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how is vx calculated? post and replies

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how is vx calculated?
Lev Yampolsky
5 Posts

I would imagine that for each time point t, vx should be calculated as

vx(t) = (x(t) - x(t-d))/d,

where d is the time step.

I see that it is not calculated this way.
In a set of measurements done at 0.5 sec steps I found out that vx is calculated as

vx(t) = x(t+d) - x(t-d).

Strange, but ok. One can argue that this sliding averaging over two consecutive measurements is a good idea.

But when I look at more detailed data, with every frame being a step (d=0.04 sec)

the same formula does not apply, i.e.,

vx(t) != (x(t+d) - x(t-d))/0.08.

(I found in help that this is the default algorithm, but it is clearly not in my data. Help mentioned that there is another algorithm, Bounce - maybe I accidentally turned it on? I could not figure out how to switch between algorithms).

I also checked if maybe velocities are averaged not over two two consecutive measurements, but over the whole 1 second. Would be very detrimental to capturing quick changes in velocity, but ok. But this is not the case either. Sometimes x clearly increases over several observations, but vx changes sign! Sometimes x is changes direction, but vx does not change sign. Something is not right.

I mean, I really don't have to care, let's assume that x and y are reported correctly and I'll calculate velocities myself, but just curious.

Thanks in advance!

Replies to how is vx calculated?

Re: how is vx calculated? -
Lev Yampolsky
5 Posts

I have a feeling that the observed problem is somehow related to the mysterious statement in the Autotracker help:

The specific track and point marked at the target position must be selected prior to autotracking.

What is a track and point and how do I mark it on the target? It is not mentioned anywhere downstream in the help. I am worrying that I did not select it prior to autotracking and thus all velocities are calculated relative to some wrong reference point.

Please advise...

ok, I see what is happening. -
Lev Yampolsky
5 Posts

after a lot of experiments I am convinced that my problem is with the mysterious  track and point thing. A lot of times, even when I clicked at the correct object, Autotracker keeps calculating velocities of the reflection (hence opposite signs relative to the actual object's trajectory). This happens because when I shift-control-click on the image I set the target, but I do not set the new "track and point" (should show un on the video as a thick cross). A couple of times I saw this cross while clicking, but could not figure out how to make it show up. No combination of shift-control-alt-command brings it in.

Sounds like a mighty important feature of the Autotracker and totally counterintuitive (why is it separate from the target and why can it be offset from it?)- wouldn't it be a good idea to describe it in help?

Thanks in advance for any advise!!

Re: ok, I see what is happening. -
Douglas Brown Avatar
Douglas Brown
339 Posts

You set the track and point right in the autotracker dialog itself (pick them from the dropdowns). When the track is a point mass there is only one point you can choose  (its position).

The velocity algorithm is indeed what you identified and it is impossible that it is using the bounce algorithm unless you change it. I suggest you recheck your calculations.

Autotracker has no effect on the v algorithm, it simply helps you mark positions.

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