Slow-motion camera program with on-screen timer?

[Dan Linder]

Brainstorming for ideas here - I'm open to nearly any platform option (Linux, Windows, Android, iOS, OSX)...

I'm working with some middle-school students and we need to estimate the speed of a tennis ball as it is launched from a sling-shot sort of device, plus they need to know how long it is in-flight (launch to impact).  Having an on-screen timer synchronized with the video is a must.  We've tried putting a stopwatch in the frame, but we found out most LCD screens don't refresh at 1/100'th a second. :)

I've seen other schools using apps on iPads/iPhones that take high-speed videos and provide a stopwatch overlaid with the video to facilitate the math.

Can anyone recommend an inexpensive program that does this?

Thanks!

Dan

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[Denis Fitzpatrick]

Although not what you asked for, this may be of interest:
http://www.cabrillo.edu/~dbrown/tracker/

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[Eric Kaplan]

Have you tried the Mythbusters approach, which is to have a grid pattern in the background?  Then you just measure how many frames it takes for the object to pass across the grid.  If you know the spacing of the grid and the frames-per-second of your camera, it's simple math to calculate the speed (and distance).

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On Thu, Mar 5, 2015 at 9:40 AM, Dan Linder <d...@linder.org> wrote:

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[Dan Linder]

We may have to re-try that.  We originally had a ruler and stopwatch in-frame, but that's when we found that the speed of the stopwatch LCD update was "chunky".  (i.e. displaying 1/100's of second resolution, but only updating in .10 to .15 second increments at times).

We'll keep plugging along.  I want to look at the "Tracker" software that Denis mentioned.

Dan

[Eric Kaplan]

You background grid could be made with square-foot vinyl floor tiles.  Some of them have patterns that are evenly spaced, giving you an actual grid of 6" squares or 4" squares.

Years ago, I worked with a Biomechanics research team.  We built a background using black and white 6" tiles, then used that to analyze the motion of Olympic athletes.

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[David Knaack]

Each video frame will have a specific timecode associated with it. Do you just need a way to single-step through the video while displaying the time code?

Pretty much any video editing program should be able to do that. I use VirtualDub on Windows, because it is free. Not sure if it supports the video format you have though.

[Adam Shaver]

Is this still an open problem? Do you want the middle school students to do the estimation, or, would you want some processing filter to estimate the velocity, etc. Tracking algorithms and statistics are my wheel house.

[Adam Shaver]

I think that tracker software was what we used in physics labs at UNL, either the same or very, very close.

[Dan Linder]

The issue is still open, though we don't really need the true muzzle velocity anymore.  Due to other constraints, we're looking at just collecting data for a number of runs at specific calibration points (known bullet, known launch angle, known target elevation, etc), then we'll interpolate the range.  Ultimately, their goal is to place the ball (bullet) into a small target area at a known distance and height.

Still, having a slow-motion camera with an on-screen timer (1000th second) would be extremely beneficial so we could get a good velocity reading.

And I *STILL* need to find some time to look into the tracker (http://www.cabrillo.edu/~dbrown/tracker/) software that Denis mentioned.   The web page makes this product look extremely interesting.

Thanks,

Dan

On Wed, Mar 11, 2015 at 3:22 PM, Adam Shaver <adam....@gmail.com> wrote:

Is this still an open problem? Do you want the middle school students to do the estimation, or, would you want some processing filter to estimate the velocity, etc. Tracking algorithms and statistics are my wheel house.

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[Adam Shaver]

If the purpose is just to quantify muzzle velocity *and* you can aim the device straight upward, you can gather the initial velocity with the closed form solution (p(t) = 1/2*g*t^2+v*t where p(t=0) =0 and g  grav. Thus, you can find the muzzle velocity by solving for v, if t_launch and t_impact is known. Also, because the dt from launch to impact will be quite large, any sampling time (such as a stopwatch) will not affect it by much. 

A much bigger issue though is the transfer of energy from the launch device to the ball. If you're using something like compressed gasses, then you can assume it to be constant + noise. However, if you're using something like elastic bands, then the transfer of energy is significantly (strongly) dependent upon the mass of the ball and the angle at which it is launched. That all being said, once the object leaves the launch device, it's flight is ballistic + drag. 

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[Dan Linder]

If they end up going down the route of needing velocity, we'll use the shoot straight up method.  Thankfully the launcher is air powered and they appear to be getting a consistent percentage of the energy transferred each shot, so hopefully that's one variable that won't fluctuate too much.

Thanks again for all the brainstorming!

Dan