Hacker News Reader

This analysis assumes the camera was aligned perfectly parallel to the finish line, but offset from it.

Unfortunately, there's no reason to assume this was true. Almost the opposite! If the operator did set up the camera correctly initially, and we know the far end ended up pointing to the wrong panel (got a knock?), this makes it more likely it was skewed so as to advantage the most distant rider. And that was Van Aert.

Supporting evidence for your point comes, I think, from the fact that, for each bicycle, the left end of the handlebars appears ahead of the right one - meaning that it crossed the plane of the camera view first. I don't think the wheels could have been noticably misaligned from straight ahead here, could they? - in which case, this would imply that the camera plane is skewed as you suggest.

If this is correct, then it suggests a way to estimate, and correct for, the skew, as we have other photos and videos showing approxinately how far the riders were from each other and from the camera, as well as their trajectories as they crossed the line.

Having read the entire article, I believe the point was related to the photo-finish camera being positioned some distance before what is officially the "finish line", approx 20cm+ they say.

The argument is that if Tom was travelling faster, the distance could have been made up in that time, potentially for a win.

You're right, the camera was in front of the actual finish line.

My point is that Wout was ahead for the /entire/ duration where the front wheel intersected the camera line. The wheel's diameter is bigger than 20cm, so the moment when Wout passed the actual finish line is included in this interval. Therefore, Wout was ahead when the winner passed the actual finish line. QED.

Got it, thanks for elaborating, I failed to understand that point from your initial comment.

Stated otherwise,

If there was an overtake -during- the moment of the photo finish then one bicycle would eclipse the other. The overtaken cycle would be entirely longer than the overtaking bicycle.

Line judges go for wheel contact, so for an overtaking to count, the -front wheel- of the overtaking rider would have to be fully within the front wheel of the other.

As long as the rider and their bike is much wider than the misalignment, the only effect is to shift which point in the picture corresponds to the moment they pass the finishing line.

As the OP demonstrates Wout was ahead even at landmarks over 20 cm from the front of the wheel the only way Tom could win was if his front wheel physically elongated over 20 cm in those last milliseconds.

If it was 20 cm off it might just as well have been a few degrees off from being perpendicular to the road/parallel to the line that marks the finish.

The rules are the outcome of reluctant evolution since back in the 19th century, it's hardly surprising that the camera isn't necessarily considered the ultimate source of truth.

(history of techno and a road cycling race on the first lines of hn, today it's really looking as if tailored to mock my interests)

I suspsect the camera was most likely positioned exactly on the finish line, but aimed ≈20cm behind it. This implies the rider furthest from the camera would be given an unfair advantage.

If it was nudged by a gust of wind or something, it seems very unlikely that the camera would have moved a full 20cm but remained parallel to the finish line.

I agree that you're probably correct that Wout did win. However, I don't think you've correctly lined up the center of the wheels, nor the rear of Wouts wheel.

For the center of the wheel, Wouts looks slighty too early and I think due to the larger size of Pidcock's hubs it's hard to determine the center here too. As for the rear of the wheel, Wouts gum wall tyre is hard to see.

So all in all, it does seem likely Wout held the lead. However, a well times bike throw can change a riders speed momentarily for a short period of time and based on the video Pidcock's bike throw was significantly more effective than Wouts. So while I'm not going as far to say Pidcock would have won, I'd say there is a chance Pidcock had the lead momentarily, and that moment may have occurred as Pidcock crossed the actual finish.

Assuming Pidcock was in front on the actual finish line, his hub would have been about 5cm in front of the photo-finish line. That leaves about 3ms for him to finish the bike throw and return his hub behind Wouts hub. Seems like a stretch, but still a possibility :-)

Maybe you could also look at how deformed the wheel perimeter compared to a perfect ellipse to estimate the speed at different points?

EDIT: Or you could calculate whether it's humanly possible to move a bike a few centimeters forwards and back again in about 15ms, which I think the hypothetical bike throw must have been for Pidcock to be the actual winner?

The wheel perimeter idea I did suggest (cmd + f for the '40%' bit down the bottom of the article) but I think the tolerances are likely too small to do it properly. Would love someone to give it a go though.

I think you're right. Stated in simpler terms, both the front and the back of Wout's wheel passed the line of the camera before the front or back of Tom's bike.

Even if the camera were 10 cms off, the fact that the back of Wout's front wheel passed that mark before the back of Tom's front wheel must means that the front of his wheel was still ahead at that moment. (Assuming same sized wheels.)

Thanks, I was thinking about doing the same plot as you. Watching the race, I believe Wout even appeared to be faster after the finish line, if I recall correctly.

The fact that the gap between the blue and green lines is increasing over time confirms your hypothesis.

the rule is that the person ahead in the photo is the winner. doesn't matter if the photo is wrong.

The rules actually state you need _two_ 3500 frames/second cameras aimed at each other.

side issue- the distortion of the spokes is screaming "rolling shutter effect", which tells me that the sensor was only read from one edge to another. If it was top-to-bottom, then the top pixels really could be older than the bottom ones, and vice versa for bottom-to-top scans. If it was left-to-right it's not clear to me whether that could cause false impressions, but it's certainly not a perfect representation of a discreet instant of reality.

It is a kind of rolling shutter effect, but you're misunderstanding the finish camera's design - they capture a single column of pixels and the horizontal axis is actually time. This is why the background of photofinishes is a bunch of streaks.

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

Your comment clearly shows that you haven't read the article.

The main point of the post is that the photo you are showing is not the photo at the time the two guys crossed the line

> Your comment clearly shows that you haven't read the article.

This was my first thought. Then I thought about it some more, and became persuaded I was wrong. I definitely did read the article.

According to the explanation in the article, it’s a narrow slice, so the back of the wheel would be photographed when that passed the finish line, even if ~30cm out – at which point the front would be clearly over the finish line.

The point being that the wheel is bigger than 30cm. It’s subtle, but I’m persuaded – unless I’m missing something, like them not having or not actually enforcing wheel-size regulations.

I personally find it very difficult to reason about something that looks so much like a photograph where only vertically-aligned points were taken at the same time.

I think you misunderstood how a photo finish camera works. Please try re-watching the explanation video. The photo isn't a snapshot of the race at a specific time, the photo shows an entire interval where the top cyclers passed the finish line. It's like having long shutter time and a narrow slit and slowly moving the camera such that you get the view of the slit projected along the entire camera sensor.

You don't understand his point. He states "Each pixel column is a specific point in time." He's saying you can observe when the BACK of the wheel crosses the too-early "camera line" and determine the winner.

Unfortunately this isn't true if the camera line is not parallel to the finish line, which it probably was not.

No, I also thought about it and found it obvious that he won.

I think this photo pretty clearly shows the winner:

https://www.tglyn.ch/blog/images/amstel_barrier_closer.png

The left bike is on the white paint while the right is not. Clear that the left one is slightly ahead of the right

The right wheel appears to be elevated, per another article they both threw their bikes forward at the finish. That would impact the view from this angle to the point where I don’t see that as definitive.

Could that camera be using a horizontal rolling shutter?