/unity-animation

Blend trees, layers & humanoid IK (Unity 6.3 LTS)

Depth for unity-animation. Verified against the Unity Manual Animation section and ScriptReference/Animator.

Blend trees

A Blend Tree blends several clips by one or two Float parameters, so continuous motion (idle → walk → run, or 8-direction movement) is one node instead of many states.

• 1D — one parameter (e.g. Speed). Place clips along the axis (0 idle, 2 walk, 6 run); the tree cross-fades by where the parameter sits.
• 2D Simple Directional — clips at distinct directions (forward, back, strafe).
• 2D Freeform Directional / Cartesian — arbitrary clip placement in a 2D field (MoveX, MoveY); Freeform Cartesian suits non-directional pairs (speed × turn).

Drive it from script exactly like any float parameter, ideally damped:

_anim.SetFloat("Speed", speed, 0.1f, Time.deltaTime);   // dampTime smooths the blend
_anim.SetFloat("MoveX", moveX);
_anim.SetFloat("MoveY", moveY);

Tip: keep blend thresholds matched to the clips' real root speed so feet don't slide.

Animation layers + Avatar Masks

Layers let an upper-body action (aim, throw) play over a base locomotion layer.

• Each layer has a weight (0–1) and a Blend mode: Override (replaces) or Additive (adds on top of the base pose).
• An Avatar Mask restricts a layer to selected bones (e.g. spine + arms), so the legs keep walking while the arms aim.

int aimLayer = _anim.GetLayerIndex("UpperBody");
_anim.SetLayerWeight(aimLayer, isAiming ? 1f : 0f);   // fade the layer in/out

Humanoid IK (OnAnimatorIK)

IK adjusts hands/feet/look toward targets after the animation pose is sampled. Requirements: Humanoid Avatar, and IK Pass enabled on the layer.

[SerializeField] private Transform lookTarget, rightHandTarget;

// Called by the Animator once per frame, per layer, when IK Pass is on.
private void OnAnimatorIK(int layerIndex)
{
    if (lookTarget)
    {
        _anim.SetLookAtWeight(1f);                 // overall look-at weight
        _anim.SetLookAtPosition(lookTarget.position);
    }
    if (rightHandTarget)
    {
        _anim.SetIKPositionWeight(AvatarIKGoal.RightHand, 1f);   // 0..1 blend toward target
        _anim.SetIKRotationWeight(AvatarIKGoal.RightHand, 1f);
        _anim.SetIKPosition(AvatarIKGoal.RightHand, rightHandTarget.position);
        _anim.SetIKRotation(AvatarIKGoal.RightHand, rightHandTarget.rotation);
    }
}

AvatarIKGoal values: LeftFoot, RightFoot, LeftHand, RightHand. Set the weight back toward 0 to release the IK smoothly.

Foot IK for sloped ground

• Enable Foot IK on the locomotion states.
• Raycast down from each foot, then SetIKPosition/SetIKRotation to the surface point and normal so feet plant on slopes instead of clipping or floating.

Gotchas

• IK calls outside OnAnimatorIK are ignored.
• SetLookAtWeight has extra body/head/eyes/clamp parameters — the single-arg overload is a shortcut for the common case.
• Layer weights and IK weights are not auto-reset; lerp them toward 0 when disengaging or the pose stays stuck.
• Additive layers need a reference pose set on the clip's import settings, or they drift.