simtk.org

Posted: **Wed Jun 02, 2021 10:00 am**

Hello,

I am trying to get the rotation of a body frame, either relative to the global frame, or to another body frame, using Matlab.

I have been able to get the global positions of the bodies, and convert them to Matlab arrays, using this method:

Code: Select all

%% get global body position
hand_L_pos = osimVec3ToArray(HAND_L.getPositionInGround(start)); % take in current state
hand_R_pos = osimVec3ToArray(HAND_R.getPositionInGround(start)); % give global position
handle_L_pos = osimVec3ToArray(HANDLE_L.getPositionInGround(start));
handle_R_pos = osimVec3ToArray(HANDLE_R.getPositionInGround(start));

However, using a similar command for rotation defined in the methodsview "body.getRotationInGround(state)" gives an object of type

SWIGTYPE_p_SimTK__Rotation_T_SimTK__Real_t

Is there a way to convert this to a usable form in Matlab? Or is there another method that I could use to get the body rotation/orientation?

I am trying to define the initial offset between the two bodies and the orientation matters in this case. Any thoughts/recommendations/advice would be greatly appreciated.

Thanks for your help,
Evan

Posted: **Wed Jun 02, 2021 10:26 am**

Hi Evan,

You can use

Code: Select all

getTransformInGround()

call, and then extract the Rotation part from the Transform. The tools we use to expose the C++ methods to Matlab appear to be have missed this call (likely due to missing a Template expansion). Please file a bug and we'll look into it.

Best regards,
-Ayman

Posted: **Wed Jun 02, 2021 2:20 pm**

Hi Ayman,

Thanks for the speedy response.

I reported the bug and have tried your suggestion.

Code: Select all

% handle L
x = HANDLE_L.getTransformInGround(start);
y = x.R;
handle_L_rot = y.convertRotationToBodyFixedXYZ();

This gives me the X,Y,Z rotation angles from ground to each body, which is great. I think I need to get one rotation in terms of the parent body for the bushing I'm creating and was wondering if there's a way to get the rotation out/into a Matlab array?

I can take the rotation to a Mat33 object (y.toMat33), but I cannot convert this to a Matlab array. Am I just missing something in the documentation?

Any advice for this issue?

Thanks for all your help,
Evan

simtk.org

How to get orientation of body in Matlab?

How to get orientation of body in Matlab?

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