Root motion implementation

[Hilpogar]

Objective

Closes: #23355
Final step for the root motion implementation !

Solution

Add the RootMotion component that stores the informations about the root motion for the current frame:

pub struct RootMotion {
    /// Translation delta with the previous frame.
    pub translation_delta: Vec3,
    /// Rotation delta with the previous frame.
    pub rotation_delta: Quat,
}

Add the RootMotionMode to control how the RootMotion is extracted. There are 2 modes, one with only translation and one with translation and rotation :

pub enum RootMotionMode {
    /// Extract only translation from the root target.
    Translation,
    /// Extract both translation and rotation from the root target.
    #[default]
    TranslationAndRotation,
}

Add the field root_motion_target and root_motion_mode to the AnimationPlayer:

pub struct AnimationPlayer {
    <...>
    root_motion_target: Option<AnimationTargetId>,
    root_motion_mode: RootMotionMode,
}

Testing

The test_root_motion tests the extraction of the root motion in a configuration with 2 blended clips. The test is done once with the clips running forward and once backward.

---

Showcase

You can run the root_motion example to see it in action.

[mockersf]

Seems like it would always be true here, maybe the match is not needed?

[Hilpogar]

Yes it's always true. I put the match in case we add more modes in the future so we don't forget to change here. I think it will be optimize by the compiler anyway. But I can replace it with just a true if you think it's not useful.

[mgi388]

Note for someone to check if this asset is allowed to be committed or if it needs to be a remote/web asset.

[Hilpogar]

Is this because of potential copyright or licensing issues with the model, or is it just to avoid overloading the repository? If it’s a licensing issue, please note that this is simply the existing fox model to which I’ve added root motion to the running animation.

[mgi388]

It’s to do with repo size / bloat. E.g., see #23567 (comment)

I don’t know the bytes threshold but AFAIK, I think bevy wants to avoid putting more assets in this repo if possible.

[Hilpogar]

I realized that removing the AnimationPlayer from an entity could "leak" the RootMotion component. So I added a hook to remove it with the AnimationPlayer. I've also added the cleaning behaviors in the tests.