Model coordinate frame

[Kunal Singhal]

Hi all,
I am new to the area of MS modeling and trying out with OpenSim. I read through the tutorials and the user guide but could not figure out few things.
We use a lab coordinate system and the user guide asked us to change it to model coordinate system. It is different for different models.
Q1. How to find out where exactly the model coodinate frame is so that experimental data can be transformed?

Q2. How to edit the model coordinate frame values?

Q3. Is it true that the marker data is with respect to the body reference frame? in that case where are these reference frames fixed? so that we can provide our own experimental marker data.

Q4. If we cannot edit model coordinate frame, then, we can surely convert the marker data from static files but then the values for dynamic trials have to be converted to the same origin as the one with static file, for eg in mid of the foot?

Thanks.

[Tim Dorn]

Hi Kunal,

The global model coordinate system is fixed (X:anterior/postior; Y: superior/inferior; Z: medial/lateral), however each segment has its own coordinate system with respect to this model coordinate system. The markers that have been placed on the model are given in the coordinate system of the segments (see 3D2392_Scale_MarkerSet.xml). However the marker data you collect in the lab should be expressed in the global model coordinate system through a series of rigid body transformations.

Note one minor point though: in terms of translations, OpenSim does not care where your lab origin is. Depending on where you put your origin in your lab will simply affect where the model is placed in the OpenSim workspace (they may not necessiarily be standing on the platform).

I have a toolbox in Matlab to extract your gait data to suitable C3D formats: http://www.timzone.net/biomech.html
I will be adding this as a simTK project in the very near future.

Cheers,

Tim

[Kunal Singhal]

Hi Tim

Thanks alot. The marker data set does provide with the location of markers but the values are calculated with respect to a refernce frame. On looking at the file I realized that marker locations were calculated from a point fixed on the body/segment itself (an assumption).
In case we have to provide an experimental marker data set from the static trial we need to both transform the data into model coordinate frame and also with respect to the body/segment on which the marker is placed (if my previous assumption is true). In that case where are the origin of the body/segment located? can they be changed?

[Michala Cadova]

Note one minor point though: in terms of translations, OpenSim does not care where your lab origin is. Depending on where you put your origin in your lab will simply affect where the model is placed in the OpenSim workspace (they may not necessiarily be standing on the platform).

Hi,
I have a small comment on that – for me the scaling didn’t work properly if I don’t do the transformation to the same origin.

I tried two cases:

1) the coordinates from the motion capture are expressed in the Opensim global coordinate system (using rotation of the coordinate system) AND translation was done as well.

2) only the rotation was done (that means that the marker’s coordinates from the motion capture are in the same global coordinate system as the model in the Opensim, but they have different origin)

-> the second method didn’t work properly. Therefore, I think, I will always do the rotation and the translation.

Michala