Unity Animation (Animator / Mecanim)

Control animation state with Unity 6.3 LTS's Animator and Animator Controllers: parameters,
transitions, blend trees, layers, and humanoid IK. Targets Unity 6.3 LTS (6000.3).

When to use

• Use when connecting animation clips into a state machine, driving them from script via
  parameters, blending locomotion (idle→walk→run), layering an upper-body action over
  movement, or adding foot/hand IK on a humanoid rig.
• Use when the project has *.controller (Animator Controller) and *.anim assets, or a
  rigged model with an Avatar.

When not to use: simple non-skeletal value tweens (UI fades, position lerps) are better
done with a tween/coroutine — see unity-csharp-scripting. Timeline cutscenes are a separate
tool. 2D sprite frame animation also uses the Animator but with sprite keyframes.

Core workflow

1. Add an Animator to the model and assign an Animator Controller; for a humanoid model,
   set its rig to Humanoid so it has an Avatar (enables retargeting and IK).
2. Define parameters on the controller — Float (Speed), Bool (IsGrounded), Int,
   Trigger (Jump) — and states with transitions whose conditions read those parameters.
3. Set parameters from script, never poke states directly: SetFloat, SetBool,
   SetInteger, SetTrigger. The state machine resolves transitions for you.
4. Blend continuous motion with a Blend Tree (one Float like Speed drives idle↔walk↔run)
   instead of many discrete states + transitions.
5. Layer additive/override motion (e.g. an upper-body "aim" layer with an Avatar Mask) and
   control its layerWeight.
6. Verify in the Animator window during Play mode — the live state highlights and parameter
   values update, so you can see exactly which transition fired (or didn't).

Patterns

1. Drive locomotion + a one-shot action from script

using UnityEngine;

[RequireComponent(typeof(Animator))]
public class CharacterAnim : MonoBehaviour
{
    private Animator _anim;
    // Cache parameter hashes — faster and typo-proof vs string lookups every frame.
    private static readonly int Speed     = Animator.StringToHash("Speed");
    private static readonly int IsGrounded= Animator.StringToHash("IsGrounded");
    private static readonly int Jump      = Animator.StringToHash("Jump");

    private void Awake() => _anim = GetComponent<Animator>();

    public void Tick(float planarSpeed, bool grounded)
    {
        _anim.SetFloat(Speed, planarSpeed);     // drives a 1D blend tree (idle/walk/run)
        _anim.SetBool(IsGrounded, grounded);    // gates a falling/landing transition
    }

    public void DoJump() => _anim.SetTrigger(Jump);  // fire-and-forget; auto-resets after use
}

2. Smooth a noisy input into a blend parameter

// dampTime smooths Speed so the blend tree doesn't snap; great for analog sticks.
_anim.SetFloat(Speed, targetSpeed, 0.1f /* dampTime */, Time.deltaTime);

3. Play / cross-fade a state directly (bypassing parameter conditions)

// Useful for hit reactions where you want an immediate, explicit transition.
_anim.CrossFade("Hit", 0.1f);                    // blend over 0.1s normalized
// Or jump instantly:  _anim.Play("Hit");

4. Wait until the current state finishes

private System.Collections.IEnumerator AfterAttack()
{
    var info = _anim.GetCurrentAnimatorStateInfo(0);   // layer 0
    yield return new WaitForSeconds(info.length);      // approximate clip length
    // ...follow-up logic
}

Pitfalls

• SetTrigger missed or "sticks" — triggers are consumed by the next satisfied transition
  and auto-reset; if no transition consumes it, it can fire later unexpectedly. Use
  ResetTrigger to clear, or prefer a Bool when the condition is a sustained state.
• String parameter typos fail silently — a misspelled name just does nothing. Use
  Animator.StringToHash and cache the int hashes.
• Transition feels laggy — Has Exit Time makes the transition wait for the clip to reach
  a normalized time. Uncheck it for responsive, condition-driven transitions (jump, hit).
• Character slides or won't move — Apply Root Motion is on but your code also moves the
  transform (or vice versa). Decide: root motion or scripted movement, not both.
• Upper-body layer overrides the whole body — set the layer's Blend mode (Override vs
  Additive), assign an Avatar Mask, and tune layerWeight (0–1).
• IK does nothing — IK only applies inside OnAnimatorIK, requires "IK Pass" enabled on
  the layer, and needs a Humanoid Avatar.

References

• For blend trees (1D vs 2D Freeform/Directional), animation layers + Avatar Masks,
  and humanoid IK (OnAnimatorIK, SetIKPositionWeight, SetIKPosition, look-at), read
  references/blend-trees-and-ik.md.
• Primary docs: Unity Manual "Animation" section and ScriptReference/Animator.

Related skills

• unity-csharp-scripting — the MonoBehaviour and coroutine timing used above.
• unity-physics — moving the body that the animation visualises.
• game-ai — deciding when to play which animation state.