Scale inconsistencies with default Blender features due to unit scale conversion

[eisegesis-623]

I'm encountering a pipeline issue when animating custom rigs alongside Metahumans imported with the Polyhammer suite.

• The tool imports and exports using Blender units (which default to meters) and appears to handle conversions to Unreal units (centimeters) itself
• Thus, the characters adhere to Blender-expected units.
• Thus, they do not adhere to the 100x scale other characters are authored at, which is necessary to use the built-in FBX exporter.
• Thus, to animate synced animations (such as wrestling), one or the other characters must be scaled 100x or 0.01x
• This makes it difficult to restore the original scale for export, as common animation techniques (like space switching) rely on world space. (This becomes especially challenging if the animation requires temporarily constraining one character's controllers to the other.)

If there are any known solutions to this problem, I'd be grateful.
Otherwise, if I could request a feature, perhaps there could be a setting to take the rig's object scale into consideration before export?

(This is arguably just as applicable to the Character DNA addon, but only one post category was available.)

[jack-yao91]

Hi @eisegesis-623, I would recommend working in scene scale 1. Even if you use 100x scale in blender to fix the issue with FBX and SkeletalMesh, the bind pose will still have non-1 scale factors.

There is a free addon "Send to Unreal" here that you can use to export FBX files with the correct corrections to the bind pose scale (while working in Blender scene scale 1.) There are alot of things like physics, etc. that get messed up when scene scale is not 1 in Blender.
https://poly-hammer.github.io/BlenderTools/send2ue/introduction/quickstart/

Another option is if you are animating using Rigify as your control rig in Blender, then the Character Control Rig addon handles this export back to Unreal for you directly from the visual transforms and scene scale 1.

Lastly, can you clarify why you couldn't create a discussion on the Character DNA category? (I just want to make sure we have permissions configured correctly)

[eisegesis-623]

Hi,
Apologies if I didn't describe it clearly. I do not have the other rig's scale set to 100 in Blender, it is just rigged at that larger size as if standard Blender Units were Unreal Units. All scale is still set to 1 to prevent those issues on Unreal's side. Scene scale is also set to 1 (though the FBX export scale settings are adjusted, which as far as I am aware does the same thing without the simulation artifacts). So a character that is 100cm tall in Unreal is 100m tall in Blender, and the import pretty smoothly.

With that out of the way, the core issue I'm describing is ultimately:

A lack of compatibility between the Polyhammer import/export and the built-in FBX exporter. (Due to the built-in exporter requiring a different approach to unit scale.)

And the proposal being:

A setting to optionally take the Polyhammer control rig's object scale into consideration before export.

(i.e. the Metahuman control rig could have a non-standard transform, such as a scale of 100 or 50 or 3.1416, and still export animations without any scale issues on Unreal's side. Perhaps this could even extend to non-zero translation and rotation, though that's probably unnecessary scope.)

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I wasn't aware of there being an active fork of Send2Unreal, thank you for pointing that out! Is it listed anywhere on the main Polyhammer website? That might be good for discoverability for other users.
Regardless, I don't think it would work for our project for a number of reasons, though it's good to have in the back pocket for other usecases.

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Oh yes, I could have created one in the Character DNA category, it's just that this issue might apply to both the Character DNA and the Control Rig addons and I had to choose only one of those categories for the post. Decided on putting it in Control Rig as it seems slightly more relevant to it than the other. Sorry for any confusion!

Thank you for your patience!

[jack-yao91]

Ok gotcha, so if I am understanding correctly. You want the exporter to read world transforms directly and then do the math to export the animation "visually as is" regardless of what the scale is on the "Control Rig" object in Blender. Right?

[eisegesis-623]

Exactly, yes.