1 Topic by AnthonyG

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I am a coach trying to measure the leg speed of a football kicker's leg.  I have played with the tracking tool, but have been unable to figure out exactly how to get the measurement. 

I set it to measure in MPH, but as I start moving the frames the tracking gives me px/f.  I have no idea what that is or how to convert it. 

I am capturing the foot as it comes into frame and tracking it (sometimes manually) until it strikes the ball (usually the highest px/f).

Can someone help me understand how to read the speed of that motion?

Thanks,
Anthony

2 Reply by joan

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You need to have a calibration reference in the video. Add a line drawing on top of something of a known length, right click the line and go into Calibrate measure to set the physical length of the object.

The accuracy of the measure will depend on several factors.

The line of known length should be on the plane of motion otherwise there will be perspective error.

The calibration object should be as large as possible and as close to the center of the image as possible to limit error due to camera lens distortion.

It should also be as close as possible to where the trajectory will take place. One idea would be to use the leg of the kicker if static points are clearly visible on it during the kick and assuming it doesn't deform (see also reservations about rotation).

To limit this camera lens distortion avoid using fisheye type lens by all means. With regular camera you will want to move back and then zoom in, this will flatten the scene and reduce perspective errors. Use a tripod and remote control the recording to minimize camera movement between calibration and measure. This is especially important if you take several attempts with the same calibration frame.

The camera optical axis should be perpendicular to the plane of motion to avoid perspective errors. At the least you will want to make sure it's parallel to the ground, with a small spirit level for example.

I'm not too familiar with how much rotation component is there during the kick. Not all of the motion will take place on a single plane of motion. You need to keep that in mind when comparing speed of several subjects if they have different kicking style.

The next version is coming along and will have several improvements on the topic. (sub pixel tracking, changing tracker parameters, acceleration, angular speed, etc.)

3 Reply by Chas Tennis (edited by Chas Tennis 2014-06-05 14:55:09)

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Term "football"?  Not sure if you mean American "football" or Non-American "football" as in most of the World ?

Do you want to measure the final foot speed at impact only?

Or foot speed vs time throughout the kick?

Is the kicking motion mostly in a single plane? 

Or is it considerably in 3 dimensions like a tennis stroke?

The basic problem is that a single camera does a good job showing 2D. The video can be calibrated if the object's motion can be viewed from a camera angle so that its motion is in those 2 dimensions.  This can be done in many applications such as bicycling, running, walking, etc., where the motion is mostly in one plane.   

For 3D object motion, whenever the object has a velocity component toward or away from the camera, that component is not well measured by the camera, it more difficult to calibrate and often calibrations might not be possible.   

I have no experience working with this flexible hose calibration technique -

For 3D calibration, the first thought that comes to mind might be to use a flexible tube and place tape on it at 10 cm spacing.  After the kick arrange the tube to the kick trajectory as best you can using the video of the kick and the help of the player showing you the kick's path.   Video it.   You should see useful calibrations across the frame corresponding to locations on the foot's path.   This approach with varying calibration scales across the frame needs more work......  It might be workable for calibrating simpler 3D motions such as kicks or baseball bat swings. To use this information to automatically produce speed calibrations, Kinovea would have to have a way to apply different scale calibrations to different frame locations. 

If the plane of the kick is mostly at one height and if you have control of the site you might consider placing the camera directly above.  This is a lot of trouble and has safety considerations.  I find this view very informative in tennis.

Anatoly Antipin composite picture from video frames.
http://i43.tinypic.com/2rot1g3.jpg

Use of a second video camera viewing at a right angle to get the 3rd dimension might also be helpful.

A convenient scale in some circumstances might be the diameter of the ball.   Also, the length of the player's shoe.

I sometimes use the appearance and length of the tennis racket as an indicator of how tilted it is away from the camera.

4 Reply by joan

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Good thinking on the calibration by hose, it would reduce the problem to one dimension instead of three, tracking the position of the point along the curve. However I think it would be very hard to get right. Holding the calibration hose in mid-air so that it matches the trajectory. Even with the help of the actual video overlaid on top of the live view, it would be mostly guess work to position and curve it properly, with no real way of knowing the bias relative to the actual trajectory in 3D space.

The positions between two marks would also not increment linearly due to perspective, although this could be mitigated by interpolating.

Another way might be to get a depth map with a Kinect-like device. The athlete might have to wear markers reflective to IR. A way to actually calibrate that third axis would still be needed though, but maybe the Kinect already provides this information.