Finding Joint Angle from Motion Capture

Hi!

I'm working right now with data from a motion capture device, essentially picking up data for three markers placed on the head neck and shoulder in an x y z plane.

My difficulty trying to develop a code to calculate the joint angles between these three markers. I am able to animate the three coordinates to play as an animation, but I'm more interested in the calculation of the angles between these points changing over time.

Any ideas on how to go about this?

Thanks

Answer 1

[Roger Stafford]

At any time, let H = [x,y,z], N = [x,y,z], S = [x,y,z] be coordinate vectors of the head, neck, and shoulder markers at that time.  Then the three respective inner angles within the triangle HNS then are:

 h = atan2(norm(cross(N-H,S-H)),dot(N-H,S-H));
 n = atan2(norm(cross(S-N,H-N)),dot(S-N,H-N));
 s = atan2(norm(cross(H-S,N-S)),dot(H-S,N-S));

(The sum of these should always be pi to within round-off accuracy.)

Roger Stafford

[Mary]

Thank you so much on your quick response, I'm still working on getting things working, but I will keep you posted

[Mary]

Thank you again for the response! I managed to get that aspect of my motion capture data working. Most of the trouble I had was conceptually figuring out how to break down my data into each [x,y,z] coordinate.

Would you happen to know how I can take these same data sets of three points in separate x, y, and z columns and figure out the velocity and acceleration of these moving joints? I'm hoping to compare different data sets of the moving head neck and shoulder to identify which is cause the most stress internally, so looking at the motion capture data is becoming somewhat more difficult than working with my empirical biomechanical model!

Thank you (I hope I'm not asking too much!!!)

[George Vendo]

Hi, Thank you for the answer. Could you please explain why the sum should always be pi?

With 3D coordinates how can we specify the position in space, by the angles? thank you

[TAEYUN]

u r a awesome dude!!!! i luv u!!!

[Mousumi Nag]

Can I get back the S,N,H from the angle .In other words, How can I get the vector points where angle is given

[Suranjita Ganguly]

The sum of the angles should be Pi (approx), because the three angles are forming a triangle, and sum of all internal angles in a triangle is 180 degrees i.e. pi