Data Export: C3D
C3D Export
Tracking data can be exported into the C3D file format. C3D (Coordinate 3D) is a binary file format that is widely used especially in biomechanics and motion study applications. Recorded data from external devices, such as force plates and NI-DAQ devices, will be recorded within exported C3D files. Note that common biomechanics applications use a Z-up right-hand coordinate system, whereas Motive uses a Y-up right-hand coordinate system. More details on coordinate systems are described in the later section. Find more about C3D files from https://www.c3d.org.
General Export Options
| Option | Description |
|---|---|
Frame Rate | Number of samples included per every second of exported data. |
Start Frame | Start frame of the exported data. You can either set it to the recorded first frame of the exported Take or to the start of the working range, or scope range, as configured under the Control Deck or in the Graph View pane. |
End Frame | End frame of the exported data. You can either set it to the recorded end frame of the exported Take or to the end of the working range, or scope range, as configured under the Control Deck of in the Graph View pane. |
Scale | Apply scaling to the exported tracking data. |
Units | Sets the length units to use for exported data. |
Axis Convention | Sets the axis convention on exported data. This can be set to a custom convention, or preset convetions for exporting to Motion Builder or Visual3D/Motion Monitor. |
X Axis Y Axis Z Axis | Allows customization of the axis convention in the exported file by determining which positional data to be included in the corresponding data set. |
C3D Specific Export Options
| Options | Descriptions |
|---|---|
Use Zero Based Frame Index | C3D specification defines first frame as index 1. Some applications import C3D files with first frame starting at index 0. Setting this option to true will add a start frame parameter with value zero in the data header. |
Export Unlabeled Markers | Includes unlabeled marker data in the exported C3D file. When set to False, the file will contain data for only labeled markers. |
Export Finger Tip Markers | Includes virtual reconstructions at the finger tips. Available only with Skeletons that support finger tracking (e.g. Baseline + 11 Additional Markers + Fingers (54)) |
Use Timecode | Includes timecode. |
Rename Unlabeled As _000X | Unlabeled markers will have incrementing labels with numbers _000#. |
Marker Name Syntax | Choose whether the marker naming syntax uses ":" or "_" as the name separator. The name separator will be used to separate the asset name and the corresponding marker name in the exported data (e.g. AssetName:MarkerLabel or AssetName_MarkerLabel or MarkerLabel). |
C3D Axes
Common Conventions
Since Motive uses a different coordinate system than the system used in common biomechanics applications, it is necessary to modify the coordinate axis to a compatible convention in the C3D exporter settings. For biomechanics applications using z-up right-handed convention (e.g. Visual3D), the following changes must be made under the custom axis.
X axis in Motive should be configured to positive X
Y axis in Motive should be configured to negative Z
Z axis in Motive should be configured to positive Y.
This will convert the coordinate axis of the exported data so that the x-axis represents the anteroposterior axis (front/back), the y-axis represents the mediolateral axis (left/right), and the z-axis represents the longitudinal axis (up/down).
MotionBuilder Compatible Axis Convention
This is a preset convention for exporting C3D files for use in Autodesk MotionBuilder. Even though Motive and MotionBuilder both use the same coordinate system, MotionBuilder assumes biomechanics standards when importing C3D files (negative X axis to positive X axis; positive Z to positive Y; positive Z to positive Y). Accordingly, when exporting C3D files for MotionBuilder use, set the Axis setting to MotionBuilder Compatible, and the axes will be exported using the following convention:
Motive: X axis → Set to negative X → Mobu: X axis
Motive: Y axis → Set to positive Z → Mobu: Y axis
Motive: Z axis → Set to positive Y → Mobu: Z axis
Notes on Timecode Export
There is an known behavior where importing C3D data with timecode doesn't accurately show up in MotionBuilder. This happens because MotionBuilder sets the subframe counts in the timecode using the playback rate inside MotionBuilder instead of using the rate of the timecode. When this happens you can set the playback rate in MotionBuilder to be the same as the rate of the timecode generator (e.g. 30 Hz) to get correct timecode. This happens only with C3D import in MotionBuilder, FBX import will work fine without the change to the playback rate.
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