Hacker News Reader

Comment by porphyra | Hacker News Reader

Photo finishes are typically done with a line scan camera. It only captures a single column of pixels at a time. So the horizontal axis in the image is actually time, not space. Super cool stuff.

View on HN

I find line scan cameras and strip photography fascinating.

https://en.wikipedia.org/wiki/Strip_photography

Early film versions used a highspeed spinning slit aperture to film fast objects. This paper from 1931 shows some very impressive results for ballistics, including the shockwave: https://royalsocietypublishing.org/doi/pdf/10.1098/rspa.1931...

Outside sports, digital line scan cameras are used in various quality control applications (objects on conveyer belts, vehicle mounted road/rail scanners, etc). This unit can film an 8k px strip at a rate of 80khz.

https://www.youtube.com/watch?v=OXUwJOJ7fMk

Oh yeah I took that picture on the Wikipedia article! I have a couple more on my website [1]. One of these days I want to go to Atherton station with my line scan camera to scan some Caltrains.

[1] https://daniel.lawrence.lu/photos/

Small world! You took some amazing images. Can I ask - what hardware did you use?

Alkeria Necta N4K2-7C

Very cool. Have you thought about filming a rocket launch? The smooth acceleration should cause very a interesting distortion.

How did you buy it? Can't find any shops online. Not on eBay either.

They only do business to business sales. You'd email them and then wire them some money (about $2000 in 2017) from your company.

What does that cost?

About $2000 I think, I bought it in 2017.

Very cool photos! Just as an FYI, the link to the full sized "Victorian house in San Francisco, CA, 2020" image 404s for me.

Weird, I'll fix that later but meanwhile you can also find it here: https://commons.wikimedia.org/wiki/File:Victorian_house_on_W...

Long train is long!

It's actually hilarious what you really need to scroll it to see it on your Wiki page.

That seems to be what flatbed scanners do.

For a while, handheld scanners were a thing: hand-held strip cameras with roller wheels you would swipe across a page.

There is a similar technique in astronomy, though much slower, called drift-scan imaging. Typically when you take a long exposure astronomical image the telescope has to rotate to track the star as it moves across the sky. So in traditional imaging you track the object, make your exposure, and then when the exposure is done you read out the image. The downside is that while you're reading out the image you can't do anything else and for an astronomical CCD it can take on the order of a minute or so. So you lose ~5% of your observing time just to reading out images. (It's more if you have to slew to different locations on the sky.)

In drift-scan imaging you keep the telescope pointed at a fixed location and you continuously read out the image at a rate that matches the motion of the stars across the field of view. This allows you to continuously collect imaging from a strip across the sky.

Fun fact! This is actually how many earth observation satellites work too! Except the motion comes not from the subject, but the satellite orbit itself. It's called a pushbroom camera.

HiRISE in MRO is a pushbroom imager too… so we have sent the tech beyond Earth’s orbit :)

Yes. And this is a terribly uninformative article which says nothing interesting about any of that.

Shouldn’t the screen (Olympics logo) in the background be a constant blur then?

It's also 1 pixel wide, and varies over time. https://youtu.be/7Xnr805bm4E?feature=shared&t=417

Agreed. Better article IMO: https://www.hodinkee.com/articles/olympics-2024-mens-100m-ph...

To the question of the Omega logo and Olympic rings asked below, this article states "In fact, the entire photo is literally the finish line viewed one pixel at a time (the Omega and Olympic branding is added at the top)."

IIRC, they have a single pixel wide display that strobes the logos, similar to a persistence of vision display.

I had to combine Tuna-Fish comments and yours to understand what's going on.

> And the Omega/Olympics flag "banner" behind the runners is not a banner, it is a led screen, one pixel wide.

So the one pixel wide scan line camera outputs and image that has time as horizontal axis, and the one pixel screen that draws the omega banner is a 40kfps single line video that does marketing one the photo finish that likely serves as a clock sync check.

Ah I was wondering why you can't actually see the finish line in the image. This wasn't really described well. Are the shadows of the runners artificial then?

You can, it's the white bit. It just doesn't move.

The shadows are what's projected on the line from the stadium lights, and the shape is how they changed in time.

Oh I see, it's the entire "track" that is rendered in the image.

Again, not well explained in the article.

Because this is a line camera, the entire image is the finish line. That is, each vertical column of pixels is what was on the finish line at that particular point in time.

That’s also the reason for the distortions. It’s not a single frame taken at one time.

shouldn’t the runners be facing left then?

Time goes backwards on the x axis.

> Are the shadows of the runners artificial then?

As the runners' shadows cross the finish line, they are recorded by the camera in the same way that the runners are.

>This wasn't really described well.

This short video might be a better explanation of the "line camera" concept of multiple photos of a single line being stretched out over the x-axis :

https://www.youtube.com/watch?v=Ut0nKdLCAEo&t=0m23s

Is the advertising board behind faked then? Shouldn't that be static relative to the finish line, and therefore not really show up?

According to Reddit thread on this, the led board behind is specifically animated with the right timing so the content shows up correctly in the line camera.

There is a special advertising board that only the camera can see.

There's a narrow display behind the finish line with a quickly-scrolling banner ad.

I was there at the time, and I could clearly see how the advertising board works. It's a vertical line of LEDs that constantly rotates through the columns of the logo. To the naked eye it just seems to be flickering randomly.

I was curious what the advertising board would have looked like to the athletes -- it must be a bit distracting to see a display scrolling at ~10m/s!

You can see it here at 9m50s:

https://youtu.be/7Xnr805bm4E?feature=shared&t=590

It's just a single animated strip, one pixel wide. I assume the camera array is on the opposite side.

Video no longer available :-(

NBC is very quick to DMCA Olympics clips.

NBC isn't DMCA-ing the official Olympics channel.

NBC is very quick to DMCA Olympics clips.

Writing a check for $12,000,000,000.00 will do that to you.

https://marketrealist.com/p/how-much-did-nbc-pay-for-the-oly...

Region blocked from the US.

Thanks for pointing that out! This video doesn't seem to be available in the US, so you can also see it in the slow motion footage here, right on the finish line:

https://www.youtube.com/watch?v=tcxyXnPIF4o#t=2m45s

(You can see it in normal speed too, but I can feel the formation of shapes better in the slow-mo, instead of it just feeling like blinking)

Ha, and that video isn't visible outside the US!

I guess pick one or the other depending on your country (or if neither works, search for one that does).

This makes total sense as a way to quickly validate the cameras calibration visually. “Does the ad look correct?”

Clever.

It’s weird that it still comes out distorted though. You’d think they’d know the correct rate to play it back at, since they’re the ones capturing it.

They would need to know beforehand the precise span between the first and last athletes.

I think they did a pretty good job, it's only slightly stretched for an event when there was only 0,12 seconds between the first and the last runners, which is unusual.

I don’t see why the distance between athletes has anything to do with it; they record at a fixed speed which presumably is the estimated speed of the winner crossing the line. So it’s known in advance, and the stretch factor if they get that speed exactly right is 1 anyway.

And of course when I say ‘speed’, I mean [width of strip] / [time between the capture of each strip].

This smells of a cascading dongle tape wrapped and marked "sprints #3"

Oh thanks for this! I had the same question and assumed the Omega branding and Olympic rings were superimposed onto the image; I never assumed it would actually be an animated advertisement timed perfectly to match the runners' pace! I assume they had a different backdrop speed for the women's race, according to their projected pace? Or would the ad look squished for them? (or alternatively, the runners stretched?)

Now that I look at it, the logos in the men's photo finish look stretched, so maybe it's calibrated for the women's pace.

When I read "line scan camera" I thought of "rolling shutter" which in digital cameras works out to be a horizontal line at a time, which causes interesting artifacts with quick moving objects like propellers or windshield wipers.

Questions that came to mind and answers as far as I can tell:

  Q: Are the scan lines parallel to the plane of completion?
  A: There is only one scan line and it is parallel to the plane of completion. "In track, the cameras are only focused on the 5 mm near the finish line." [0]

  Q: If yes, is the sequence of lines scanned in the same direction as the competitor movement?
  A: There is only one scan line.

0. https://www.axios.com/2024/08/05/noah-lyles-wins-gold-track-...

Also interesting: https://www.hodinkee.com/articles/olympics-2024-mens-100m-ph...

Note that the rolling shutter effect is not limited to digital photography: it also affects physical focal plane shutters at high shutter speeds due to the time it takes for shutter curtains to move across the frame.

This effect is the origin of the "fast race car slants forward" trope:

https://about.usps.com/news/national-releases/2011/images/pr...

https://imsmuseum.org/adoptable_car/1911-winner/

First I was wondering why there are ad banners visible in the image. With a line scan camera the background which is standing still should actually only have horizontal stripes.

But since the company Omega produced this photo it actually makes sense to artificially add their logo next to the Olympic rings.

i read that that actually a led screen that basically is synced to the camera so it appears as a 2d banner when in actuallity its a 1d banner that changes over time which when displayed like it is looks like a 2d banner. kinda neat. In a weird way similar to the the adverts painted on the pitch or trackside that look like 2d on screen dogs but only look that way from a certain camera angle

This is genius, thanks for pointing that out!

You can see it flash in the video of the event: https://youtu.be/7Xnr805bm4E?feature=shared&t=417

Thanks for the link! This is so great so actually see how the line camera and this line screen work together to create an ad in the finish line photo.

It’s slightly misleading to say it’s time rather than space, though, because of course the subjects are moving and so each column does correspond to a different position (with respect to each one). With respect to the track, however, moving horizontally is obviously just moving in time.

Another weird (until you think about it) thing that’s true: every pixel in the image is at the finish line.

The 'vertical' axis is spatial, representing position across the track, but I would argue that if we take your final sentence and append "...at sequential time intervals" to it, we end up making the case that the horizontal axis in the image is actually time, not space. Each column is the finish line at a different time, and perhaps counter-intuitively (given the usual convention), the right-most column is the earliest.

This is clearer, I think, from considering how the banner is made: as others have pointed out, it is produce by having a single row of LEDs in line with the finish line, each one being turned on and off in a sequence which results in the banner shown in the picture.

Reply on news.ycombinator.com