Convert C3D to BVH

A motion-capture system may produce C3D while a game engine or character-animation package accepts only BVH. Converting the files requires solving marker trajectories onto a skeleton; it is not a simple file-format rewrite.

What the C3D format is

C3D is a binary motion-capture container used for synchronized 3D marker trajectories, analog channels, force-plate measurements, events, and trial metadata. Vicon Nexus, Qualisys Track Manager, Motion Analysis Cortex, BTS systems, and many biomechanical applications create or export C3D files.

A trial commonly contains labeled markers such as LASI, RASI, LKNE, and RKNE, but C3D does not require a complete articulated skeleton, bone hierarchy, joint limits, or solved joint rotations. Marker labels and coordinate conventions also vary between capture systems.

What the BVH format is

BVH, or Biovision Hierarchy, stores a skeleton hierarchy followed by per-frame channel values, normally root translation and Euler joint rotations. It is supported by Blender, Autodesk MotionBuilder, many game and animation packages, and other software that consumes skeletal motion.

BVH is useful when the destination needs animation-ready bones. Its text-based structure is relatively easy to inspect and exchange, but it normally cannot preserve C3D marker labels, force data, analog signals, residuals, camera-quality information, or most trial metadata. Keep the original C3D as the archival source.

How to convert C3D to BVH

The most reliable workflow uses a motion-capture or animation application that can import C3D, solve markers, and export BVH. Autodesk MotionBuilder is suited to optical-data cleanup, marker-set mapping, character definition, solving, retargeting, and BVH export. Blender can perform the same process at lower cost, but it needs a compatible C3D importer and more manual setup.

1. Open the trial in the capture or biomechanical software that created it. Verify the frame rate, units, axis directions, marker labels, gaps, and frame range. Repair mislabeled or missing markers and filter noisy trajectories before solving.
2. Use BTK or ezc3d for local inspection and preprocessing when the original application is unavailable. Install ezc3d with pip install ezc3d; it can read C3D points, analog data, parameters, and timing, but it does not infer a skeleton or produce valid BVH by itself.
3. In MotionBuilder, import the C3D through the available optical-data import command. Load or create the subject's marker set and map labels to body landmarks such as pelvis, knees, ankles, shoulders, and elbows. Exact menu names can differ by MotionBuilder version and installed import options.
4. Create the optical or marker-to-skeleton solve. Define the rest pose, subject proportions, bone lengths, parent-child hierarchy, root marker, and any required marker filters. A generic marker cloud is not enough to determine a reliable skeleton.
5. Inspect the solved motion for swapped left and right limbs, incorrect facing direction, foot sliding, knee inversions, shoulder errors, and discontinuities caused by occlusion. Correct the marker data or solve settings before baking.
6. Characterize the solved skeleton, bake the joint animation, set the required frame range and sample rate, then export the motion as BVH. Select the correct root, translation channels, axis convention, and Euler rotation order for the destination software.

In Blender, install a C3D importer that supports the installed Blender version, import the marker trajectories, create or import an armature with suitable proportions, constrain the armature to the markers, bake the resulting action, and use File → Export → Motion Capture (.bvh). Blender's BVH exporter writes an existing armature; it does not reliably turn unlabeled marker points into a human skeleton.

Quality and compatibility checks

• Units: C3D data is often expressed in millimetres, while BVH readers may interpret values as centimetres or arbitrary scene units. Apply the required scale to bone lengths and root translations, then verify the subject's height and travel distance.
• Axes: confirm the up axis, forward direction, handedness, and coordinate origin in both applications. A mismatch can rotate, mirror, or invert the motion.
• Rotation order: BVH declares each joint's channel order in its hierarchy. An importer that assumes another Euler order can create apparent limb twisting even when the source solve is correct.
• Root motion: decide whether the BVH should contain horizontal translation, vertical translation, and facing rotation. Some destinations expect locomotion on the root; others use an in-place animation.
• Frame timing: preserve the C3D frame range and sampling rate unless resampling is intentional. Confirm that the BVH header's frame time matches the destination's playback rate.
• Gaps and occlusions: missing or mislabeled markers can produce joint pops, implausible rotations, and foot sliding. Fill gaps or exclude affected segments before baking.
• Data loss: BVH generally retains only a skeleton and animation channels. Export a cleaned C3D or another analysis format separately if force plates, analog signals, events, marker residuals, or original trajectories are needed later.

Online converters and alternatives

There is no broadly reliable generic online C3D-to-BVH converter because the conversion needs a subject-specific marker model and skeletal solve, not just a format parser. Generic file-conversion sites cannot infer bone hierarchy or joint rotations. Use the capture vendor's software, MotionBuilder, or a local Blender workflow; avoid uploading identifiable or proprietary motion-capture data to unverified services.