Do we have test scenes?

I'll look into those.

@fire I used this model in quick check to show axes where everything is a sibling of root:

constraint_demo.zip

@fire For now, since the feature freeze is nearing, I separated the PR only from the refactoring part, which will cause break compatibility; Since LookAtModifier is added in 4.4, if this PR is carried over to 4.5 it will occur.

• Move enum BoneAxis to SkeletonModifier from LookAtModifier #101148

I will write a CI test later, because this module does not depend on external factors such as time, but purely mathematical transformations, so it is possible to check for them.

For reference, see also this GDScript implementation of VRM constraints I made for godot-vrm: https://github.com/V-Sekai/godot-vrm/blob/master/addons/vrm/node_constraint/bone_node_constraint.gd

Also, I made an example model here: vrm-c/vrm-specification#444 Before merging this PR, we should write a draft PR to godot-vrm that uses this PR and test that it works correctly.

This PR looks much more comprehensive than what I wrote and seems to handle everything in VRM and more, awesome! Feel free to request a review from me once this PR is ready to go.

Improved handling of zero division in ConvertTransformModifier. Also, "target bone" has been renamed to "reference bone".

While "target" in Aim is clear, the word "target" in Convert and Copy is a bit confusing (although Blender consistently calls it "target"). This rename would make Aim "target" to "reference", but I think it's still understandable.

If anyone has any other better names, suggestions are welcome.

It's april, my time to work on this is now.

I have already shared this with the team, but I will put a demo project for compatibility with VRM (by manually porting settings).

constraint_demo.zip

Model:
https://hub.vroid.com/characters/3609285562418162352/models/5465162707471684141

Animation (please download and replace to solve dependency):
https://booth.pm/ja/items/5512385

Finger settings are not ported. There are too many, but I decide that the few files animating the fingers are sufficient without them for testing. If you find anything odd, please compare Unity's UniVRM and Godot's Modifier settings. There may be a mistake in seeing the amount or something else depending on my eyes.

However, Constraint implementation itself should be correct since this PR add and pass the random transforms test with MathCheck.

Rebased to follow #106622 (comment).

Thanks!

It might be because I'm misunderstanding the system but is there a way to apply the transform in the local space of the apply bone?

@scotdoria Copy/ConvertTransform mod transfers are performed in local space. Several values (local by based rest, based current pose, based initial transform) can be calculated by combining the Relative/Additive options. You can find the explanation in the class reference.

In the future, model/global spaces possibly also be available, but it will take time to implement many safety calculations. The current implementation focuses on achieving natural character posing by covering compatibility with the VRM format and some Blender settings.

By the way, if local space is not your intended space after retargetingg rest may be overridden by retargeter, so please set up the Advanced import option to use RetargetModifier.

• Add RetargetModifier3D for realtime retarget to keep original rest #97824

A 2D version is needed