The Complete Guide to Retargeting Mocap to Custom Character Rigs

You've found the perfect motion capture animation, but there's a problem: it was created for a different skeleton than your custom character. Maybe you're using stylized characters with exaggerated proportions, or you've built a unique creature that doesn't match standard humanoid rigs.

This is where animation retargeting comes in—one of the most powerful (and sometimes frustrating) techniques in game development. In this comprehensive guide, we'll walk through retargeting workflows for all three major engines and solve the common problems that trip up even experienced developers.

What is Animation Retargeting?

Animation retargeting is the process of transferring motion data from one skeleton (the source) to another skeleton (the target) with a different bone structure, naming convention, or proportions.

Think of it like this: the source skeleton is an actor performing movements, and the target skeleton is a puppet that needs to mimic those movements—even though the puppet might have longer arms, shorter legs, or completely different joint positions.

Modern retargeting systems use Inverse Kinematics (IK) to solve this problem. Instead of directly copying bone rotations (which would break horribly with different proportions), IK solvers calculate where bones should end up to achieve similar poses and contact points.

When Do You Need Retargeting?

You'll need to retarget animations when:

• Different skeleton structures – Your character has more or fewer bones than the source animation (extra twist bones, different finger setups, etc.)
• Different proportions – Your character is taller, shorter, or has different limb ratios than the source
• Different bone naming – Source uses "LeftArm" while your rig uses "arm_L" or "L_Arm"
• Different hierarchy – Bones are parented differently (spine structure, hand bone chains)
• Cross-engine transfers – Moving animations between Mixamo, UE5 Mannequin, or custom rigs

Retargeting in Unreal Engine 5

UE5 introduced the IK Retargeter system, which is significantly more powerful than UE4's bone-based approach. Here's the complete workflow:

Retargeting in Unity

Unity's Mecanim system handles retargeting through its Humanoid Avatar system. It's more automatic but less flexible than UE5's approach.

Retargeting in Blender

Blender requires add-ons for proper retargeting. The most common solutions are Rokoko Studio Live, Auto-Rig Pro, or the free Rigify retarget tools.

Common Issues & Solutions

Even with proper setup, retargeting can produce unexpected results. Here are the most common problems and their fixes:

Foot Sliding / Floating

Problem: Character's feet slide across the ground or float above it during walk/run cycles.

Causes:

• Height difference between source and target skeletons
• Missing or misconfigured foot IK
• Root motion mismatch

Solutions:

• UE5: Enable "Force Root Lock" on the animation asset, or use Control Rig with IK foot solvers
• Unity: Enable "Foot IK" on the Animator component, adjust "Feet Spacing" in avatar config
• All engines: Scale animation or adjust retarget settings to match character height

Broken Fingers / Hands

Problem: Fingers twist unnaturally, collapse into fists, or point in wrong directions.

Causes:

• Different finger bone counts (3 vs 4 bones per finger)
• Different finger hierarchy or naming
• Source animation lacks finger data

Solutions:

• Create separate finger chains in your IK Rig (not just whole-hand)
• If source lacks finger data, use procedural finger poses or hand IK
• For simpler hands, exclude fingers from retargeting entirely

Twisted Spine / Broken Hips

Problem: Character's torso twists incorrectly, or hips flip during certain movements.

Causes:

• Different spine bone counts or positions
• Incorrect retarget root assignment
• Gimbal lock in Euler rotations

Solutions:

• Ensure retarget root is the actual hip/pelvis bone, not a root motion bone
• Match spine chain lengths (if source has 3 spine bones, configure 3 in target chain even if you have 5)
• Enable quaternion interpolation where available

Animation Plays at Wrong Scale

Problem: Movement distance is too large or too small—character takes tiny steps or huge leaps.

Solutions:

• Check FBX import scale settings (Unity: 0.01 vs 1.0, Unreal: automatic vs manual)
• Adjust root motion scale in animation settings
• Use animation layers to blend with scaled movement

Best Practices

Follow these guidelines to minimize retargeting headaches:

1. Start with a Standard Base

When creating custom characters, build on standard skeleton templates (UE5 Mannequin, Mixamo) and add extra bones rather than starting from scratch. This makes most marketplace animations work with minimal adjustment.

2. Test Early with Placeholder Animations

Before purchasing an entire animation pack, test one or two animations with your character. Identify issues early when you can still adjust your rig.

3. Document Your Bone Mappings

Create a reference document showing which bones map to what. This saves hours when setting up new characters or troubleshooting months later.

4. Use Additive Layers for Fixes

Instead of editing retargeted animations directly, use additive animation layers to correct issues. This preserves the original and allows non-destructive adjustments.

5. Batch Process When Possible

Once you have a working retarget setup, batch process all animations at once. This ensures consistency and saves time compared to one-by-one adjustments.

Wrapping Up

Animation retargeting is one of those skills that seems daunting at first but becomes second nature with practice. The key is understanding that you're not just copying data—you're translating movement intent between different body structures.

Start with simple animations (idles, walks) to verify your setup works before tackling complex combat or acrobatic sequences. And when things go wrong—because they will—systematically check your bone mappings, IK chains, and root configuration.

Modern engines have made retargeting dramatically easier than it was even five years ago. UE5's IK Retargeter in particular is a game-changer for handling proportion differences that would have required manual keyframe adjustment in the past.

Have questions about retargeting animations to your specific character? Reach out to our team—we're happy to help troubleshoot.