Comment by heavyset_go

A lot of classic perceptual hash algorithms do "squinty" comparisons, where if an image kind of looks like one you've hashed against, you can get false positives.

I'd imagine outside of egregious abuse and truly unique images, you could squint at a legal image and say it looks very much like another illegal image, and get a false positive.

From what I'm reading about PhotoDNA, it's your standard phashing system from 15 years ago, which is terrifying.

But yes, you can add heuristics, but you will still get false positives.