velocity of a joint between two frames using kinect sdk - kinect

Question

I'm stack for along time in this problem and i will really appreciate if any one could help me in that.
I asked many times in many forums, i've searched alot but no answer that really helped me.
i'm developping an application where i have to calculate the velocity of a joint of skeleton body using vs c# 2012 and kinect sdk 1.7
i have first to be sure of the logic of things before asking this question so,
if I understood correctly, the delta_time i'm looking for to calculate velocity, is not the duration of one frame (1/30s) but it must be calculated from two instants:
1- the instant when detecting and saving the "joint point" in the first frame
2- the instant when detecting and saving the same "joint point" in the next frame
if it's not true, thank you for clarifying things.
starting from this hypothesis, i wrote a code to :
detectiong a person
tracking the spine joint ==> if it's is tracked then saving its coordinates into a list (I reduced the work for the moment on the Y axis to simplify)
pick up the time when saving the coordinates
increment the framecounter (initially equal to zero)
if the frame counter is > 1 calculate velocity ( x2 - x1)/(T2 - T1) and save it
here is a piece of the code:
System.Diagnostics.Stopwatch stopWatch = new System.Diagnostics.Stopwatch();
double msNow;
double msPast;
double diff;
TimeSpan currentTime;
TimeSpan lastTime = new TimeSpan(0);
List<double> Sylist = new List<double>();
private int framecounter = 0;
private void KinectSensorOnAllFramesReady(object sender, AllFramesReadyEventArgs allFramesReadyEventArgs)
{
Skeleton first = GetFirstSkeleton(allFramesReadyEventArgs);
if (first == null) // if there is no skeleton
{
txtP.Text = "No person detected"; // (Idle mode)
return;
}
else
{
txtP.Text = "A person is detected";
skeletonDetected = true;
/// look if the person is totally detected
find_coordinates(first);
/*******************************
* time computing *
/*******************************/
currentTime = stopWatch.Elapsed;
msNow = currentTime.Seconds * 1000 + currentTime.Milliseconds;
if (lastTime.Ticks != 0)
{
msPast = lastTime.Seconds * 1000 + lastTime.Milliseconds;
diff = msNow - msPast;
}
lastTime = currentTime;
}
//framecounter++;
}
void find_coordinates(Skeleton first)
{
//*modification 07052014 *****/
Joint Spine = first.Joints[JointType.Spine];
if (Spine.TrackingState == JointTrackingState.Tracked)
{
double Sy = Spine.Position.Y;
/*******************************
* time starting *
/*******************************/
stopWatch.Start();
Sylist.Add(Sy);
framecounter++;
}
else
return;
if (framecounter > 1)
{
double delta_Distance = Sylist[Sylist.Count] - Sylist[Sylist.Count - 1];
}
}
to be honnest, i dont really know how ti use timespan and stopwatch in this context ( i mean when there are frames to process many times/s)
i will be thankfull for any help !

Answer 1

First:
The SkeletonFrame has a property called Timespamp that you can use. It's better to use that one than to create your own timesystem because the timestamp is directly generated by the Kinect.
Second:
Keep track of the previous Timestamp and location.
Then it's just a matter of calculation.
(CurrentLocation - PreviousLocation) = Distance difference
(CurrentTimestamp - PreviousTimestamp) = Time taken to travel the distance.
For example you would get 0.1 meter per 33 miliseconds.
So you can get the meters per seconds like this = (1 second / time taken to travel) * distance difference. In the example this is = (1000/33)*0.1 = 3.03 meter per second.