Anyway to determine STA metrics in OpenSim

[Radhakrishnan Vignesh]

Hi,

I am trying to see if I can determine STA metrics in OpenSim as defined by Grimpampi. I thought the position of the marker in body reference frame would change, so I added a function to SimBody to determine that during IK (image added). But the position remains the same and the transformation changes.

Is there a way to determine these metrics?

[Mohammadreza Rezaie]

Hi, If I understand correctly, you are trying to find the position of markers in local reference frame. If this is the case, please see this example (Python):

Code: Select all

model = osim.Model('subject01_simbody.osim')
state = model.initSystem()
ground = model.getGround()
for marker in model.getMarkerSet():
	vec = marker.getLocationInGround(state)
	frame = marker.getParentFrame()
	vec2 = ground.findStationLocationInAnotherFrame(state, vec, frame)

[Radhakrishnan Vignesh]

Hi Mohammadreza,

Thank you. Just to confirm this would give me the marker position in the body reference frame (anatomical reference frame) for each point? And it would not be a constant value ?

Thanks

[Mohammadreza Rezaie]

Yes, in body reference frame. You need to update coordinates' value for every IK time frame and call those in a loop.

[Radhakrishnan Vignesh]

Thank you

[Radhakrishnan Vignesh]

Hi,

any idea why the command marker.getParentFrame();, throws up an error in C++. It states exception, ComoinentHasNoSysten at memory location

[Mohammadreza Rezaie]

Hi, I know nothing about C++. Try this one, the output is already in local reference frame and thus, no conversion would be required.

Code: Select all

vec = marker.get_location()

[Radhakrishnan Vignesh]

Hi,

Thanks. I tried the get_location() function but the output is always constant.

Thanks

[Mohammadreza Rezaie]

Yes, the position of model's markers remains constant in local reference frame. They are registered relative to body's frame and won't change during motion.

[Radhakrishnan Vignesh]

So is the transformation matrixes of the body reference frame change from time instant to time instant ?