Root motion in Unity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this root motion code snippet?

Consider the following Unity C# code inside an Animator's OnAnimatorMove() method. What will be the final position of the GameObject after this frame?

Unity

void OnAnimatorMove() {
    Vector3 rootMotion = animator.deltaPosition;
    rootMotion.y = 0;
    transform.position += rootMotion * 2f;
}

A The GameObject moves twice the animator's deltaPosition on the XZ plane.

B The GameObject moves by the animator's deltaPosition including Y axis.

C The GameObject does not move because rootMotion.y is set to zero.

D The GameObject moves by half the animator's deltaPosition on the XZ plane.

💡 Hint

Remember that animator.deltaPosition gives the root motion movement for this frame, and modifying the Y component affects vertical movement.

The code takes the animator's deltaPosition, zeroes out the Y component to ignore vertical movement, then multiplies the XZ movement by 2 before adding it to the transform's position. So the GameObject moves twice the horizontal root motion.

🧠 Conceptual

intermediate

Which statement about root motion in Unity is correct?

Choose the correct statement about how root motion works in Unity animations.

A Root motion is automatically applied even if <code>Animator.applyRootMotion</code> is false.

B Root motion only affects the rotation of the GameObject, not its position.

C Root motion applies the movement of the animated root bone directly to the GameObject's transform.

D Root motion can only be used with Mecanim's legacy animation system.

💡 Hint

Think about what root motion means in terms of animation and GameObject movement.

Root motion means the movement of the root bone in an animation is applied to the GameObject's transform, moving it in the scene. This happens only if applyRootMotion is true.

🔧 Debug

advanced

Why does this root motion code cause the GameObject to jitter?

Examine the following code snippet inside OnAnimatorMove(). The GameObject jitters when moving. What is the cause?

Unity

void OnAnimatorMove() {
    transform.position += animator.deltaPosition;
    transform.rotation = animator.rootRotation;
}

A The rootRotation is not normalized, causing jitter in rotation.

B The code adds deltaPosition manually but does not set <code>animator.applyRootMotion</code> to false, causing double movement.

C The code should multiply deltaPosition by Time.deltaTime to smooth movement.

D The code should use <code>transform.Translate()</code> instead of modifying position directly.

💡 Hint

Check the applyRootMotion setting and how root motion is applied automatically.

If applyRootMotion is true, Unity applies root motion automatically. Manually adding deltaPosition causes the GameObject to move twice, resulting in jitter.

📝 Syntax

advanced

Which option correctly overrides OnAnimatorMove to apply root motion only on the XZ plane?

Choose the code snippet that correctly overrides OnAnimatorMove() to apply root motion only on the horizontal plane (X and Z), ignoring vertical movement.

A

void OnAnimatorMove() {
    Vector3 motion = animator.deltaPosition;
    motion.y = 0;
    transform.position += motion;
    transform.rotation = animator.rootRotation;
}

B

void OnAnimatorMove() {
    Vector3 motion = animator.deltaPosition;
    transform.position += motion;
    transform.rotation = animator.rootRotation;
}

C

void OnAnimatorMove() {
    Vector3 motion = animator.deltaPosition;
    motion.y = 0;
    transform.Translate(motion);
    transform.rotation = animator.rootRotation;
}

D

void OnAnimatorMove() {
    Vector3 motion = animator.deltaPosition;
    motion.y = 0;
    transform.position = motion;
    transform.rotation = animator.rootRotation;
}

💡 Hint

Remember that transform.position += motion adds movement, while transform.position = motion sets absolute position.

Option A correctly zeroes out the Y component and adds the horizontal root motion to the current position. Option A incorrectly sets position absolutely, causing snapping. Option A uses Translate(motion), which applies the motion in local space (default Space.Self) instead of world space, making it incorrect for root motion deltaPosition. Option A does not ignore vertical movement.

🚀 Application

expert

How to synchronize root motion with physics Rigidbody movement?

You want to use root motion to move a character with a Rigidbody component for physics interactions. Which approach correctly applies root motion to the Rigidbody without breaking physics simulation?

A Set <code>rigidbody.position += animator.deltaPosition</code> and <code>rigidbody.rotation = animator.rootRotation</code> directly in <code>Update()</code>.

B Apply root motion by setting <code>transform.position</code> and <code>transform.rotation</code> directly, ignoring Rigidbody.

C Disable root motion and move the Rigidbody manually using forces based on animation speed.

D In <code>OnAnimatorMove()</code>, call <code>rigidbody.MovePosition(rigidbody.position + animator.deltaPosition)</code> and <code>rigidbody.MoveRotation(animator.rootRotation)</code>.

💡 Hint

Think about how to move Rigidbody objects properly to keep physics stable.

Using rigidbody.MovePosition and MoveRotation inside OnAnimatorMove() applies root motion while respecting physics. Directly setting position or rotation breaks physics and causes jitter.