit conversion option.
3. Did not use any additional `Transform` nodes, relying solely on the `FBX Character Import` and `FBX Character ROP` nodes for unit handling.

**Observations:**
- The bone hierarchy and scale appear unchanged.
- The issue seems to be specifically related to the skinning weights when the file is re-uploaded to Mixamo.
- The `FBX Character ROP` node does not provide an option to choose the FBX version, which might be contributing to the issue.

**Questions:**
1. Has anyone encountered similar issues when round-tripping FBX files between Mixamo and Houdini?
2. Are there specific settings or nodes in Houdini that are critical for preserving skinning weights during export?
3. Are there any known issues with the `FBX Character ROP` node that might affect compatibility with Mixamo?
4. Any additional debugging steps or tools in Houdini that could help identify the source of the problem?

Any insights or suggestions would be greatly appreciated!

Drasko Ivezic
- The `FBX Character ROP` node does not provide an option to choose the FBX version, which might be contributing to the issue.

Filmbox FBX render node [www.sidefx.com]

The Filmbox FBX ROP node allows specification of the SDK via parameter.

Note from the Docs

The FBX format specification generally does not support animated objects with a changing number of points or changing connectivity, such as Houdini’s particle systems or fluids. It is only possible to export these types of objects by breaking them up into individual triangles (although still within one FBX object node).

PHENOMDESIGN

Drasko Ivezic
- The `FBX Character ROP` node does not provide an option to choose the FBX version, which might be contributing to the issue.

Filmbox FBX render node [www.sidefx.com]

The Filmbox FBX ROP node allows specification of the SDK via parameter.

Note from the Docs

The FBX format specification generally does not support animated objects with a changing number of points or changing connectivity, such as Houdini’s particle systems or fluids. It is only possible to export these types of objects by breaking them up into individual triangles (although still within one FBX object node).

Hi, this is not really helpful. To use skinned characters together with the shared hierarchy, you have to use character fbx rop. And with this one, there are no parameters you can use to select the version of fbx. I tried to use the normal fbx rop, but with that I only get the skin, and when I try to use this in Mixamo, it behaves as if there is no skeleton hierarchy in the file, so there is no way to use it. I am also aware of the limitations that the topology cannot be changed, but that is not the problem here. The problem is that I have a fbx in Houdini and something else comes out, but I have neither requested nor authorised this change. So if you ask me, it's a buggy fbx character rop because it generates some data that mixamo obviously doesn't like. I even tried reskinning the character with the jointcapturebiharmonic node, and in Houdini the deformations look fine, but back to mixamo and the skin weights are broken.

It's really hard to say what Mixamo isn't liking from the output FBX. Have you tried contacting Mixamo support with your FBX file? I'm assuming that if you import said exported FBX into Houdini again, everything is working?

edward
It's really hard to say what Mixamo isn't liking from the output FBX. Have you tried contacting Mixamo support with your FBX file? I'm assuming that if you import said exported FBX into Houdini again, everything is working?

Hi, there is no support page for mixamo, unless Adobe has hidden it so well that even their own support team can't find it. And looking in the Adobe user forums doesn't help either, as their user base isn't connected to the Houdini user base. Do you have a link on their mixamo user support page where I can upload files and ask for help? I couldn't find any.

No, I have no idea about Mixamo. I just assumed they have some way of contacting.

Just some things to try while you are connecting with Mixamo would be to follow this example for op referencing in the hierarchy into the filmbox ROP?

Drasko Ivezic
Hi, this is not really helpful. To use skinned characters together with the shared hierarchy, you have to use character fbx rop. And with this one, there are no parameters you can use to select the version of fbx.

Drasko Ivezic
I tried to use the normal fbx rop, but with that I only get the skin, and when I try to use this in Mixamo, it behaves as if there is no skeleton hierarchy in the file, so there is no way to use it.

Drasko Ivezic
The problem is that I have a fbx in Houdini and something else comes out, but I have neither requested nor authorised this change. So if you ask me, it's a buggy fbx character rop because it generates some data that mixamo obviously doesn't like.

Drasko Ivezic
there is no support page for mixamo,

PHENOMDESIGN
Just some things to try while you are connecting with Mixamo would be to follow this example for op referencing in the hierarchy into the filmbox ROP?

Hm, thanks, I find it a bit strange to use it this way when I have imported a character together with the skeleton hierarchy in Sops with Character Import Sop. The hierarchy is already set there to work in sops, so I need a sops solution to export fbx with bones and all. I tried importing the character as fbx hierarchy and exporting with fbx rop, but the bones and skinning attributes were not available.

Thats a bummer.

I found this info on the Docs:
"Houdini bones are converted to joints on export, since most applications seem to export/import just joints, and not bones, from FBX (it can represent both)."

Here is one more example using op: but a bit different.

Houdini Tutorial: Export FBX with Children from one SOP node [www.youtube.com]

I reported the bug for the fbx char rop and when the developers starts to test it maybe it will get fixed. It is probably just some minor mistake. And I guess the devs would probably get some more useful info from Adobe is they ask for it. It is worthy to pressure adobe to introduce the usd support to mixamo.