Seamless looping animations stutter at the last frame before looping

[alfredbaudisch]

Godot version

4.0 alpha 16 and also 3.5-stable

System information

Windows 10 Pro 64-bits, RTX 3080 Ti (516.94)

Issue description

• Godot does not seem able to seamless loop skeletal mesh rotation animations that rotate from 0 to 360 degrees.
• Godot stops for a frame in the last frame before looping.
• To make sure it's not a problem with my animation, I tested the same animation in Blender, Unity, Unreal Engine, Godot 3.5 and Godot 4-alpha16, and only in Godot (both versions) that it doesn't loop perfectly, freezing at the last frame before looping.
• Interpolation is set to Linear (everything as default).

• After more investigation, I noticed Godot duplicate the 1st frame when importing animations where the 1st frame is the same as the last frame - this is what causes the apparent stutter:

Animation Setup

1. The 1st setup I tried: animation composed of a bone where the 1st keyframe is at 0º and the last at 354º - this is the one looping seamlessly in the other engines.
2. 2nd setup tried: 1st keyframe at 0º and last at 360º (wrong, because the last frame and 1st frame are at the same position).

Expected

The rotation animation should loop without pauses.

For example, in UE (in this GIF the animation loops 2 times before the end of the GIF, the freeze at the end is due to the GIF. You can follow the looping by moving your cursor on top of one of the blades and following it - see how it's seamless for 2 revolutions):

Biggest proof that it just works everywhere else. The wheel animation in my game trailer below (starts at 0:11) is EXACTLY the same unaltered in UE, it's the same mesh and Blender NLA action (the difference in the sample project attached below is that I removed the other parts of the mesh from the same .blend file):

https://twitter.com/PardallGames/status/1560658418431434753

Steps to reproduce

1. In Blender, create an armature, add a bone and create a new animation (Action).

2. Set the bone rotation to Euler Angles:
   

3. Set the 1st keyframe rotation to Y: 0º and last keyframe (60th frame) to 354º.

4. Assign the bone to any kind of mesh, as long as you can see the mesh rotating.

5. Push Down the Action as a NLA Track and export either as FBX or GLTF.

6. Import the mesh and animation into Godot and play it looping. The last frame freezes for a frame before looping.

Minimal reproduction project

Godot 4-alpha16 project and Blend file:

LoopingMachineFreezing.zip

An additional project with a Humanoid with Mixamo animations, which loop correctly in Blender and Unity, but in Godot it has duplicate initial frames:
Godot4-LoopingHumanoid.zip

Regardless of it being Godot4-alpha16, THE SAME happens in Godot 3.5 - which is the worrisome part.

Temporary Solutions / Hacks

1. Adjust every animation manually: Seamless looping animations stutter at the last frame before looping #65513 (comment)
2. Use my Godot's editor option "Remove Duplicated First Frames" (no need to change meshes and animations): Seamless looping animations stutter at the last frame before looping #65513 (comment)

[cybereality]

I have tested @alfredbaudisch files on my machine (Ubuntu 22.04 and AMD RX 6800 XT) and I see the same thing here. So I don't believe it is OS or GPU/driver related.

[TokageItLab]

Let me clear up the issue, this is a loop problem, not a rotation problem, right?

[alfredbaudisch]

Let me clear up the issue, this is a loop problem, not a rotation problem, right?

Yes, correct, a looping problem.

[alfredbaudisch]

I managed to make it work, but this feels like a hack/workaround because it requires all meshes/animations to be changed - not a solution to the bug.

Summary of the workaround: the 2nd frame of the looping animation should be moved to be the 1st. And the current 1st frame should be moved to frame 0, but not exported.

PS: if you want to preserve your meshes and animations check my other temporary solution in another comment: #65513 (comment).

Temp Fix / Hack

• Move the keyframe that is related to rotation at 0 degrees to keyframe 0 .
• The NLA strip/the animation itself should start at frame 1 (rotation at 5.9º)
• And it should end at whatever frame you want, as long as the last value has the rotation at 360º.
• Now it loops seamless.
• If the looping animation is not related to rotation, the same logic applies, the 2nd frame should actually be the first, and the current 1st frame should be moved to frame 0 - outside of the exported animation.
• This is not the consistent behaviour with any other 3D application or engine.

Updated sample:
engine-sample.zip

[alfredbaudisch]

I found more details about the bug. Updated the main issue.