Marker's visualisation, .trc vs. .mot

[Michala Cadova]

Hi everybody,
I have just started with the OpenSim, and I would like to make some parts of the modeling/simulation process clear. So far, I went through the OpenSim manual, the SIMM manual and the forum, but with something I am still confused.

1) What is the difference between .trc and .mot files? As I understand it, the .trc file contains the positions/trajectories of the markers (which I can get with motion capture) and the .mot files the kinematics (angles, etc) and the external forces. So, if I only want to visualize the motion, which I have from the motion capture recording, the .trc files should be enough? Then, if I have the model with the markers I can use the “File  preview motion data” and it should work? (actually, it is not, but maybe I have some errors in the trc file).

2) Is there a way to visualize the marker set in the model without running IK? What if I have several marker sets and I forgot which one is which (how individual marker sets are positioned). I need to choose the best marker set for the current motion capture session. So I want to see which marker set is the closest one / the best on.

Thank you for any information.

Michala

[Ayman Habib]

Hi Michala,

- trc files are generated by the motion capture systems and so are model agnostic and have no gen-coordinate values, only trajectories of markers in space, while .mot files usually contain gen-coordinate values that are specific to a model.
Other than importing/exporting models between SIMM and OpenSim the two software packages are practically unrelated though they use similar file formats in some cases.
- Preview motion data works on the raw trc files.
- You can change the marker set in the model using XML file editing for now.

Hope this helps and please let us know if you continue to have questions.

Best regards,
-Ayman

[Michala Cadova]

Thank you for the answer,

But one more question I have to ask – so how do I computed/get the coordinates in the .mot file when I have the .trc file?

Michala

[Ayman Habib]

Hi Michala,

That's exactly the job of the Inverse Kinematics (IK) tool. If you have a model scaled to the size of the subject in your experimental setup, you use IK to compute the best fit joint angles to match marker positions/trajectories.

Hope this explains,
-Ayman