Preparing Marker Coordinate Data

Presented by Adam Galloy¹, Kayt Frisch PhD¹, and Dylan Schmitz²

¹Dordt College, ²University of Wisconsin-Madison

Contact: kayt.frisch@dordt.edu

3D marker coordinate data exported from a motion capture system will likely require some preparation before it is compatible with an OpenSim model. This page provides an overview of the different preparations that you may need to make before marker data is compatible with an OpenSim model. The process of preparing data has been made significantly easier by the tools developed by the OpenSim community. These tools simplify and automate much of the data preparation. These can be found in the Tools for Preparing Motion Data page.

Preparing File Format

OpenSim only reads marker coordinate data formatted into .trc files. Marker coordinate files in other data formats, such as .c3d files, will need to be converted into .trc files before they can be used in OpenSim. This includes having a proper .trc header.

For more information on the .trc file format and how it is laid out see the Marker (.trc) Files page.

Preparing Marker Names

OpenSim associates digital markers attached to an OpenSim model to experimental marker data from .trc files by matching the experimental markers with model markers that have the same name. Because of this matching method, care must be taken to make sure that the marker names in the .trc file match the model marker names. Experimental marker names are taken directly from the column headings in the .trc file.

Motion capture systems often give markers names like "Subject36_LASI" that have both a subject identifier and a marker location. These subject-specific names may not match the names of the markers attached to the model. Model markers can be easily renamed within OpenSim to match the marker names in the .trc file. However, changing every model marker name manually in OpenSim for every subject becomes time consuming. To save time, renaming the .trc column headings to more general names such as "LASI" during data processing is recommended over changing model marker names.

The process of changing the column headings in .trc files is generally easier to automate than renaming model marker names since the files containing experimental data are being edited anyway. Many tools exist for automated text-editing of tab-delimited files.

Preparing the Marker Coordinate System

Motion capture systems may export data in a different coordinate system than the OpenSim model desired for analysis. If the lab coordinate system is rotated from the model coordinate system, then analysis will require a coordinate transformation beforehand. However, if the lab coordinate system is only translated from the model coordinate system then a coordinate transformation may not be necessary as long as external forces are applied in a way that agrees with the marker data. Details on how to reconcile coordinate systems are given in the Coordinate Systems in OpenSim page.