godot: AnimationTree: Improper blending of bone location when blending more than two animations

Godot version: Godot 3.2.4 rc1

OS/device including version: Windows 10 version 1909

Issue description:

When blending more than two animations using two separate Animation Nodes (such as a Blend2 being consumed by another Blend2), the location of bones isn’t properly interpolated if more than 2 animations are being blended together. This is most noticeable when transitioning from a Blend Amount less than 1 to exactly 1, which causes a noticeable “snapping”, because values arbitrarily close to 1 have a noticeably different location. It seems rotations are interpolated smoothly. I’ve also tested this with a BlendSpace2D within a BlendSpace1D, and using transitions to have a similar effect. Both have the same problem, where there’s a jarring transition when reaching an animation. However, here’s a strange exception: if the blended node is put into the in slot of a Blend2 and the third animation the blend slot, it works fine.

video of the problem

Steps to reproduce:

1. Create a model with three skeletal animations. For two of them, translate one or more bones (make sure to not just rotate them).
2. Import the model and animations and create an AnimationTree using the model’s AnimationPlayer. Set its root as an AnimationNodeBlendTree
3. Create a Blend2 of the translated animations. I also tested with a BlendSpace1D and BlendSpace2D. Set the blend_amount between 0 and 1 (exclusive)
4. Create another Blend2. Set the in slot to use the third, non-translated animation, and set blend to the translated animation blend.
   • (Important info: if you swap the in and blend inputs, the problem goes away in this example. This could be a useful workaround, unless you’re blending between two blending nodes, which is my real use case, in which case the issue is present regardless of the input order).
5. Slowly move the blend_amount of this new node from 0 to 1
6. Watch as the animation snaps into place when going from 0.99 to 1

Minimal reproduction project:

project.zip

Things to do in this project:

• Open the AnimationTree and slowly move the blend amount of problem_blend from 0 to 1. The box will snap when going from 0.99 to 1 and back.
• Set the blend amount of down to 0 or 1 and repeat the previous step. The problem disappears
• Swap the inputs of in and blend for problem_blend and repeat step 1. The problem disappears