puck speed

[Timberwoof]

My friend took some burst-mode photos of me while I was playing hockey at a tournament in Toronto last weekend. I found a sequence that I used to calculate the speed of a typical shot: two nearly identical photos where the puck was a blur in distinct positions. I used Photoshop to superimpose the images, then make one transparent so the puck shows through from the other image. I increased the contrast in that area to make it easier to measure the distance between the puck in the two pictures. I also measured the height of my stick and of the crest on my jersey for scale. Some careful calculations in Excel and I got my answer.

• movement of puck: 293 pixels
• knob to black on stick: 402 p.
• inclination of stick: 60 deg. est.
• stick height adjusted: 464 p.
• knob to black on stick: 1.34 m
• height of crest: 54 p.
• height of crest: .15 m
• scale: .00278 m/pixel (based on crest measurements)
• movement of puck: .81 m
• frame rate: 30 / sec
• interval: .033 sec
• speed of puck: 24.4 m/sec, 88 km/h, 54 mph

[guest1]

That's pretty damned cool....

[junior1cats]

Cool Timby...nice jersey BTW

[Redness]

Originally Posted by Timberwoof

My friend took some burst-mode photos of me while I was playing hockey at a tournament in Toronto last weekend. I found a sequence that I used to calculate the speed of a typical shot: two nearly identical photos where the puck was a blur in distinct positions. I used Photoshop to superimpose the images, then make one transparent so the puck shows through from the other image. I increased the contrast in that area to make it easier to measure the distance between the puck in the two pictures. I also measured the height of my stick and of the crest on my jersey for scale. Some careful calculations in Excel and I got my answer.

• movement of puck: 293 pixels
• knob to black on stick: 402 p.
• inclination of stick: 60 deg. est.
• stick height adjusted: 464 p.
• knob to black on stick: 1.34 m
• height of crest: 54 p.
• height of crest: .15 m
• scale: .00278 m/pixel (based on crest measurements)
• movement of puck: .81 m
• frame rate: 30 / sec
• interval: .033 sec
• speed of puck: 24.4 m/sec, 88 km/h, 54 mph

Can't you just divide the blur length by the shutter speed? The burst rate of your camera may vary, but the shutter speed should be bang on. (Also, then you only need one photo.)

[Timberwoof]

The blur length alone includes the size of the puck, which you may not know. The blur length is shorter than the amount of movement from one frame to the next, so your error in measuring it is greater with just the blur length. But either way you have to convert the length in pixels to real distances, which is why I needed to calibrate with the jersey crest.

[TartanBill]

Good effort!

If you know the shutter speed, you could use the length of the streak to provide a separate estimate. Your calculated speed seems in the right range.

[wortchillergoal]

Two things.
First, that is impressive work.

Second, with that much time on your hands, are you gainfully employed?

[rony]

Nerd....

Oh, wait, I'm just as bad...

[Timberwoof]

Gainfully employed? Yes ... but I was on vacation. Nerd? My roommate brought a Netgear wireless access point as a gift for our host and then installed it so that we could surf the web and post about hockey puck speeds from our laptop computers.

Anyway, the puck bounced toward my defenseman who cleared it.

And yes, that jersey is from Junior1cats.

[Redness]

Originally Posted by Timberwoof

The blur length alone includes the size of the puck, which you may not know. The blur length is shorter than the amount of movement from one frame to the next, so your error in measuring it is greater with just the blur length. But either way you have to convert the length in pixels to real distances, which is why I needed to calibrate with the jersey crest.

The 'gap' length also includes the length of the puck, since the gap is measured from the front of the puck in the first shot to the back of the puck in the second shot. I guess you could include both shutter lengths as well for better accuracy, assuming that the shots were made from a tripod.

I'm assuming you know the length of the puck, but I guess if it came at you face first (so that it hits you flat instead of on edge) that would mean a smaller puck size. Sssssoooooo.... assuming the frame rate is accurate, as is the shutter speed, we should include the gap and one blur for best accuracy.

[Timberwoof]

I don't know the shutter speed at which the images were taken, so I have to calculate it, and then it gains me nothing new. For the movement I measured from the front in one image to the fornt in the other, then from the back of the blur in one image to the back in the other. I averaged the two distances and got the distance traveled.

Even if I knew the shutter speed, I could not know the angle of the puck, and that would affect its length in the pictures.

[TartanBill]

Originally Posted by Timberwoof

Even if I knew the shutter speed, I could not know the angle of the puck, and that would affect its length in the pictures.

No matter about the shutter speed, it's just helpful to calculate a couple ways to make sure the results are consistent.

The angle problem will also affect your calculation, it's the same problem and same angle. Just estimate, the variance it won't be that big an effect. I'd guess you would re-estimate speed a bit 60 mph.

[Redness]

Originally Posted by Timberwoof

I don't know the shutter speed at which the images were taken, so I have to calculate it, and then it gains me nothing new. For the movement I measured from the front in one image to the fornt in the other, then from the back of the blur in one image to the back in the other. I averaged the two distances and got the distance traveled.

Even if I knew the shutter speed, I could not know the angle of the puck, and that would affect its length in the pictures.

Your camera probably records the shutter speed in the properties of the jpg file. The shutter speed is not variable, but the frame rate is. In burst mode, your camera performs as follows: First shot only - compose picture (focus, white balance, etc), subsequent pictures - shutter open, shutter close, read sensor, compress data, write data to card, clear sensor.

The compressability of the data will affect both the processing speed as well as the amount of data to be written. The frame rate is an average, not a timed event like the shutter.

Either way, I don't think the results are going to vary all that much. Once you find out the shutter speed, do the calculations again, and let us know.