There are different "problems" as you're saying that could tackle your needs.

- Gesture recognition to detect different hand gestures

- Skeleton detection to infer people's positions and whether they have a raised hand

- Object detection (e.g., YOLO) to detect hands

I believe in this case you could go for skeleton detection as you get information from which you could infer raised hands.

right but can skeleton detection do hundreds of people at once?

You can always slice the images into smaller ones, run detection on each tile, and combine results. Supervision has a utility for this - https://supervision.roboflow.com/latest/detection/tools/infe..., but it only works with detections. You can get a much more accurate result this way. Here is some side-by-side comparison: https://github.com/roboflow/supervision/releases/tag/0.14.0.

I always hated the "raise your hand if..." because it's so biased by the huge number of people who are too embarrassed or lazy to raise their hand.

It should be "raise your right hand if... and your left hand if not...", then you can exclude people who don't raise any hand.

Slightly more difficult to count though.

Exploit the dimension of time, like PIPS:Lab's Diespace (but using modern face tracking now instead of simple LED tracking 11 years ago). Nod your head for yes, shake your head for no. That's a "hands free interaction", and less flamboyant and spectacular than waving your hands around, for shy and lazy people.

Hi swyx! The easiest way would be to train a custom model to detect raised hands. I found one on Roboflow - https://universe.roboflow.com/search?q=raised%20hand. I'm not sure how good it would be on your images, so I'd recommend adding some of your pictures. Then you just detect hands and detect people and calculate the ratio.

How are these people arranged in the image? It is doable with good models and some sanity checks, comparing hands per square meter and heads per square meter.

just all over the place, perhaps loosely in rows, imagine i'm on stage at a conference and there's 500 people in front of me (real scenario), i'd like to point a camera at them and do a quick poll, feel like it'd be a great demo

you could imagine "massively multiplayer computer vision" is pretty useful for some real life scenarios like switzerland direct democracy elections

Check out PIPS:Lab's "DieSpace" performance art, in which the audience participates by gesturing and drawing letters with LEDs that spell out words like their names, answer yes or no questions by nodding or shaking their head with the LED held to their forehead, and the gestures are recovered along with images of their faces, and reconstructed into their faces and names into a 3D Die Space Cloud! That was at least 11 years ago, but it's pretty obvious how it works, and how fun it is, which is why it works so well! You can see them automatically registering all the faces in the audience on the laptop on stage in real time during the performance.

Diespace internet community for the deceased HD:

https://www.youtube.com/watch?v=aQz_irTiqGE

PIPS: lab at TEDxAmsterdam

https://www.youtube.com/watch?v=ApyDSq_DbQo

PIPS lab Showreel 2017 @ 1:24:

https://youtu.be/FNzTWqraCDU?t=84