Store Kinect's v2.0 Motion to BVH File

Question

I would like to store the motion capture data from Kinect 2 as a BVH file. I found code which does so for Kinect 1 which can be found here. I went through the code and found several things that I was not able to understand. For example, in the mentioned code I've tried to understand what exactly the Skeleton skel object, found in several places in the code, actually is. If not, are there any known application available to accomplish the intended?

EDIT: I tried to change Skeleton skel to Body skel which I think is the correspondant object for kinect SDK 2.0. However I've got an error when I try to get the position of the body:

tempMotionVektor[0] = -Math.Round( skel.Position.X * 100,2);
tempMotionVektor[1] = Math.Round( skel.Position.Y * 100,2) + 120;
tempMotionVektor[2] = 300 - Math.Round( skel.Position.Z * 100,2);

I've gotten errors when calling the function Position for the Body skel. How can I retrieve the X, Y, Z of the skeleton in sdk 2.0?? I tried to change the above three lines to:

tempMotionVektor[0] = -Math.Round(skel.Joints[0].Position.X * 100, 2);
tempMotionVektor[1] = Math.Round(skel.Joints[0].Position.Y * 100, 2) + 120;
tempMotionVektor[2] = 300 - Math.Round(skel.Joints[0].Position.Z * 100, 2);

EDIT: Basically I managed to store the a bvh file after combining bodyBasicsWPF and kinect2bvh. However, it seems that the skeleton I am storing is not efficient. There are strange movements in the elbows. I am trying to understand if I have to change something in the file kinectSkeletonBVH.cp. More specifically, what are the changes in the joint axis orientation for the kinect 2 version. How can I change the following line: skel.BoneOrientations[JointType.ShoulderCenter].AbsoluteRotation.Quaternion; I tried to change that line with skel.JointOrientations[JointType.ShoulderCenter].Orientation. Am I right? I am using the following code to add the joint to BVHBone objects:

BVHBone hipCenter = new BVHBone(null, JointType.SpineBase.ToString(), 6, TransAxis.None, true);
BVHBone hipCenter2 = new BVHBone(hipCenter, "HipCenter2", 3, TransAxis.Y, false);
BVHBone spine = new BVHBone(hipCenter2, JointType.SpineMid.ToString(), 3, TransAxis.Y, true);
BVHBone shoulderCenter = new BVHBone(spine, JointType.SpineShoulder.ToString(), 3, TransAxis.Y, true);

BVHBone collarLeft = new BVHBone(shoulderCenter, "CollarLeft", 3, TransAxis.X, false);
BVHBone shoulderLeft = new BVHBone(collarLeft, JointType.ShoulderLeft.ToString(), 3, TransAxis.X, true);
BVHBone elbowLeft = new BVHBone(shoulderLeft, JointType.ElbowLeft.ToString(), 3, TransAxis.X, true);
BVHBone wristLeft = new BVHBone(elbowLeft, JointType.WristLeft.ToString(), 3, TransAxis.X, true);
BVHBone handLeft = new BVHBone(wristLeft, JointType.HandLeft.ToString(), 0, TransAxis.X, true);

BVHBone neck = new BVHBone(shoulderCenter, "Neck", 3, TransAxis.Y, false);
BVHBone head = new BVHBone(neck, JointType.Head.ToString(), 3, TransAxis.Y, true);
BVHBone headtop = new BVHBone(head, "Headtop", 0, TransAxis.None, false);

I can't understand where inside the code the axis for every Joint is calculated.

Answer 1

The code you used for Kinect 1.0 to obtain a BVH file use the joints information to build bone vectors by reading the Skeleton.

public static double[] getBoneVectorOutofJointPosition(BVHBone bvhBone, Skeleton skel)
{
    double[] boneVector = new double[3] { 0, 0, 0 };
    double[] boneVectorParent = new double[3] { 0, 0, 0 };
    string boneName = bvhBone.Name;

    JointType Joint;
    if (bvhBone.Root == true)
    {
        boneVector = new double[3] { 0, 0, 0 };
    }
    else
    {
        if (bvhBone.IsKinectJoint == true)
        {
            Joint = KinectSkeletonBVH.String2JointType(boneName);

            boneVector[0] = skel.Joints[Joint].Position.X;
            boneVector[1] = skel.Joints[Joint].Position.Y;
            boneVector[2] = skel.Joints[Joint].Position.Z;
..

Source: Nguyên Lê Đặng - Kinect2BVH.V2

Except in Kinect 2.0, Skeleton class has been replaced by the Body class, so you need to change it to deal with a Body instead, and obtain the joints by following the steps quoted below.

// Kinect namespace
using Microsoft.Kinect;

// ...

// Kinect sensor and Kinect stream reader objects
KinectSensor _sensor;
MultiSourceFrameReader _reader;
IList<Body> _bodies;

// Kinect sensor initialization
_sensor = KinectSensor.GetDefault();

if (_sensor != null)
{
    _sensor.Open();
}

We also added a list of bodies, where all of the body/skeleton related data will be saved. If you have developed for Kinect version 1, you notice that the Skeleton class has been replaced by the Body class. Remember the MultiSourceFrameReader? This class gives us access on every stream, including the body stream! We simply need to let the sensor know that we need body tracking functionality by adding an additional parameter when initializing the reader:

_reader = _sensor.OpenMultiSourceFrameReader(FrameSourceTypes.Color |
                                             FrameSourceTypes.Depth |
                                             FrameSourceTypes.Infrared |
                                             FrameSourceTypes.Body);

_reader.MultiSourceFrameArrived += Reader_MultiSourceFrameArrived;

The Reader_MultiSourceFrameArrived method will be called whenever a new frame is available. Let’s specify what will happen in terms of the body data:

1. Get a reference to the body frame
2. Check whether the body frame is null – this is crucial
3. Initialize the _bodies list
4. Call the GetAndRefreshBodyData method, so as to copy the body data into the list
5. Loop through the list of bodies and do awesome stuff!

Always remember to check for null values. Kinect provides you with approximately 30 frames per second – anything could be null or missing! Here is the code so far:

void Reader_MultiSourceFrameArrived(object sender,
            MultiSourceFrameArrivedEventArgs e)
{
    var reference = e.FrameReference.AcquireFrame();

    // Color
    // ...

    // Depth
    // ...

    // Infrared
    // ...

    // Body
    using (var frame = reference.BodyFrameReference.AcquireFrame())
    {
        if (frame != null)
        {
            _bodies = new Body[frame.BodyFrameSource.BodyCount];

            frame.GetAndRefreshBodyData(_bodies);

            foreach (var body in _bodies)
            {
                if (body != null)
                {
                    // Do something with the body...
                }
            }
        }
    }
}

This is it! We now have access to the bodies Kinect identifies. Next step is to display the skeleton information on-screen. Each body consists of 25 joints. The sensor provides us with the position (X, Y, Z) and the rotation information for each one of them. Moreover, Kinect lets us know whether the joints are tracked, hypothsized or not tracked. It’s a good practice to check whether a body is tracked before performing any critical functions.

The following code illustrates how we can access the different body joints:

if (body != null)
{
    if (body.IsTracked)
    {
        Joint head = body.Joints[JointType.Head];

        float x = head.Position.X;
        float y = head.Position.Y;
        float z = head.Position.Z;

        // Draw the joints...
    }
}

Source: Vangos Pterneas Blog - KINECT FOR WINDOWS VERSION 2: BODY TRACKING