Whats the oddest thing about the wiper tech is that we've had the tech for automated wipers since at least the 70s. As a kid my neighbor's Cadillac had it.

tl;dr: you can use optics to determine if there's rain on a surface, from below, without having to use any fancy cameras or anything, just a light source and light sensor.

If you're into this sort of thing, you can buy these sensors and use them as a rain sensor, either as binary "yes its rained" or as a tipping bucket replacement: https://rainsensors.com