For anyone wondering if the speed of light is a concern, a good mnemonic is that a foot is roughly equal to a nanosecond. So a millisecond is long enough (by a factor of ~50000) that camera placement doesn’t matter.
Well, camera placement matters in the obvious way: it has to be on the finish "line". Where "line" is really a plane extending vertically from the line drawn on the surface.
I vaguely remember an article about a bike race photo finish that showed that this is not a given and reverse engineered where the camera was pointing (based on background) and who the likely winner was.
But in the end, if I am not mistaken the UCI rules state that the photo finish line is the final arbiter, not the paint on the ground. I don't think this is a great injustice - it's a sport and the rules are arbitrary to a great extent (e.g. male riders 180 cm to 189.9 cm tall must use a smaller bike than those 190 cm and taller in time trials - which sucks for those 189.9 cm tall).
I used to work with a company who insisted on timestamping their sensor data with nanosecond accuracy. Mind you they were not working on anything high speed, or explosive in nature. Just robots moving around at best with highway speeds.
I asked them if they care so much about the accuracy will they also adjust their data for time-of-flight delays. Since by the time the light bouncing from an object hits your lidar/camera meters away what you are sensing is several nanoseconds the past.
To top it off they stored the nanoseconds elapsed since the unix epoch as a float double. So floating point weirdness immediately destroyed all the precision they worked so hard on.
I do appreciate that lightspeed is "about a billion feet per second" and it's also a number that's known exactly, since the foot is defined in terms of the speed of light in a vacuum.
As Grace Hopper so elegantly pointed out. She gave me one of her many nanoseconds, which were lengths of scrap telephone wire cut to a little under a foot long.
This is brilliant, thanks for posting. Things like this make me miss my deceased father. I would love to discuss this with him. He was the only person I knew of that genuinely loved thoroughbred horse racing for the beauty of the animals and for the sport. We would camp out right on the finish line at Belmont park (and often on random weekends when the place was mostly empty) and stay put for hours waiting for the Belmont Stakes. He was an amateur photographer and once got a press pass to photograph from the trench along the outside of the track. I noticed the tall narrow mirror mounted to the finish line pole, and he said it was for the photo finish camera but, not much elaboration beyond that.
Another interesting takeaway is that because you are taking a single line capture the pixel distance equates to time not to distance like a normal image.
What that means is that if you pick two points on the car, you can measure that distance in pixels and multiply by the frequency to get change in time.
Divide the actual physical distance of those two points by that result and you know how fast the car was traveling.
I was surprised to see they use the front bumper splitter as the "start" of the car and not the hood.
You would expect teams to game the system and use longer splitters if photo finishes happen often enough. Tried taking a look at the regulations and it seems that they're not publicly available.
That’s mitigated a bit by the airflow implications of longer splitters, where a few extra cm might cost you more in drag or downforce reduction than you’d gain in a photo finish like this.
My beef is that NASCAR uses a rolling start. This isn't a horse race or Olympic 50m dash where all racers start in the same place under a starters pistol.
Buescher started 4 rows behind Larson. Doesn't that mean he raced the 400 miles faster than Larson?
NASCAR is not meant to be logical, it's meant to be an exciting spectacle. Just look at the "lucky dog" rule: it allows the first of the lapped drivers to be put in the main lap during a yellow. The goal is to create a race within the race for this spot.
In effect, you can win a race while having traveled 1 or 2 laps less than the others if you benefited from the lucky dog rule once or twice.
> In effect, you can win a race while having traveled 1 or 2 laps less than the others if you benefited from the lucky dog rule once or twice.
The driver still has to travel the full number of laps to win the race, same as any other winner would. The lucky dog just gets to drive faster than anyone else is allowed to during caution. The driver actually has to drive their car all the way around the track. It's the same as if the driver was able to drive faster than the leader in the middle of the race and get their lap back the hard way. No one would say he traveled fewer laps. They would say he drove faster.
Splitter length is very strictly regulated, as is every part of the stock car. There have been big penalties for much more subtle infractions, especially in the last few seasons with the current car design.
> You would expect teams to game the system and use longer splitters if photo finishes happen often enough. Tried taking a look at the regulations and it seems that they're not publicly available.
NASCAR cars are standardized and measured before and after every race. Failing a race inspection severely penalizes the team (loss of points, driver or crew chief suspensions, fines). Failing a pre-race inspection means the car doesn't race until it passes inspection. Teams aren't allowed to install splitters with different lengths than their competitors.
NASCAR, however, will mandate different splitter measurements for different racetracks. Still, all cars at each racetrack must use splitters that match NASCAR's requirements.
NASCAR fan here. Over the years the measurements of the car have gone from metal templates laying over the car to using lasers, with 1/32 of an inch making a difference.
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[ 1.3 ms ] story [ 68.7 ms ] threadI vaguely remember an article about a bike race photo finish that showed that this is not a given and reverse engineered where the camera was pointing (based on background) and who the likely winner was.
edit: found it https://www.tglyn.ch/blog/amstel_gold/
But in the end, if I am not mistaken the UCI rules state that the photo finish line is the final arbiter, not the paint on the ground. I don't think this is a great injustice - it's a sport and the rules are arbitrary to a great extent (e.g. male riders 180 cm to 189.9 cm tall must use a smaller bike than those 190 cm and taller in time trials - which sucks for those 189.9 cm tall).
I used to work with a company who insisted on timestamping their sensor data with nanosecond accuracy. Mind you they were not working on anything high speed, or explosive in nature. Just robots moving around at best with highway speeds.
I asked them if they care so much about the accuracy will they also adjust their data for time-of-flight delays. Since by the time the light bouncing from an object hits your lidar/camera meters away what you are sensing is several nanoseconds the past.
To top it off they stored the nanoseconds elapsed since the unix epoch as a float double. So floating point weirdness immediately destroyed all the precision they worked so hard on.
https://americanhistory.si.edu/collections/nmah_692464
https://www.donttakepictures.com/dtp-blog/2019/5/8/winning-b...
(which IIRC was a plot point in a Columbo episode one time - but wasn't every interesting piece of tech?)
What that means is that if you pick two points on the car, you can measure that distance in pixels and multiply by the frequency to get change in time. Divide the actual physical distance of those two points by that result and you know how fast the car was traveling.
You would expect teams to game the system and use longer splitters if photo finishes happen often enough. Tried taking a look at the regulations and it seems that they're not publicly available.
https://www.reddit.com/r/NASCAR/comments/jncj3f/why_is_the_n...
I don't think the teams are allowed to change the splitter size.
Buescher started 4 rows behind Larson. Doesn't that mean he raced the 400 miles faster than Larson?
In effect, you can win a race while having traveled 1 or 2 laps less than the others if you benefited from the lucky dog rule once or twice.
The driver still has to travel the full number of laps to win the race, same as any other winner would. The lucky dog just gets to drive faster than anyone else is allowed to during caution. The driver actually has to drive their car all the way around the track. It's the same as if the driver was able to drive faster than the leader in the middle of the race and get their lap back the hard way. No one would say he traveled fewer laps. They would say he drove faster.
NASCAR cars are standardized and measured before and after every race. Failing a race inspection severely penalizes the team (loss of points, driver or crew chief suspensions, fines). Failing a pre-race inspection means the car doesn't race until it passes inspection. Teams aren't allowed to install splitters with different lengths than their competitors.
NASCAR, however, will mandate different splitter measurements for different racetracks. Still, all cars at each racetrack must use splitters that match NASCAR's requirements.