why Kinect application runs a moment then bugs

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C#

Kinect

SDK

Hello,

I'm developping an application using Kinect SDK 1.7 where i want to implement a fall detection algorithm.
based on the joints coordinates, i have to calculate the maximum and the minimum of all the joints to determinate the 2 dimensions(H and WD) of a 3D bounding Box.

i start with reducing the frames rate to 15FPS (in the KinectSensorOnAllFramesReady event) and then process frames to save the joints coordinates and calculate H and WD.
here is a peice of code of what i did.

private static float[][] JointTab = new float[20][];
        List<float> Hlist = new List<float>();
        List<double> WDlist = new List<double>();
        List<float> JlistX = new List<float>();
        List<float> JlistY = new List<float>();
        List<float> JlistZ = new List<float>();
        static int framesCounter = 0;
 public MainWindow()
        {
// other process

private void KinectSensorOnAllFramesReady(object sender, AllFramesReadyEventArgs allFramesReadyEventArgs)
        {
            
            framesCounter = framesCounter++ % 2; // 2 is the divisor to work with 15 FPS

            if (framesCounter == 0)
            {
                // other process to manipulate depth images


            Skeleton first = GetFirstSkeleton(allFramesReadyEventArgs);

            if (first == null) // s'il n y pas de squeulette 
            {
                txtP.Text = "No Person detected"; //  Idle mode)
                return;
            }

            if (first.TrackingState == SkeletonTrackingState.Tracked)
            {
                txtP.Text = "A person is detected";
                
                save_joints(first);
              

            }

         }
        }
 //************************************************************************************************************

        // Save and display joints coordinates

        //************************************************************************************************************
        /// <summary>
        /// method to save and display joints coordinates detected in a list
        /// </summary>
        /// <param name="first"></param>
        /// <returns></returns>
        private void save_joints(Skeleton first)
        {
           
            foreach (Joint joint in first.Joints)
            {
                float jx = joint.Position.X;
                float jy = joint.Position.Y;
                float jz = joint.Position.Z;
                JlistX.Add(jx);
                JlistY.Add(jy);
                JlistZ.Add(jz);
            }

            JlistX.Sort();
            JlistY.Sort();
            JlistZ.Sort();
           float  xMin = JlistX[0];
           float  xMax = JlistX[JlistX.Count - 1];
           float  yMin = JlistY[0];
           float  yMax = JlistY[JlistY.Count - 1];
           float  zMin = JlistZ[0];
           float  zMax = JlistZ[JlistZ.Count - 1];
  
            float W = System.Math.Abs(xMin - xMax);
            float H = System.Math.Abs(yMin - yMax);
            float D = System.Math.Abs(zMin - zMax);
            double WD = System.Math.Sqrt(Math.Pow(W, 2) + Math.Pow(D, 2));
            Hlist.Add(H);

            //WDlist.Add(WD);

            //***********************************************************************//
            //                                                                       //
            //                        display min et max               //
            //                                                                       //
            //***********************************************************************//
            
            xmax.Text = xMax.ToString();
            ymax.Text = yMax.ToString();
            zmax.Text = zMax.ToString();
            xmin.Text = xMin.ToString();
            zmin.Text = zMin.ToString();
            ymin.Text = yMin.ToString();
            dimH.Text = H.ToString();
            dimWD.Text = WD.ToString();
           

   

           //*************************************************************************************************************************//
        }

when running the code, there are mani problems:
1/ zmin is always at 0 ..
2/ the application runs a moment, i can see max and min values changing and then it stay fixed while i continue to move and make gestures.
3/ Hlist has always one only value which i don't understand.

Can anyone please help me ?
i'm stuck here for a while and can't see where is the error !
thanks for any help