Guiding Ships with Moire Patterns

Posted by Eridanus2 7 hours ago

Guiding Ships with Moire Patterns(tinkerings.org)

48 points | 10 comments

dbuxton 2 hours ago|

Many runways have something called PAPI (precision approach path indicator) which does something similar. Basically you have four (or two sometimes for less precise version) lights which appear either red or white depending on whether you are high or low on the glidepath and you aim to get two reds and two whites - more whites means too high, more reds too low.

I can see that this would be not very intuitive for left/right guidance but are similar systems used in ship navigation?

Terr_ 3 hours ago||

This reminds me of pre-GPS systems like LORAN [0], where it also involves ship navigation by demarcating everything into patterned regions.

[0] https://en.wikipedia.org/wiki/LORAN

devanl 5 hours ago||

It's really neat that the author took the time to fabricate their own light.

I've been looking for something similar in concept for when I back into my garage from a side street with basically no driveway. It's a very narrow single-car garage. I can consistently back in well enough for myself, but if I'm not dead-on and I have passengers, passengers on one side of the car are going to have to squeeze to get out.

Essentially it can be hard to tell if I'm perfectly parallel with the garage walls and whether I'm off center, or rather if I'm within tolerance to back in even if the angle isn't perfect.

I've looked into building something like range lights [1], but I don't think the math scales down to short distances. Plus if I'm looking in the rearview mirror or my backup camera, neither of which are perfectly aligned with the direction of travel, that might mess with optical navigation aids.

I might have to dig into this moire approach to see if it would be more feasible.

Yes, obviously this is a minor inconvenience that I'm able to deal with on a regular basis, but it's fun to theorycraft.

1: https://en.wikipedia.org/wiki/Leading_lights

AlotOfReading 3 hours ago||

You can probably get better angular resolution with lenticular sheets. You see one image if you're dead on, and the other images if you're off to the left or right.

Some napkin math says a couple inches of precision should be achievable at reasonable distances?

FabHK 4 hours ago|||

Low-tech solution in my driveway: Put the car into the desired position, then hang up a couple of old tennis balls on strings to just touch it.

devanl 4 hours ago||

Do hanging tennis balls (and their expensive gadget relatives, laser pointers) help with backing in straight?

In my mental model, the tennis ball tells you when you've backed in / pulled in the appropriate distance. I'm not seeing how it would help me back in straight / centered in my garage, or at least not until I've already backed in so far that I'd need to pull out again to try again.

twentyfiveoh1 4 hours ago|||

Usually "good enough". Garages aren't big enough to be completely crazy in angles. People pick a consistent location for the tennis ball. Their face, a sticker, a mirror, etc. If they are off, its not going to line up correctly.

tlavoie 2 hours ago|||

My fire hall has spray-paint lines to align the rear wheels with in the mirror, including a short cross-line at the stopping point when backing in. Works great!

sublinear 4 hours ago||

Could you maybe arrange some hints for yourself in the surrounding landscaping?

I have done similar in the past with what I call "taking mental snapshots", but it's always easier if there are things you can change to simplify the process. i.e. make sure to align the side mirrors with the flower pots when I'm backed up enough to begin straightening out (direction of the misalignment tells which way to adjust).

The extreme end of this would, of course, be putting lines down for the rear tires. I'm just curious if there's a better way using what you already have so it doesn't look dorky.

Ultimately the only trick to backing up perfectly is the distance to each side being equal as you reverse in as straight of a line as you can, so I'm not sure it takes more than two surrounding reference points being compared to another two points on the car. Distance to each side is comparing the lower corners of the rear window to the lower corners of the garage opening. Flower pots or something else further away gives you better "angular resolution" when you're straightening up (no idea what else to call it).

moffkalast 33 minutes ago||

> "it's difficult to research something if you don't know the name of it"

> - Tom Scott, 8 years ago

Ah, research in the pre-LLM era was quite different. You actually had to know which questions to ask.

Going one level down further, it's really hard to imagine what it was like doing research into something you have zero clue about in the pre-Google era. I guess you just asked a librarian and hoped they'd point you roughly towards the right section and then just started to read random books?