Best Practices for Effective Motion Capture Retargeting

Motion Capture Retargeting Image

Retargeting is a technique that transfers motion data from a source (usually a live performer or keyframed animation) to a target (a digital character). This isn’t a copy-paste; retargeting requires adjustments and refinements to ensure that the movements of the digital character authentically replicate the source data.

This technique allows animators to adapt motion data to different character shapes, sizes, and proportions. For instance, in a fantasy game, movements captured from a human performer can be retargeted onto a monster or character with different proportions. 

We’ve put together this guide to help aspiring animators understand what retargeting is and how they can ensure it’s effective in their projects.

Why is Retargeting Important?

Authenticity and Realism: The primary goal of retargeting is to achieve high levels of realism in character movements. By accurately mapping the motion data to the digital character, retargeting helps create believable and lifelike animations.

Versatility in Character Design: Retargeting allows animators to apply the same set of motion data to various characters, regardless of their physical differences. This versatility is especially valuable in projects with a high character count.

Efficiency: It significantly reduces the time and effort required to animate complex characters and movements manually. Using motion capture app data, animators can achieve detailed and dynamic movements more quickly than traditional keyframe animation.

Consistency: Retargeting ensures consistency in the character’s movements throughout the animation. It maintains the integrity of the original performance, delivering a cohesive and fluid motion that aligns with the character’s personality and physical attributes.

Adaptability: It allows for the adaptation of motion capture data to characters different to the source. This adaptability is essential when working with fantastical or exaggerated character designs that do not match the proportions of the human performer.

Best Practices for Effective Retargeting

Understanding Source and Target Models

Analyse Character Proportions: Before beginning the retargeting process, it’s crucial to understand the differences in proportions and anatomy between the source and target models. This understanding helps in making informed adjustments during the retargeting process.

Study Movement Dynamics: Understand the unique movement dynamics of both the source and target characters. This includes their weight, balance, and how they interact with their environment.

Motion Capture Data Handling

1. Preparation

Capture Motion Data: First, you need the motion capture data. This is usually done in a mocap studio with an actor wearing a mocap suit. The data is captured as a series of movements corresponding to the actor’s joints and body parts.

Select Software: Choose the software you will use for retargeting. Different software might have slightly different workflows.

2. Import Mocap Data

Import to Software: Import the mocap data into your chosen 3D animation software. This data typically comes in formats like BVH (Biovision Hierarchy) or FBX (Filmbox).

3. Handling Mocap Data

Clean Motion Capture Data: Ensure that the motion capture app data is clean and free of errors before retargeting. This includes

  1. Removing any noise or artefacts i.e.  jitter or unrealistic movements that may have been captured and ensures that these issues don’t get transferred or amplified during the retargeting process. 
  2. Simplifying the Retargeting Process: cleaner data makes the retargeting process smoother. If data is noisy or erratic it can lead to unpredictable results when applied to the character rig.
  3. Universal Corrections: Cleaning the data before retargeting means you’re making corrections that are universally applicable, regardless of the target character. This is especially useful if the same mocap data will be used for multiple characters.
  4. Optimise Data for Retargeting Tools: Many retargeting tools and software work best with clean, consistent data. Pre-cleaning helps in reducing the computational complexity and potential errors during retargeting.

Note In some cases, you might also need to clean up data after retargeting:

Character-Specific Adjustments: Some issues may only become apparent after the data has been applied to a specific character, especially if the character’s proportions or rig differ significantly from the mocap actor.

Fine-Tuning and Refinement: After retargeting, you might need to make fine adjustments to ensure the animation looks natural on the new character, which can include cleaning up minor glitches or unnatural movements.

4. Prepare the Target Character

Rigging: Ensure your target character is properly rigged with a skeleton that matches the mocap data in terms of hierarchy and naming conventions, as much as possible.

T-Pose: Start with your character model in a T-pose or a neutral pose that matches the starting pose of the mocap data.

5. Map the Skeleton

Mapping Joint Hierarchies

Joint hierarchies form the backbone of any character rig, whether human or non-human. As such, it’s crucial to map the joint hierarchies from the source model to the target model accurately. This mapping ensures that the motion data drives the character’s movements correctly.

Every software option will have slightly different tools for retargeting, but the best practices for ensuring accurate results remain the same:

Hierarchy Alignment: Start by aligning the basic hierarchies of the source and target models. Ensure that primary joints like hips, shoulders, knees, and elbows correspond. This involves having both source and target in a ‘T’ pose and matching joints.

Detailed Joint Matching: For finer control, match secondary joints like fingers and facial joints. This step is essential for capturing nuanced expressions and detailed hand movements.

Joint Orientation: Pay close attention to the orientation of joints. Incorrect orientations can lead to unnatural movements, especially in joints with significant rotational motion.

Flexible Rigging: Employ rigs that are adaptable. Rigs that can be easily adjusted to different joint hierarchies or proportions are invaluable in retargeting.

Scaling and Proportion Adjustments

Differences in size and proportion between the source and target models are common challenges in retargeting. These differences can distort the motion when applied directly.

Uniform Scaling: When the overall size differs, but proportions are similar, uniform scaling of the motion data is an.

Proportional Scaling: For differing proportions, scale the motion data for each joint or limb, ensuring the movements remain natural for the target model’s anatomy.

Selective Scaling: In cases where only certain parts of the body are disproportionately different, apply scaling selectively to those areas.

6. Retargeting

Apply Mocap Data: Use the retargeting tool in your software to apply the mocap data to your character’s rig. This process transfers the motion of each joint from the mocap data to the corresponding joint in your character’s skeleton.

Refine Animation: Check the animation on your character. Look for any sliding feet, interpenetration of limbs, or unnatural movements.

7. Fine-Tuning

Edit Keyframes: Manually adjust keyframes to fix any issues. This might involve tweaking the position of limbs, adjusting timing, or smoothing out jerky movements.

Use Additional Constraints: Sometimes, adding constraints (like IK handles for feet and hands) can help maintain contact with the ground or other objects.

8. Testing and Iteration

Test in Context: If the character is for a game or a scene, test the animation in the actual environment to see how it looks and interacts.

Iterate as Needed: Make adjustments based on testing, and iterate until the animation looks natural and meets your requirements.

9. Exporting

Export Animation: Once you’re satisfied with the retargeted animation, export it in the required format for your project, such as FBX for use in a game engine.

Dealing with Non-Human Characters

Retargeting to non-human characters introduces unique challenges due to different anatomies and joint structures.

Techniques for Non-Human Characters

Anatomy Study: Understand the non-human character’s basic anatomy and movement mechanics. This understanding helps in creating a more believable rig and motion.

Creative Joint Mapping: In cases where the target has a different number of joints to the source (like tails, wings, or additional limbs), map these to the closest human equivalent or add custom joints in the rig.

Motion Adaptation: Adapt the motion data to fit the character’s anatomy.

Additional Considerations

Advanced Retargeting Techniques

Layered Animation: After retargeting, use layered animation to add character-specific nuances or correct any unnatural movements.

Physics-Based Adjustments: Employ physics simulations for elements like cloth or hair to ensure they react realistically to the character’s movements.

Feedback Loops: Implement a feedback loop where the retargeted animation is periodically reviewed and refined, considering both technical and artistic perspectives.

Quality Control

Consistency Checks: Regularly check for consistency in movements, especially when retargeting the same actor’s data across different characters.

Motion Review with Actors: When possible, involve the original performance actors in reviewing the retargeted motion. Their insights can be invaluable in maintaining the integrity of the performance.

Tips for Advanced Techniques in Retargeting

Handling Complex Motions

Complex motions such as facial expressions, hand movements, and intricate gestures present significant challenges in retargeting due to their detail and subtlety.

Strategies for Complex Motions

High-Resolution Capture: Use high-resolution motion capture systems for capturing detailed movements, especially for facial expressions and finger movements.

Layered Approach: Apply a layered approach in animation – start with broad movements and then add layers of finer, detailed motions.

Morph Targets/Blend Shapes for Faces: Use morph targets or blend shapes for facial animation. This method allows for the combination of pre-set facial expressions and nuances captured from the performance.

Motion Cleaning: Clean and smooth out the data for complex motions to remove jitter or anomalies that can occur during capture.

Utilising Inverse Kinematics

Inverse Kinematics (IK) is an essential tool in animation and retargeting, particularly useful for adjusting limb positions and ensuring contact points (like feet on the ground) are maintained realistically.

Implementing Inverse Kinematics

IK for Limb Correction: Use IK to correct limb orientations and positions, especially when the character interacts with objects or other characters.

Foot and Hand Placement: Ensure that feet and hands are placed correctly relative to the environment, utilising IK to adjust for discrepancies in the original capture.

Balance and Weight Transfer: Use IK to adjust the character’s balance and weight transfer, adding to the realism of the movement.

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