Author Topic: Measuring Speed  (Read 432 times)

wilheldp

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Measuring Speed
« on: September 18, 2017, 12:39:10 PM »
Is there a proper name for the backing board that you can place behind a fast-moving subject to measure its speed?  The Mythbusters used them all the time, and I've seen them on some ping pong cannon videos too.  Basically, it is black and white lines of known width (inch/foot, or metric markings).  If you know the frame rate of the camera and playback frame rate, you can measure the exact distance that an object moves between frames, and calculate its speed.

I'd like to do some of those experiments with my Chronos.  I can probably make one, but if they are relatively cheap to buy, I'd rather go that route.  I don't have a lot of spare time at the moment, and I'd be worth it to me to just buy such a board unless they are exorbitantly expensive.  As of yet, I've failed to find a string of search terms on Google or Amazon that gets me what I want.

NoDak

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Re: Measuring Speed
« Reply #1 on: September 18, 2017, 01:24:24 PM »
That is a good question. I know exactly what you are talking about, but have no idea what they are called.

I don't think something like that has a high enough volume to be ecenomic to mass produce. The high speed market is small and everyone will need different sizes and graduations for their particular project.

I made one out of a yardstick and tape years ago. My camera at the time wasn't up to the task so I never got far with it.

tesla500

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Re: Measuring Speed
« Reply #2 on: September 18, 2017, 07:24:44 PM »
I've been calling those things a "Scale" or "Scale sheet". For small ones, you can just print something out on a printer, calibrated to whatever your favorite unit is, 1cm wide lines work well for something that fits on a normal printer. It's really easy to draw out the scale lines in Paint or a similar program, then you need to figure out the print scaling, this usually requires a few test prints and measuring the size that actually came out.

For bigger ones, the best option is to mark out and mask off a piece of wood with masking tape, and paint the stripes on.

wilheldp

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Re: Measuring Speed
« Reply #3 on: September 19, 2017, 03:58:30 AM »
Yeah, I'm starting to think that nobody sells them commercially.  The NHTSA website references "inch tape" with alternating yellow and black, 1" bands, but I couldn't find anybody selling that.  I found some photo scales on forensic supply websites, but the biggest ones I could find are 1-1/4" wide and 12 inches long.  I guess I'm going to have to make my own.  I really hate painting.

NoDak

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Re: Measuring Speed
« Reply #4 on: September 19, 2017, 05:09:45 AM »
Just measure it and use duct tape. That's what Mythbusters and company have done on occasion. Nobody says it has to be cable TV grade. Just mark at one foot intervals and put the edge of the tape on that line. Then you'll have 12", 10", and 2" intervals. (Assuming you use 2" duct tape.)

thebishop

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Re: Measuring Speed
« Reply #5 on: September 19, 2017, 05:56:53 AM »
« Last Edit: September 19, 2017, 06:04:19 AM by thebishop »

NoDak

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Re: Measuring Speed
« Reply #6 on: September 19, 2017, 12:52:26 PM »
Maybe you'd like http://www.measureittape.com or https://www.amazon.com/Beacon-Yellow-Measuring-2-Inch-10-Yard/dp/B00JBF6ASQ? (there are many similar ones on the same theme)

Those graduations wouldn't show up depending on the distance and resolution.


wilheldp

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Re: Measuring Speed
« Reply #8 on: September 19, 2017, 03:07:21 PM »
https://www.amazon.com/Stripe-Black-White-Fabric-Yard/dp/B00QFHHK32/ref=pd_sbs_201_3?_encoding=UTF8&psc=1&refRID=80BQSVXYM05QWKWJXACC ?

 :)

That might be crazy enough to work.  I could cut off some of the black strips and make 1-foot segments to put below the 1-inch segments.  I wonder how accurate their 1" strips are...not that any of this is going to be super-precise anyway.

NoDak

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Re: Measuring Speed
« Reply #9 on: September 19, 2017, 09:45:14 PM »
https://www.amazon.com/Stripe-Black-White-Fabric-Yard/dp/B00QFHHK32/ref=pd_sbs_201_3?_encoding=UTF8&psc=1&refRID=80BQSVXYM05QWKWJXACC ?

 :)

That might be crazy enough to work.  I could cut off some of the black strips and make 1-foot segments to put below the 1-inch segments.  I wonder how accurate their 1" strips are...not that any of this is going to be super-precise anyway.

Lets add on a few more cars to this crazy train. ;D

Get a length of this fabric.

Hem each end into a loop.

Get some black pipe and make a coat rack. Horizontal members slightly longer than the fabric. (Don't cheap out and use PVC, this thing will need some weight to keep the wind from making it into a kite. :o )

Put a piece of black pipe into the hems that is long enough to stick out each end.

Loop the hemmed fabric over the top and under the bottom of the coat rack. Stripes facing outward

Connect the two pieces of black pipe with bungie cords/tarp straps on the "back side" to keep the fabric tight.

Weigh the whole thing down with sandbags, cinder blocks, or whatever else you have available so it does not become a kite in the wind.

Film your high-speed stuff in front of your stripes. 8)


For options, I also found some fabric with 2" stripes. It might show better. https://www.amazon.com/gp/product/B017AKWDP2/ref=ask_ql_qh_dp_hza

I couldn't find anything with 3"or 4" stripes that wasn't a stupid price. :(

thebishop

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Re: Measuring Speed
« Reply #10 on: September 20, 2017, 12:12:09 AM »
https://www.amazon.com/Stripe-Black-White-Fabric-Yard/dp/B00QFHHK32/ref=pd_sbs_201_3?_encoding=UTF8&psc=1&refRID=80BQSVXYM05QWKWJXACC ?

 :)

That might be crazy enough to work.  I could cut off some of the black strips and make 1-foot segments to put below the 1-inch segments.  I wonder how accurate their 1" strips are...not that any of this is going to be super-precise anyway.

The important thing is that the striping is uniform, then you can simply adjust by measuring the exact width.

NoDak

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Re: Measuring Speed
« Reply #11 on: September 20, 2017, 05:09:44 AM »
https://www.amazon.com/Stripe-Black-White-Fabric-Yard/dp/B00QFHHK32/ref=pd_sbs_201_3?_encoding=UTF8&psc=1&refRID=80BQSVXYM05QWKWJXACC ?

 :)

That might be crazy enough to work.  I could cut off some of the black strips and make 1-foot segments to put below the 1-inch segments.  I wonder how accurate their 1" strips are...not that any of this is going to be super-precise anyway.

The important thing is that the striping is uniform, then you can simply adjust by measuring the exact width.

True, that is the big thing. One would need to have some in front of them and measure with a calipers to find the actual width.

pstest

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Re: Measuring Speed
« Reply #12 on: October 22, 2017, 08:31:35 PM »
cheap and dirty way to make a checkered fiducial backdrop is in MS Excel - Choose the cell size desired. Blacken cell A1 and then every other cell in that column  go to column B and blacken cell B2 and then every other cell int hat column - repeat until you have a grid - size the grid for your application and print as many copies required to complete your backdrop -  glue the printed grids to a piece of cardboard required for your backdrop - measure distance of camera lens to backdrop. 

gyppor

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Re: Measuring Speed
« Reply #13 on: October 23, 2017, 12:39:06 AM »
If you need something long a telescoping staff for a laser level could work, can be had relatively cheap and off the shelf from a survey place. The only issue I can see is they're not very wide.

http://www.ebay.ca/itm/2-pcs-of-CST-4M-Metric-Telescoping-Aluminum-Rod-Metric-E-06-804MM-with-Bag-/142216661926?hash=item211cc60ba6:g:-pQAAOSw241YVvGl

AimedResearch

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Re: Measuring Speed
« Reply #14 on: November 27, 2017, 07:11:45 PM »
You might be looking for a quilter's ruler or cutting mat.  They are sold clear (for your own background) or white among other colors.

Honestly, if a TV show is using a grid (behind or outside the plane of travel) it is only for show.  If you are interested in accuracy, you use an image calibration in the plane of travel.  This means you have a second image file for your calibration grid in the plane of travel and reference it for your tests.  You update the calibration every time you move the camera or adjust the lens.  If you only have video editing software, then you make the calibration image transparent and overlay it to your test videos.  If you have decent motion analysis software, then you can have on-screen point tracking.