Line scan camera image processing for train photography

Okay I was stumped about how this works because it's not explained, as far as I can tell. But I guess the sensor array has its long axis perpendicular to the direction the train is traveling.

Line scan sensors are basically just scanners, heck people make em out of scanners .

Usually the issue is they need rather still subjects, but in this case rather than the sensor doing a scanning sweep they're just capturing the subject as it moves by, keeping the background pixels static.

Thanks, I added a section called "Principle of operation" to explain how it works.

You use a single vertical line of sensors and resample "continuously". When doing this with film, the aperture is a vertical slit and you continuously advance the film during the exposure.

For "finish line" cameras, the slit is located at the finish line and you start pulling film when the horses approach. Since the exposure is continuous, you never miss the exact moment of the finish.

What's your FPS/LPS in this setup? I've experimented with similar imaging with an ordinary camera, but LPS was limiting, and I know line-scan machine vision cameras can output some amazing numbers, like 50k+ LPS.

Absolutely fascinating stuff! Thank you so much for adding detailed explanations of the math involved and your process. Always wondered how it worked but never bothered to look it up until today. Reading your page pushed it beyond idle curiosity for me. Thanks for that. And thanks also to HN for always surfacing truly interesting reading material on a daily basis!

It only works for trains because the image of train at t+1 is basically image of train at time t shifted over by a few pixels, right? It doesn't seem like this would work to capture a picture of a human, since humans don't just rigidly translate in space as they move.

If the human is running and doesn't frantically shake it decently works. There's samples of horse race finishing line pics in the article, and they look pretty good IMHO.

It falls apart when the subject is either static or moves it's limbs faster than the speed the whole subject moves (e.g. fist bumping while slowly walking past the camera would screw it)

The analogue equivalent (a slit scan camera) is easier to understand, I think https://www.lomography.com/magazine/283280-making-a-slit-sca... https://petapixel.com/2017/10/18/role-slit-scan-image-scienc...

You can also get close in software. Record some video while walking past a row of shops. Use ffmpeg to explode the video into individual frames. Extract column 0 from every frame, and combine them into a single image, appending each extracted column to the right-hand-side of your output image. You'll end up with something far less accurate than the images in this post, but still fun. Also interesting to try scenes from movies. This technique maps time onto space in interesting ways.

Depends what you're going for.

https://en.wikipedia.org/wiki/Slit-scan_photography#/media/F...