Setting marker weight in z direction to 0

[Radhakrishnan Vignesh]

Hello,

I am trying to project a marker onto the bone longitudinal axis. While I can do that for the marker in segment coordinate system by setting z=0, I am not able to do it with the experimental marker.

How would I be able to do that? Or alternatively, is it possible to set individual weights for the x,y,z coordinates. That way I can set z = 0

Thanks

[Thomas Uchida]

You could try the following:
1. Find the coordinates of the marker (x,y,z) with respect to the segment coordinate system.
2. Set z=0.
3. Express the new marker location with respect to Ground.
If you're using the API, you can use the Frame class to perform steps 1 and 3; see Frame::findStationLocationInAnotherFrame (https://simtk.org/api_docs/opensim/api_ ... aa78c66f79) and Frame::findStationLocationInGround (https://simtk.org/api_docs/opensim/api_ ... c727289274).

[Radhakrishnan Vignesh]

Hi Thomas,

Thank you. Just to confirm:

1) I load an mot file and the model and at each time instant realise the model
2) Set the z axis to 0 and convert to ground

Is that correct?

May I ask if the output of this would be different from applying point kinematics? For example, if you know the location of the point in segment coordinate system, using an mot file, determine its location in the ground refernece same?

Is there any way to do it if there is no mot file?

Thanks

[Radhakrishnan Vignesh]

Hi,

I just wanted to confirm whether the operation is similar to Point kinematics tool? And if not, what is the difference (since you are calculating the new coordinates from computed joint angles)?

Thanks,
Vignesh

[Radhakrishnan Vignesh]

Hi,

I have a question regarding projection of markers and the bone GUI. I would like to project a skin-marker onto the bone surface and I have done this by defining a point using pointkinematics and determining its position in the global frame. But I have a question regarding the coordinate system of the bone.

I have attached 3 images:

1) Original location of the marker

normal.JPG (9.55 KiB) Viewed 842 times

2) Marker's z coordinate =0 in bone reference frame (local coordinate system)

z0.JPG (9.68 KiB) Viewed 842 times

3) Marker's z coordinate is projected close to the bone surface (calculated using fluoroscopy).

boneSurface.JPG (9.2 KiB) Viewed 842 times

I would like to know:
1) How is the coordinate system defined that z=0 is towards the medial aspect of the leg?
2) If my aim is to get the projection of the marker onto the bone, would the second or third image be more accurate?
3) In a previous post (viewtopicPhpbb.php?f=91&t=8598), the calculated the length of the bone using the location of the coordinate system. Is there anyway to determine the thickness of the bone to project on its surface?

Thanks