Inductor

That's literally just an XLR3 plug. It's used for analog audio and DMX lighting controls.

Don't know why you'd put it in a rock though.

Unfourtunately, I couldn't find a source stating it would be required. AFAIK it's been assumed that they would use perceptual hashes, since that's what various companies have been suggesting/presenting. Like Apple's NeuralHash, which was reverse engineered. It's also the only somewhat practical solution, since exact matches would be easily be circumvented by changing one pixel or mirroring the image.

Patrick Breyer's page on Chat Control has a lot of general information about the EU's proposal.

Matched using perceptual hash algorithms that have an accuracy between 20% and 40%.

relevant xkcd

It is open source, licensed under the Apache License 2.0.

EDIT: Found it on f-droid: https://f-droid.org/en/packages/app.organicmaps/

Have you tried Organic Maps? Works pretty well for me.

base12 has the advantage of being divisible by 2, 3, 4 and 6, while base10 is only divisible by 2 and 5.

Optical Character Recognition. Basically just extracting text from an image.

You can do this in VLC, though it's not very practical. VLC's equalizer has a preamp slider, it's just not great if you want to change it all the time.

Here's a circular rainbow from ~~an aircraft~~ a skydiver:

EDIT: image embedding didn't work

EDIT 2: not from a plane

EDIT 3: sorry for all the edits, fixed image

I'm not an expert, but I guess it would depend on the speed of sound in the rod.

Fun fact about that: in morse code, SOS is a prosign. This means it gets its own special rules.

Rather than being three seperate letters (...

...), it's one letter without any letter spaces (...---...). This is something that applies to all prosigns in morse code, though most of them are just two letters long.

Also, when sending it on repeat you just continue the pattern without any spaces. Instead of ...---... ...---... (with a letter space) or ...---.../...---... (with a word space), you send ...---...---...---...---... and just keep continuing the pattern. iirc SOS is the only prosign where this is a thing.

Other prosigns are for example HH (........) to indicate a correction to something previously sent, and SK (...-.-) (silent key) to signal that you have finished with the current conversation and the frequency is now clear.