How to set up player movement in the Unity engine where the speed is set by a script and animation root motion is not applied, but synchronize the two in order to avoid issues such as foot sliding.

The problem

In animation blend trees, root motion isn’t linearly blended. Blending halfway towards a forward movement animation will not result in a root motion with half that animation’s speed.

This likely has to do with things like foot phase synchronization. More info on this is available at https://kybernetik.com.au/animancer/docs/manual/blending/mixers/synchronization/.

The solution

The key aspect to work around non-linear root motion blending is to not blend root motion at all. Instead, we can manipulate the speed of animations with known root motion velocity in order to match in-game player velocity.

We still use a blend tree to simplify transitioning between different movement animations, even though we aim to always end up with a single active animation.

Here’s the step by step approach:

1. Set up blend tree
   • 2D simple directional (no idle animation)
   • 8 directional animations (minimize velocity difference when blending - for example, blending a forward with a sideways animation can result in diagonal movement that is slower than either of the other animations individually. Blending animation root motion is poorly documented.)
   • Adjust animation speeds so that each has a resulting velocity of 1m/s
     1. Compute positions -> Velocity XZ
     2. Divide animation speed by norm of velocity
   • Reset animation positions to unit circle coordinates
   • Set blend tree animation direction through X and Y parameters connected to user input
   • Adjust blend tree speed by setting a parameter to be equal to the player’s speed
2. Transition to/from idle animation
   • Idle animation cannot be in blend tree as the blend tree’s speed will be modified
   • Blending between idle and movement animations is unreliable anyway in terms of resulting velocity (unpredictable, non-linear)

This is what the resulting blend tree looks like:

It may be possible to still blend between movement and other animations by nesting blend trees, although copy pasting a blend tree you previously created requires a simple workaround.