Physics interpolation doesn't interpolate Skeleton2D animation correctly, resulting in wobbly visuals

[Calinou]

• Possibly related to Transform2D::interpolate_with result produces unintended skew #71974.

Tested versions

• Reproducible in: 4.3.dev b7feebe, 3.6.beta 945a490

System information

Godot v4.3.dev (b7feebe) - Fedora Linux 39 (KDE Plasma) - X11 - GLES3 (Compatibility) - NVIDIA GeForce RTX 4090 (nvidia; 550.78) - 13th Gen Intel(R) Core(TM) i9-13900K (32 Threads)

Issue description

Physics interpolation doesn't interpolate Skeleton2D animation correctly, resulting in wobbly visuals.

I've tried disabling interpolation on specific nodes on the SkeletalPlayer scene, but the only thing that fixes the issue is disabling interpolation on the entire CharacterBody2D – which makes any movement not interpolated. I've also tried changing the process mode of the AnimationPlayer and/or AnimationTree to Physics instead of Idle, but that didn't help either.

This issue is still noticeable at 60 tps on a 120 Hz display, so it's not just at low physics FPS.

All videos were recorded at 60 FPS.

10 ticks per second, physics interpolation enabled

10_tps_with_interpolation.mp4

10 ticks per second, physics interpolation disabled

Skeleton updates at the rendered framerate, so does it actually need to be interpolated here? I've tried disabling interpolation on the skeleton/AnimationPlayer/AnimationTree/Polygon2D nodes and it didn't help though. I've also tried changing the process mode of the AnimationPlayer and/or AnimationTree to Physics instead of Idle.

10_tps_without_interpolation.mp4

10 ticks per second, physics interpolation enabled, AnimationPlayer + AnimationTree using Physics callback mode

10_tps_interpolation_physics_callback.mp4

120 ticks per second, physics interpolation enabled

At this stage, it looks pretty much the same whether interpolation is enabled or not.

120_tps_with_interpolation.mp4

Steps to reproduce

• Create an animation that uses Skeleton2D.
• Enable physics interpolation and set physics ticks per second to 10.

Minimal reproduction project (MRP)

skeleton.zip

[lawnjelly]

Ah this is interesting, I hadn't noticed that.

It's likely it will need to the interpolated transform somewhere in the relative xforms, will investigate. 👍

[lawnjelly]

Have figured this out, took a while debugging the numbers but wasn't as bad a problem as I feared.
Will prepare PR, as it will be good to have this fixed in 3.x and 4.x.

[lawnjelly]

Fixed by #93309.

[akien-mga]

This still needs to be fixed in 4.3 IIUC?

[lawnjelly]

This still needs to be fixed in 4.3 IIUC?

Possibly, I'll be looking at this with @rburing .

There are significant differences in the 2D skinning in 4.x if I remember right, so we'll have to see whether the same issue / fix is needed.

[lawnjelly]

Re-opening as still needs to be done in 4.x (my mistake).