In this tutorial, Animation Mentor Game Animation and Character Animation mentor Natasha Krinsky walks through how to use Autodesk MotionBuilder (aka “MoBu”) to clean up and assemble motion capture for games. She shows how to bring in a character from Mixamo, hook up mocap clips, blend them in Story Mode, fix common issues like foot sliding, and prep the animation to send into a game engine.
Meet Natasha Krinsky
With experience on major game titles like Life is Strange, Madden, and Clockwork Revolution, Natasha brings a wealth of industry knowledge to her students at Animation Mentor. Her focus: helping animators master the art of nuanced, emotional performances. She mentors students in both the 3D Character Animation program and 3D Game Animation program.
You can learn more about Natasha here.
Why Use MotionBuilder for Mocap?
MotionBuilder is built specifically for working with body motion capture, and many studios use it alongside Maya in their animation pipeline.
Natasha highlights several advantages:
- Dedicated Story Mode that lets you edit and blend mocap clips almost like a video editor.
- A pose library system that makes it easy to store, reuse, and precisely match poses at the start and end of moves.
- The ability to keep multiple gameplay takes in a single file, so runs, idles, jumps, and attacks can live together.
- A workflow where you clean on a control rig first, then bake to the skeleton that will go into Unreal or another engine, preserving the original mocap.
Setting Up a Character in MotionBuilder
Natasha starts with a Mixamo character in a T‑pose and explains how to “characterize” it so MotionBuilder can drive it.
Key steps she demonstrates:
- Import the character into an empty scene and confirm there is a clean T‑pose as a separate take.
- Turn on X‑ray display to see the skeleton inside the mesh.
- From the Asset Browser, drag a Character asset onto the skeleton (not the mesh) and choose “Biped” when prompted.
- Rename the default “Character” node to something meaningful for your project.
Once the rig is characterized, it’s ready to receive mocap clips through Story Mode.
Using Story Mode to Assemble Mocap Clips
Story Mode is the heart of Natasha’s demo. She builds a mini sequence where the character:
She uses several Mixamo clips (run, jump, attack, jump off) and shows how to:
- Insert a Character Animation Track in Story Mode and assign it to the characterized rig.
- Add individual mocap files as clips on that track.
- Use the “eye” tool on the track to reposition clips in world space so the character actually runs toward the box.
- Overlap the end of one clip and the start of the next to create blends instead of hard cuts.
Because the jump mocap was recorded at ground level, she cuts the clip at the landing moment, then raises that segment so the character lands on the box instead of passing through it. She scales and blends clips to adjust timing so the runs and stops feel less abrupt.
This process is very similar to nonlinear video editing, but you’re editing motion capture in 3D.
Baking to a Control Rig and Cleaning
Once Natasha is happy with the basic motion matching in Story Mode, she switches to cleanup.
Her workflow:
- Plot (bake) the combined Story Mode motion to a Control Rig (IK/FK) so she can tweak poses and keys.
- Create a new Take, turn off Story Mode, and work directly on the plotted animation.
- Use animation layers to apply adjustments non‑destructively on top of the baked base.
She also shows how to:
- Use the pose library (Pose Controls) to capture a specific “final pose,” then apply it at both the start and end of the animation.
- Manage translation and rotation options (T and R) so the same pose can be placed at different locations in the world while matching orientation exactly.
- Zero out the pose influence after a few frames so it doesn’t “hold” for the entire shot.
This is especially useful for loops where the first and last frame must match.
Fixing Foot Sliding with Aux Effectors and IK
Foot sliding is one of the most common issues when blending mocap clips, and Natasha demonstrates how MotionBuilder helps control it.
She creates auxiliary effectors (aux effectors) for the feet and uses IK blending to lock or free them at specific times:
- When the foot is firmly planted, she keys IK on and activates the aux effector so the foot stays in place.
- As soon as the character starts to jump or step, she keys IK off and turns the aux effector off so the motion can flow naturally.
- She sometimes uses partial IK/rotation blending so planted feet still have a bit of rotation and do not feel completely cemented to the ground.
This approach lets her convert an unrealistic slide into a believable planted step leading into the jump.
Refining Motion with F‑Curves and Filters
For jittery mocap, Natasha opens the F‑Curves view (MotionBuilder’s version of the graph editor) on the base animation layer.
From there, she can:
- Manually adjust curves to smooth out unwanted noise in specific channels.
- Apply the Butterworth filter to selected keys, which acts as a smoothing filter for mocap data.
She notes that you can use this selectively—for example, on a bouncing root or noisy rotations—rather than flattening everything.
Adding a Prop with Constraints
To push the example further, Natasha creates a simple “weapon” prop and constrains it to the character’s wrist.
Process she follows:
- Create a cube in the scene and shape it into a simple rod.
- Add a Parent/Child constraint from the Asset Browser.
- In the Navigator, set the wrist bone as the Parent and the prop as the Object.
- Temporarily disable the constraint, position the prop correctly in the hand, then use “Snap” to line it up and activate the constraint.
- Optionally “Lock” the constraint so the prop cannot be moved accidentally, then unlock it when she needs to adjust it.
This is the basis of driving weapons, tools, or other held objects with a character’s animation.
Handy MotionBuilder Tricks Natasha Uses
Throughout the tutorial, Natasha calls out some small but powerful MotionBuilder features that differ from Maya:
- Pinning body parts (like the chest or hips) so you can move another part (like the head) without dragging everything else.
- Switching between full‑skeleton, partial‑skeleton, and single‑bone selection modes for finer control while keying.
- Creating “nulls” (called nuls in MotionBuilder) and motion trails for visualizing paths and refining arcs.
Together, these tools make it easier to nudge mocap into more appealing, game‑ready performances.
From MotionBuilder to Game Engine
In a production setting, the final step after cleanup would be to bake the polished control‑rig animation back down onto the skeleton that your engine uses and export that skeleton and clips to Unreal or another runtime.
Natasha emphasizes that this bake‑back step keeps the original mocap accessible while giving you a clean, engine‑ready animation on the actual game skeleton.
To go deeper into game animation workflows, including how mocap fits alongside keyframed animation, Natasha recommends checking out the Animation Mentor Game Animation program, where you can practice run, jump, combat, and other gameplay moves using professional pipelines.
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