91 Reply by Chas Tennis

(1 replies, posted in General)

I don't know much about this subject.  This thread might be related.   

http://forums.dpreview.com/forums/read. … e=38276161

For example-

"You cannot play High Definition video on slower computers. However, I use a very slow computer. It is only a Pentium 1.6ghz. Windows Media Player and VLC player will not play the HD video, strangely however, I have a program called Womble MPEG Video Wizard which will play the HD video on my slower computers. "

I don't know what your camera produces but to watch videos the computer often has to be very fast. 

Watching real time HD (correction) on my 533 Mhz computer is impossible as it takes a 3.2 Ghz computer.  It hesitates and drops many HD frames.  correction -- My slow computer can play -  at 30 fps -  high speed videos (120fps & greater)  from my Casio FH100 because they all have much lower resolution and the data rate for those videos is less.

Running Kinovea & video together on a computer that is too slow?

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92 Reply by Chas Tennis

(20 replies, posted in General)

Websites with marker information. 

Search terms - biomechanical, markers, retro-reflective, sphere, video, analysis

General       http://en.wikipedia.org/wiki/Motion_capture

Sphere application.   http://en.wikipedia.org/wiki/File:Kistler_plates.jpg

Retro-reflective sphere markers - discussion on making or purchasing the spheres.
http://biomch-l.isbweb.org/threads/1893 … on-Request

Reflective tape.
http://books.google.com/books?id=PrOKEc … mp;f=false

Markers with patterns.     http://www.youtube.com/watch?v=7JiD1mFan5E

Application of markers in dance - Techniques used briefly described for many publications for the analysis of dance motions.
http://www.citraining.com/Biomechanical … Dance.html

A company offering video analysis of pitching. Retro-reflective balls are used.
http://www.youtube.com/watch?v=BhNmUv4XkyU&NR=1

A company discussing retro-reflective balls & IR LED ring light limitations outdoors.  Uncertain of what video techniques and markers they applied outdoors.
http://w4.ub.uni-konstanz.de/cpa/articl … /4387/4078

Vicon system able to operate outdoors with IR LED illumination & retro-reflective balls -
http://blogs.discovermagazine.com/80bea … aroos-hop/

LED markers. Also, LED sources themselves could be used as markers but again lighting outdoors is an issue.  In addition, when tracking point light sources outdoors such as LEDs there will be competing sources from small solar reflections.  These frequently appear outdoors from the sun reflecting from specularly reflecting surfaces especially those that have small curved surfaces, for example, a ball bearing or curved chrome tube.  These reflections might also appear as point sources and saturate the video levels just like the LED marker.   ( A ball bearing itself might be useful as a marker in sunlight.)

Estimated beam spread from retro-reflective materials. - As a first rough guess for RR applications a collimated laser beam might be reflected from the retro-reflective materials in a cone of 15 degrees back toward the light source.  Consult specs for materials.  [Corner cube reflectors can reflect light back to the light source with much less spread.]   The camera lens must be within this cone.  To observe the cone send a laser pointer through a hole in a screen and onto the RR material, look at the reflected light on the screen.

Professional systems. At this point I see some applications with LED IR ring lights at the camera for illumination (Vicon, etc) and retro-reflective spheres on the subject.  Often expensive 3D multi-camera systems for professional use. Many applications are indoors with controlled lighting.  Lighting outdoors in sunlight is an issue for retro-reflection markers.   

Non-professional users. First impression after a brief Internet search.  For the non-professional 2D user with a single high speed camera and working outdoors, markers with patterns, such as white diffusely reflecting spheres on black backgrounds that will produce contrast from sunlight are probably the most practical.  All affordable video & high speed video cameras (Casio, etc.) have a filter that blocks IR. Therefore applications with similar cameras must be in the visible.   Indoors, where lighting might be controlled, the retro-reflective tapes, RR spheres, and LED markers are probably workable.

Other options especially outdoors at reasonable cost or high cost?   Disagree with any points above?

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93 Reply by Chas Tennis

(20 replies, posted in General)

I have just a little experience using markers for high speed film imaging and almost no experience for sports applications or tracking. No internet search yet so there may be many markers that I’m not familiar with.  This thread could use some advice on marker options from those who have professional experience selecting markers for high speed video in biomechanical research. 

1) Tape.  Contrasting tape placed in reference locations.  http://vimeo.com/21512296  If applied to the body one issue is that the tape may not move with the bones due to muscles bouncing, etc.  Did the upper arm rotate or did the muscle with tape lag the rapid upper arm internal shoulder rotation?

2) Retro-Reflective Tape or Paint. One marker candidate is retro-reflective tape.  It reflects light back in the direction that the light arrived from.  Retro-reflective tape has the same glass beads that are used in the road signs to reflect the headlights of your car back to you.  To use RRT put the markers on the object and place light sources very close and alongside the camera lens.  Lighting is an issue, easiest to use in low ambient background light. The use of RRT with considerable ambient background light and especially in full sunlight may not be practical. ?  3M has technical information on its retro-reflective products.
http://vimeo.com/21529497

3) Markers with Patterns.  I don’t know how the Kinovea tracker works.  To improve chances of tracking would it be better to have the tracker work on the contrast & shape within the marker instead of between a marker and the varying background?  For example, would high contrast markers with patterns such as these help the tracker?

  1) Black circle surrounded by white ring
  2) White circle surrounded by black ring
  3) Circle half white and half black, etc.
  4) Other, high contrast within the marker
  5) Multi-markers, 3 circles close together.
  6) A black rectangle - similar in size to the feature window? - with a white circle inside. 

4) Balls as Markers.  Spheres regardless of how they are oriented display the same shape and reflect light the same way. (A tennis ball seems to be an ideal object to track.) Therefore using small balls as markers seems like one option. Ping pong balls are very light weight.  They would have to be rigidly attached.  Velcro is an option but not completely rigid.  Epoxy the ball to a machine screw, clamp, bendable aluminum strip or strap, etc. and attach that to the object?   Ping pong balls add air resistance and that may not be negligible for some high speed applications such as on a tennis racket.  Style is an issue.

5) LEDs.  Are point light sources good candidates for tracking? These have been used in biomechanical research.  But are they bright enough to be useful, probably not in sunlight.   LEDs are manufactured in thin credit card shaped magnifiers. Do they also make other very small, light weight, self-contained LEDs with light weight, small batteries?
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“Tracking tennis/baseball balls is most probably out of reach without super high speed camera.” 

I have not tried high speed video yet using the following set up.

Generally a very useful camera location for projectiles or balls is with the camera on or very near the trajectory and viewing parallel to the trajectory. If coming toward the camera images of objects get slowly bigger and if going away they get slowly smaller – but the image of the object moves very slowly in the frame in comparison to viewing the object on its trajectory from the side.  [With bullets first surface mirrors are placed next to or on the trajectory looking back at the gun to protect the camera.]

Tracking. This set up viewing the ball going away after it has been struck or thrown is very favorable for tracking especially if the ball is viewed against a high contrast background.  This view also tends to be very informative.  It may show the effects of tennis ball spin or golf ball slice and also may show where the ball lands. 

I’ve seen some golf trajectories and believe that golf balls are routinely tracked going away by golf software. A camera position often used in golf is from just behind and 1 meter above the golf tee but not exactly along the initial trajectory.

Tennis strokes, volleyball serves, baseball pitches, etc., can be viewed from behind and exactly parallel to the initial trajectory. I have not seen this view in high speed video of tennis serves probably because the camera has to be positioned higher (3+ meters) than a standard tripod reaches (2 meters). This makes the setup inconvenient for non-professional use.  For example, I'd like to see high speed video of the initial trajectory of a kick serve as it leaves the racket.  The path of the racket strings across the back of the ball could also be seen.  Kinovea might be able to track the ball after the racket has moved off to the side especially if the court is the background.

For a video taken from behind and 1 meter below the trajectory/racket height using a Casio FH100 at 240fps -

Using video not optimized for setup I tried to track a tennis ball going away with Kinovea.  K tracked the ball for a short distance away from the racket until the contrast deteriorated and then lost it.  K reacquired the ball again after several frames when the ball passed in front of a dark green background.  I just tried tracking a few times and don't recall any settings such as the search window. Does the feature window/search window continue on its previous path after the object track is lost?   

I believe that tracking a tennis ball just after it is served could be a clean, informative and important tracking problem especially if observed exactly along the initial trajectory. 

Motion Blur.  Always use the fastest shutter speed possible to minimize motion blur and get a sharp image of the ball.   I use 1/10,000s or faster.  In addition, the along- trajectory view itself will also minimize motion blur.

If anyone is interested in tracking tennis serves or other objects along the trajectory with high speed video please describe your application. 

You’ll see me at the US Open this year with my Casio FH100.  I'll be in the stands sitting in the row with the proper height to video serves along the trajectory.  #1 on my list is Stosur’s kick serve.   Anyone going to the French Open?

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94 Reply by Chas Tennis

(20 replies, posted in General)

Definition:  By "markers" do you mean the features inherently in a scene such as a ball or the edge of a baseball bat. Or do you mean artificial things that are added for tracking such as tape, LED lights, etc.?  Or, it does not matter for the tracking problem?

I tried to track a tennis ball going away with Kinovea.  It tracked the ball for a short distance away from the racket until the contrast deteriorated and then lost it.  It reacquired it again after several/many? frames as the ball passed in front of a dark green practice hitting wall.  I just tried it a few times and don't recall any settings such as the search window. Does the search window continue on its previous path after the object is lost?

Eventually I would like to attach a marker to a point on my tennis racket and track a total path as has been done. But I don't know of effective video techniques that are practical. Attach a ping pong ball?

To serve I put black electrician's tape on my upper arm and hand.  It showed high contrast and a changing rectangular shape.   A ring of tape could be used on an arm or leg so that turning does not lose sight.

Has anyone used very light weight & bright LED markers that would show up in sunlight illumination?

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95 Reply by Chas Tennis

(2 replies, posted in Ideas and feature requests)

I have not thought out very well or for very long.......dumb mistakes, etc.......so please don't take it otherwise and spend much time   

Let us say I place an axis as best I can in the upper arm.   The axis begins at the judged center of the elbow to runs to the judged ball on top of the humerus or other end for the axis that seems easiest to judge in the image.   There is a disc always centered on the axis and by eye you move it along the axis until its center is near some marker (the tape on my arm in my video, or bony protrusion or some other reference point on the body). By eye you make the disc perpendicular to the axis and intersecting a point on the marker. That gives one reference line for the angle, such as it is, for frame #1. 

Step to a later frame and place the same exact axis & disc image oriented as copied from the frame #1 into the later frame.   The orientation for the limb & its axis may have changed some.  I think that the center of the disc will always be at the same relative position along the axis (for example, 67% of the distance from the top to the bottom).  I'm not sure of this assumption for all camera conditions, wide angle, large angle limb change, etc., assume for now.  If you carry the image from the first frame into the later frame and reoriented it manually including the disc, would that do it?  The length change for the limb should help reorient & place the axis in the later frame. One line for the angle would be from frame #1 and the another from the later frame.   Orienting the axes seems similar to your perspective grid operations.

I have no idea of the errors in placing rotation axes into limbs in videos.   

FYI - general interest on rotation and a non-video technique.

Look at the internal rotation rates for lower, intermediate & advanced level players:

http://onlinelibrary.wiley.com/doi/10.1002/jst.112/pdf

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96 Topic by Chas Tennis

(2 replies, posted in Ideas and feature requests)

I just downloaded your software this morning and am very pleased at how it works.  It is the first sports software that I have used.

Recently I realized by reading discussions on the Tennis Talk website that shoulder upper arm rotation may play a much bigger part in the tennis serve than I had previously understood.  [Pronation is defined as lower arm rotation.]   I am now interested in measuring upper arm rotation and practicing it to improve my serve.

To illustrate here is a video of two of my serves. I put pieces of black plastic tape on the protruding elbow bones of my humerus to make the upper arm rotation stand out.   Casio FH100, 240fps, 1/10,000s. 

http://vimeo.com/21512296

The tape moves suddenly indicating when I rotate my upper arm. Probably late? I can measure the motion linearly and assume a radius to the upper arm's rotation axis to get an angular rotation rate.

A similar upper arm rotation is supposed to be a significant contributor for baseball pitching.  For pitching I have read that the upper arm is capable of rotating up to 7000 degrees/sec, possibly one of the fastest rotation rates for the body parts.  The motion is powered by the stretched internal rotators, the pec and lat, large muscles.

I am not that knowledgeable in biomechanics so please double check my conclusions.

Anyway, if I could manually position a rotation axis in the upper arm to measure the position of a marker on the upper arm and repeat it on  subsequent frames that would be what I'm most interested in now.   The question is does such a tool offer any advantage over measuring the tape position linearly?   Is it possible to measure the rotation angle from a 2D video image where the axis also changes from one frame to the next?

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