AnimationUtils.makeClipAdditive Behavior

You're correct in your observation about AnimationUtils.makeClipAdditive. This method does indeed use the first frame of the animation clip as the reference pose for calculating additive animations, rather than using the skeleton's rest pose.

The method assumes that the first frame of the animation represents a neutral or base pose from which the additive animation should be calculated. This can be problematic if your first frame is already in an animated state rather than matching the rest pose.

Unfortunately, based on the available documentation, there isn't a direct parameter or option in makeClipAdditive to specify using the skeleton's rest pose instead of the first frame.

Possible Workarounds

1. Create a custom solution: You could potentially create your own version of the additive animation calculation that uses the rest pose as reference.

2. Modify your animation clips: Ensure your animation clips start from the rest pose, or create a version that does.

3. Use SkeletonUtils: While not directly solving this issue, the SkeletonUtils library provides methods like retargetClip that might help in manipulating animations to achieve the desired effect.

If you need more specific implementation details, you might want to look at the source code for AnimationUtils.makeClipAdditive and create a modified version that uses the rest pose information from your skeleton.