How to edit marker/force/analog data during c3d conversion in MATLAB (Opensim4.0 & osimC3D)

[Maria Fox]

Hi all,

I'm trying to update my c3d conversion methods for OpenSim 4.0 using the provided osimC3D methods. I can import the osimC3D object and assign the markers and forces to structures for editing, but I need help with:
1) How to save edited marker/force structures back to the osimC3D object?
2) How to access analog channels/data in osimC3D?

I've tried the following method to assign the marker structure back to the object, but I get an error. Does anyone know how to go about this?

Code: Select all

c3d = osimC3D([directory{i} c3dfileNames.(subID{i}){j}], 1);
                
[markerStruct, forceStruct] = c3d.getAsStructs();

% compute virtual markers
markerStruct = computeVirtualMarkers(markerStruct,1); % my custom function 

% save edited markers back to osimC3D object?
>> c3d.markers = osimTableFromStruct(markerStruct);
You cannot set the 'markers' property of osimC3D. 

Thanks!
Maria

[Benjamin Michaud]

Hello Maria,

I know this is a bit off topic, but if your goal is to only load/write c3d, I suggest that you have a look at ezc3d on github. It could do what you need just fine

If you do so, please let me know if anything doesn't work, I am currently doing a rush on improving the library!

Thanks

[Maria Fox]

Benjamin,

Thanks for the suggestion, but unfortunately that will not work for me. I need to do some processing to all the marker, force, and analog data. I was using the c3d2OpenSim toolbox with some custom functions to make these adjustments, but I want to update everything for OpenSim 4.0 now. It might just be easiest for me to work with my existing code and update the trc/mot file headers.

Thanks,
Maria

[Christopher Dembia]

You cannot update or modify the original C3D file. The workflow is to load the C3D file, perform the transformations you want, then write out TRC and MOT files.

[Maria Fox]

Christopher,

I'm not trying to modify the c3d files themselves (which you can actually do in Mokka, but it's not scaleable for many trials); I'm trying to modify the files once imported as structures in MATLAB. That is what I currently do with my code, but I didn't know if it was possible to have a similar protocol with the new osimC3D methods in MATLAB. I went ahead and updated the headers in c3d2OpenSim's code since those required very little change and my files seem to be working fine in OpenSim 4.0.

Thanks,
Maria