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.

Predict

I skipped this issue however If you have any solution it is good to post it here not just for me, since I noticed a lot of people searching for storing movements in bvh files. – Predict

I'm able to store the Kinect v1 and v2 information for a txt file. This BVH file is something that I just read and it will be a feature that I'll be adding to our acquisition software. If you are interested in the *.txt files just let me know. I don't have a proper BVH solution yet. – Portecochere

check: pterneas.com/2014/03/13/… – Tradesman

is there any conversion that you could do after writing the file? What files are available to write to and convert to from the written file, and which files can be converted to .bvh? – Sanfordsanfourd

I have a way to properly write the Kinect v1 and Kinect v2 streams into txt. Maybe if you use it, you can double-check then your bvh's files for the same experiment. Check my bio for more info. – Portecochere

@Tradesman If the link you posted answers this, would you mind forming that into an answer to this question (per this meta question) – Tula

@JoseRamon I've added how to solve the major part of the issue, there's still how to initialize the skeletal model, and how data is read over time based on Kinect 1.xx hardware vs 2.0 hardware. – Tradesman

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

Tradesman