Kinect Joint Angle Calculation

Question

Sorry about this but I need some conformation about this function and calculation

I currently have these Vectors:

   Vector3D ElbowLeft = new Vector3D(body.Joints[JointType.ElbowLeft].Position.X, body.Joints[JointType.ElbowLeft].Position.Y, body.Joints[JointType.ElbowLeft].Position.Z);
   Vector3D WristLeft = new Vector3D(body.Joints[JointType.WristLeft].Position.X, body.Joints[JointType.WristLeft].Position.Y, body.Joints[JointType.WristLeft].Position.Z);
   Vector3D ShoulderLeft = new Vector3D(body.Joints[JointType.ShoulderLeft].Position.X, body.Joints[JointType.ShoulderLeft].Position.Y, body.Joints[JointType.ShoulderLeft].Position.Z);

   Vector3D Head = new Vector3D(body.Joints[JointType.Head].Position.X, body.Joints[JointType.Head].Position.Y, body.Joints[JointType.Head].Position.Z);
   Vector3D Neck = new Vector3D(body.Joints[JointType.Neck].Position.X, body.Joints[JointType.Neck].Position.Y, body.Joints[JointType.Neck].Position.Z);
   Vector3D SpineShoulder = new Vector3D(body.Joints[JointType.SpineShoulder].Position.X, body.Joints[JointType.SpineShoulder].Position.Y, body.Joints[JointType.SpineShoulder].Position.Z);

I am calculating the angle between the two vectors using this function

public double AngleBetweenTwoVectors(Vector3D vectorA, Vector3D vectorB)
{
    double dotProduct = 0.0;
    vectorA.Normalize();
    vectorB.Normalize();
    dotProduct = Vector3D.DotProduct(vectorA, vectorB);

    return (double)Math.Acos(dotProduct) / Math.PI * 180;
}

And I am calling it like this:

   double LeftElbowAngle = AngleBetweenTwoVectors(ElbowLeft - ShoulderLeft, ElbowLeft - WristLeft);
   double NeckAngle = AngleBetweenTwoVectors(Neck - Head, Neck - SpineBase);

Is this correct? Im just doubting myself because When i put my arm straight or stand up straight it detects an angle of about 170 - 175 rather than 180. on both my neck and my elbow joint

Remember that you have 3 dimensions there, getting the arm or head perfectly straight might not be very easy. How does the other angles look? — Chris
– Chris, Commented Apr 23, 2015 at 20:08
Well Left/Right Elbow joint is 170-178ish, with what seems to me to be a straight arm. Neck is also about the same when I stand perfectly straight. Same with the Knees for the leg. And I haven't figured out how to do wrist calculations since the Kinect also connects the wrist to the thumb... My friend helped me confirm that everything is mathematically correct. — ShatteredPheonix
– ShatteredPheonix, Commented Apr 23, 2015 at 20:11
Yea again when I move my arm to what seems like a right angle it fluctuates between 80-90. Same for 45, they seem mostly to fluctuate by 10. Like when I put my arm completely straight it won't give me something weird like 120. — ShatteredPheonix
– ShatteredPheonix, Commented Apr 23, 2015 at 20:15
Sounds like it's working fine then. I would guess the kinects depth accuracy is simply not good enough to give you a perfectly straight 3D line. It might also be hard for it to actually track the elbow joint very well if the arm is perfectly straight. — Chris
– Chris, Commented Apr 23, 2015 at 20:18

ShatteredPheonix · Accepted Answer · 2015-04-23 20:37:45Z

1

I have confirmed that the above Algorithim is correct mathematically, however the accuracy of the device may be a bit off due to the hardware, and individual human bones may prevent from perfect joint extension which is 180.

answered Apr 23, 2015 at 20:37

ShatteredPheonix

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Comments

mudassir ahmed · Accepted Answer · 2019-08-02 19:01:21Z

0

correct me If I'm wrong but I think instead of this return (double)Math.Acos(dotProduct) / Math.PI * 180 it should be

 return (double)Math.Acos(dotProduct) * (180.0/Math.PI);

since angle returned by Math.Acos is in radian and to convert it into degrees you should be multiplying it to 180/pi

answered Aug 2, 2019 at 19:01

mudassir ahmed

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